java.lang.String 类源码解读
2024-03-25 21:39:58
- String类定义实现了java.io.Serializable, Comparable<String>, CharSequence 三个接口；并且为final修饰。
public final class String
defined
- String由char[]数组实现
/** The value is used for character storage. */
private final char value[]; /** Cache the hash code for the string */
private int hash; // Default to 0value[]用于存储字符串内容，被final修饰，说明一旦创建就不可被修改。String 声明的变量重新赋值即代表重新指向了另一个String实例对象。
实现序列化
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L; /**
* Class String is special cased within the Serialization Stream Protocol.
*
* A String instance is written into an ObjectOutputStream according to
* <a href="{@docRoot}/../platform/serialization/spec/output.html">
* Object Serialization Specification, Section 6.2, "Stream Elements"</a>
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];Serializable
serialVersionUID是记录序列化的版本号，serialPersistentFields用来存储需要被序列化的字段。
String构造方法
// String的value是不可变的，所以可以公用char[]数组表达同一个字符串
public String() {
this.value = "".value;
}
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
// Arrays.copyOf原理是声明新的char[]数组，System.arraycopy进行复制
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count <= 0) {
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
if (offset <= value.length) {
this.value = "".value;
return;
}
}
// Note: offset or count might be near -1>>>1.
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.value = Arrays.copyOfRange(value, offset, offset+count);
} // 通过codePoints 数组创建
public String(int[] codePoints, int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count <= 0) {
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
if (offset <= codePoints.length) {
this.value = "".value;
return;
}
}
// Note: offset or count might be near -1>>>1.
if (offset > codePoints.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
} final int end = offset + count; // Pass 1: Compute precise size of char[]
int n = count;
for (int i = offset; i < end; i++) {
int c = codePoints[i];
if (Character.isBmpCodePoint(c))
continue;
else if (Character.isValidCodePoint(c))
n++;
else throw new IllegalArgumentException(Integer.toString(c));
} // Pass 2: Allocate and fill in char[]
final char[] v = new char[n]; for (int i = offset, j = 0; i < end; i++, j++) {
int c = codePoints[i];
if (Character.isBmpCodePoint(c))
v[j] = (char)c;
else
Character.toSurrogates(c, v, j++);
} this.value = v;
}
/* Common private utility method used to bounds check the byte array
* and requested offset & length values used by the String(byte[],..)
* constructors.
*/
private static void checkBounds(byte[] bytes, int offset, int length) {
if (length < 0)
throw new StringIndexOutOfBoundsException(length);
if (offset < 0)
throw new StringIndexOutOfBoundsException(offset);
if (offset > bytes.length - length)
throw new StringIndexOutOfBoundsException(offset + length);
} /**
* Constructs a new {@code String} by decoding the specified subarray of
* bytes using the specified charset. The length of the new {@code String}
* is a function of the charset, and hence may not be equal to the length
* of the subarray.
*
* <p> The behavior of this constructor when the given bytes are not valid
* in the given charset is unspecified. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @param offset
* The index of the first byte to decode
*
* @param length
* The number of bytes to decode * @param charsetName
* The name of a supported {@linkplain java.nio.charset.Charset
* charset}
*
* @throws UnsupportedEncodingException
* If the named charset is not supported
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code length} arguments index
* characters outside the bounds of the {@code bytes} array
*
* @since JDK1.1
*/
public String(byte bytes[], int offset, int length, String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null)
throw new NullPointerException("charsetName");
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(charsetName, bytes, offset, length);
} /**
* Constructs a new {@code String} by decoding the specified subarray of
* bytes using the specified {@linkplain java.nio.charset.Charset charset}.
* The length of the new {@code String} is a function of the charset, and
* hence may not be equal to the length of the subarray.
*
* <p> This method always replaces malformed-input and unmappable-character
* sequences with this charset's default replacement string. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @param offset
* The index of the first byte to decode
*
* @param length
* The number of bytes to decode
*
* @param charset
* The {@linkplain java.nio.charset.Charset charset} to be used to
* decode the {@code bytes}
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code length} arguments index
* characters outside the bounds of the {@code bytes} array
*
* @since 1.6
*/
public String(byte bytes[], int offset, int length, Charset charset) {
if (charset == null)
throw new NullPointerException("charset");
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(charset, bytes, offset, length);
} /**
* Constructs a new {@code String} by decoding the specified array of bytes
* using the specified {@linkplain java.nio.charset.Charset charset}. The
* length of the new {@code String} is a function of the charset, and hence
* may not be equal to the length of the byte array.
*
* <p> The behavior of this constructor when the given bytes are not valid
* in the given charset is unspecified. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @param charsetName
* The name of a supported {@linkplain java.nio.charset.Charset
* charset}
*
* @throws UnsupportedEncodingException
* If the named charset is not supported
*
* @since JDK1.1
*/
public String(byte bytes[], String charsetName)
throws UnsupportedEncodingException {
this(bytes, 0, bytes.length, charsetName);
} /**
* Constructs a new {@code String} by decoding the specified array of
* bytes using the specified {@linkplain java.nio.charset.Charset charset}.
* The length of the new {@code String} is a function of the charset, and
* hence may not be equal to the length of the byte array.
*
* <p> This method always replaces malformed-input and unmappable-character
* sequences with this charset's default replacement string. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @param charset
* The {@linkplain java.nio.charset.Charset charset} to be used to
* decode the {@code bytes}
*
* @since 1.6
*/
public String(byte bytes[], Charset charset) {
this(bytes, 0, bytes.length, charset);
} /**
* Constructs a new {@code String} by decoding the specified subarray of
* bytes using the platform's default charset. The length of the new
* {@code String} is a function of the charset, and hence may not be equal
* to the length of the subarray.
*
* <p> The behavior of this constructor when the given bytes are not valid
* in the default charset is unspecified. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @param offset
* The index of the first byte to decode
*
* @param length
* The number of bytes to decode
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and the {@code length} arguments index
* characters outside the bounds of the {@code bytes} array
*
* @since JDK1.1
*/
public String(byte bytes[], int offset, int length) {
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(bytes, offset, length);
} /**
* Constructs a new {@code String} by decoding the specified array of bytes
* using the platform's default charset. The length of the new {@code
* String} is a function of the charset, and hence may not be equal to the
* length of the byte array.
*
* <p> The behavior of this constructor when the given bytes are not valid
* in the default charset is unspecified. The {@link
* java.nio.charset.CharsetDecoder} class should be used when more control
* over the decoding process is required.
*
* @param bytes
* The bytes to be decoded into characters
*
* @since JDK1.1
*/
public String(byte bytes[]) {
this(bytes, 0, bytes.length);
} /**
* Allocates a new string that contains the sequence of characters
* currently contained in the string buffer argument. The contents of the
* string buffer are copied; subsequent modification of the string buffer
* does not affect the newly created string.
*
* @param buffer
* A {@code StringBuffer}
*/
public String(StringBuffer buffer) {
synchronized(buffer) {
this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
}
} /**
* Allocates a new string that contains the sequence of characters
* currently contained in the string builder argument. The contents of the
* string builder are copied; subsequent modification of the string builder
* does not affect the newly created string.
*
* <p> This constructor is provided to ease migration to {@code
* StringBuilder}. Obtaining a string from a string builder via the {@code
* toString} method is likely to run faster and is generally preferred.
*
* @param builder
* A {@code StringBuilder}
*
* @since 1.5
*/
public String(StringBuilder builder) {
this.value = Arrays.copyOf(builder.getValue(), builder.length());
} /*
* Package private constructor which shares value array for speed.
* this constructor is always expected to be called with share==true.
* a separate constructor is needed because we already have a public
* String(char[]) constructor that makes a copy of the given char[].
*/
String(char[] value, boolean share) {
// assert share : "unshared not supported";
this.value = value;
}constructor
String是不可变的，所以可以公用char[]数组表达同一个字符串，从构造方法可以看出，this.value = origin.value.
构造函数可通过【codepoint】创建，这里需要另外详细了解字符编码和【bmpcodepoint】等概念。
通过byte[]创建的字符串都是由【StringCoding.decode】方法进行获取char[]。常用String方法
public int length() {
return value.length;
} public boolean isEmpty() {
return value.length == 0;
} public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}从源码可以看出，通过string直接获取的length是char的长度，但是由于string是使用utf-16对字符串进行编码存储在char数组中，
所以http://www.qqxiuzi.cn/zh/hanzi-unicode-bianma.php?zfj=kzb 对于类似这种生僻字，需要两个char才能表示，因此length长度
不能代表字符个数。codePoint代码点相关方法
/**
* Returns the character (Unicode code point) at the specified
* index. The index refers to {@code char} values
* (Unicode code units) and ranges from {@code 0} to
* {@link #length()}{@code - 1}.
*
* <p> If the {@code char} value specified at the given index
* is in the high-surrogate range, the following index is less
* than the length of this {@code String}, and the
* {@code char} value at the following index is in the
* low-surrogate range, then the supplementary code point
* corresponding to this surrogate pair is returned. Otherwise,
* the {@code char} value at the given index is returned.
*
* @param index the index to the {@code char} values
* @return the code point value of the character at the
* {@code index}
* @exception IndexOutOfBoundsException if the {@code index}
* argument is negative or not less than the length of this
* string.
* @since 1.5
*/
public int codePointAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, value.length);
} /**
* Returns the character (Unicode code point) before the specified
* index. The index refers to {@code char} values
* (Unicode code units) and ranges from {@code 1} to {@link
* CharSequence#length() length}.
*
* <p> If the {@code char} value at {@code (index - 1)}
* is in the low-surrogate range, {@code (index - 2)} is not
* negative, and the {@code char} value at {@code (index -
* 2)} is in the high-surrogate range, then the
* supplementary code point value of the surrogate pair is
* returned. If the {@code char} value at {@code index -
* 1} is an unpaired low-surrogate or a high-surrogate, the
* surrogate value is returned.
*
* @param index the index following the code point that should be returned
* @return the Unicode code point value before the given index.
* @exception IndexOutOfBoundsException if the {@code index}
* argument is less than 1 or greater than the length
* of this string.
* @since 1.5
*/
public int codePointBefore(int index) {
int i = index - 1;
if ((i < 0) || (i >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointBeforeImpl(value, index, 0);
} /**
* Returns the number of Unicode code points in the specified text
* range of this {@code String}. The text range begins at the
* specified {@code beginIndex} and extends to the
* {@code char} at index {@code endIndex - 1}. Thus the
* length (in {@code char}s) of the text range is
* {@code endIndex-beginIndex}. Unpaired surrogates within
* the text range count as one code point each.
*
* @param beginIndex the index to the first {@code char} of
* the text range.
* @param endIndex the index after the last {@code char} of
* the text range.
* @return the number of Unicode code points in the specified text
* range
* @exception IndexOutOfBoundsException if the
* {@code beginIndex} is negative, or {@code endIndex}
* is larger than the length of this {@code String}, or
* {@code beginIndex} is larger than {@code endIndex}.
* @since 1.5
*/
public int codePointCount(int beginIndex, int endIndex) {
if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {
throw new IndexOutOfBoundsException();
}
return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);
} /**
* Returns the index within this {@code String} that is
* offset from the given {@code index} by
* {@code codePointOffset} code points. Unpaired surrogates
* within the text range given by {@code index} and
* {@code codePointOffset} count as one code point each.
*
* @param index the index to be offset
* @param codePointOffset the offset in code points
* @return the index within this {@code String}
* @exception IndexOutOfBoundsException if {@code index}
* is negative or larger then the length of this
* {@code String}, or if {@code codePointOffset} is positive
* and the substring starting with {@code index} has fewer
* than {@code codePointOffset} code points,
* or if {@code codePointOffset} is negative and the substring
* before {@code index} has fewer than the absolute value
* of {@code codePointOffset} code points.
* @since 1.5
*/
public int offsetByCodePoints(int index, int codePointOffset) {
if (index < 0 || index > value.length) {
throw new IndexOutOfBoundsException();
}
return Character.offsetByCodePointsImpl(value, 0, value.length,
index, codePointOffset);
}codePoint
在第4点说道，length()方法并不能代表string的字符个数，这里可以通过codePointCount(0,str.length()) 来获取String的字符个数。
copy chars
void getChars(char dst[], int dstBegin) {
System.arraycopy(value, 0, dst, dstBegin, value.length);
} public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
if (srcBegin < 0) {
throw new StringIndexOutOfBoundsException(srcBegin);
}
if (srcEnd > value.length) {
throw new StringIndexOutOfBoundsException(srcEnd);
}
if (srcBegin > srcEnd) {
throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
}
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}getchars就是复制string的value数组。 第一个是默认修饰符，即只能java.lang包内使用，两个复制getchars的实现都是由System.arraycopy。
encode for string
public byte[] getBytes(String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null) throw new NullPointerException();
return StringCoding.encode(charsetName, value, 0, value.length);
} public byte[] getBytes(Charset charset) {
if (charset == null) throw new NullPointerException();
return StringCoding.encode(charset, value, 0, value.length);
} public byte[] getBytes() {
return StringCoding.encode(value, 0, value.length);
}getBytes
string提供了三种getBytes方式，前两种是传入编码格式参数，第三种则是使用jvm默认编码方式来获取 bytes。
equals 方法解析
// char数组进行一一索引值判断
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
} // 为阻塞方法，因StringBuffer是阻塞的。
public boolean contentEquals(StringBuffer sb) {
return contentEquals((CharSequence)sb);
} // 非阻塞，StringBuilder为非阻塞的。char数组进行一一索引值判断
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
char v1[] = value;
char v2[] = sb.getValue();
int n = v1.length;
if (n != sb.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != v2[i]) {
return false;
}
}
return true;
} // char Sequence类似char[]。char数组进行一一索引值判断
public boolean contentEquals(CharSequence cs) {
// Argument is a StringBuffer, StringBuilder
if (cs instanceof AbstractStringBuilder) {
if (cs instanceof StringBuffer) {
synchronized(cs) {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
} else {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
}
// Argument is a String
if (cs instanceof String) {
return equals(cs);
}
// Argument is a generic CharSequence
char v1[] = value;
int n = v1.length;
if (n != cs.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != cs.charAt(i)) {
return false;
}
}
return true;
} // 该相等方法与上面不同的是，该方法用的是匹配方法，上面使用的是一一比较索引值方法
public boolean equalsIgnoreCase(String anotherString) {
return (this == anotherString) ? true
: (anotherString != null)
&& (anotherString.value.length == value.length)
&& regionMatches(true, 0, anotherString, 0, value.length);
} // 指定范围进行一一索引值匹配
public boolean regionMatches(boolean ignoreCase, int toffset,
String other, int ooffset, int len) {
char ta[] = value;
int to = toffset;
char pa[] = other.value;
int po = ooffset;
// Note: toffset, ooffset, or len might be near -1>>>1.
if ((ooffset < 0) || (toffset < 0)
|| (toffset > (long)value.length - len)
|| (ooffset > (long)other.value.length - len)) {
return false;
}
while (len-- > 0) {
char c1 = ta[to++];
char c2 = pa[po++];
if (c1 == c2) {
continue;
}
if (ignoreCase) {
// If characters don't match but case may be ignored,
// try converting both characters to uppercase.
// If the results match, then the comparison scan should
// continue.
char u1 = Character.toUpperCase(c1);
char u2 = Character.toUpperCase(c2);
if (u1 == u2) {
continue;
}
// Unfortunately, conversion to uppercase does not work properly
// for the Georgian alphabet, which has strange rules about case
// conversion. So we need to make one last check before
// exiting.
if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
continue;
}
}
return false;
}
return true;
} public boolean startsWith(String prefix, int toffset) {
char ta[] = value;
int to = toffset;
char pa[] = prefix.value;
int po = 0;
int pc = prefix.value.length;
// Note: toffset might be near -1>>>1.
if ((toffset < 0) || (toffset > value.length - pc)) {
return false;
}
while (--pc >= 0) {
if (ta[to++] != pa[po++]) {
return false;
}
}
return true;
} public boolean startsWith(String prefix) {
return startsWith(prefix, 0);
} public boolean endsWith(String suffix) {
return startsWith(suffix, value.length - suffix.value.length);
}equals
大小比较compareTo方法解析
// 一一比较索引值，通过char进行 大小比较。
public int compareTo(String anotherString) {
int len1 = value.length;
int len2 = anotherString.value.length;
int lim = Math.min(len1, len2);
char v1[] = value;
char v2[] = anotherString.value; int k = 0;
while (k < lim) {
char c1 = v1[k];
char c2 = v2[k];
if (c1 != c2) {
return c1 - c2;
}
k++;
}
return len1 - len2;
} // 定义了静态忽略大小写的比较器Comparator变量
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
// 定义了私有内部类-忽略大小写的字符串比较器类
private static class CaseInsensitiveComparator
implements Comparator<String>, java.io.Serializable {
// use serialVersionUID from JDK 1.2.2 for interoperability
private static final long serialVersionUID = 8575799808933029326L; public int compare(String s1, String s2) {
int n1 = s1.length();
int n2 = s2.length();
int min = Math.min(n1, n2);
for (int i = 0; i < min; i++) {
char c1 = s1.charAt(i);
char c2 = s2.charAt(i);
if (c1 != c2) {
c1 = Character.toUpperCase(c1);
c2 = Character.toUpperCase(c2);
if (c1 != c2) {
c1 = Character.toLowerCase(c1);
c2 = Character.toLowerCase(c2);
if (c1 != c2) {
// No overflow because of numeric promotion
return c1 - c2;
}
}
}
}
return n1 - n2;
} /** Replaces the de-serialized object. */
private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
} // 忽略大小写的比较方法
public int compareToIgnoreCase(String str) {
return CASE_INSENSITIVE_ORDER.compare(this, str);
}compareTo
在CaseInsensitiveComparator私有内部类中，定义了readResolve()方法，这个方法的目的是保证CaseInsensitiveComparator在反序列化中也能保持单例。
string 重写hashCode
/**
* s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
*/
public int hashCode() {
int h = hash;
if (h == 0 && value.length > 0) {
char val[] = value; for (int i = 0; i < value.length; i++) {
h = 31 * h + val[i];
}
hash = h;
}
return h;
}string重写hashcode使用的质数31。
indexof等通过值查询索引
public int indexOf(int ch) {
return indexOf(ch, 0);
} /**
* Returns the index within this string of the first occurrence of the
* specified character, starting the search at the specified index.
* <p>
* If a character with value {@code ch} occurs in the
* character sequence represented by this {@code String}
* object at an index no smaller than {@code fromIndex}, then
* the index of the first such occurrence is returned. For values
* of {@code ch} in the range from 0 to 0xFFFF (inclusive),
* this is the smallest value <i>k</i> such that:
* <blockquote><pre>
* (this.charAt(<i>k</i>) == ch) {@code &&} (<i>k</i> >= fromIndex)
* </pre></blockquote>
* is true. For other values of {@code ch}, it is the
* smallest value <i>k</i> such that:
* <blockquote><pre>
* (this.codePointAt(<i>k</i>) == ch) {@code &&} (<i>k</i> >= fromIndex)
* </pre></blockquote>
* is true. In either case, if no such character occurs in this
* string at or after position {@code fromIndex}, then
* {@code -1} is returned.
*
* <p>
* There is no restriction on the value of {@code fromIndex}. If it
* is negative, it has the same effect as if it were zero: this entire
* string may be searched. If it is greater than the length of this
* string, it has the same effect as if it were equal to the length of
* this string: {@code -1} is returned.
*
* <p>All indices are specified in {@code char} values
* (Unicode code units).
*
* @param ch a character (Unicode code point).
* @param fromIndex the index to start the search from.
* @return the index of the first occurrence of the character in the
* character sequence represented by this object that is greater
* than or equal to {@code fromIndex}, or {@code -1}
* if the character does not occur.
*/
public int indexOf(int ch, int fromIndex) {
final int max = value.length;
if (fromIndex < 0) {
fromIndex = 0;
} else if (fromIndex >= max) {
// Note: fromIndex might be near -1>>>1.
return -1;
} if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
// handle most cases here (ch is a BMP code point or a
// negative value (invalid code point))
final char[] value = this.value;
for (int i = fromIndex; i < max; i++) {
if (value[i] == ch) {
return i;
}
}
return -1;
} else {
return indexOfSupplementary(ch, fromIndex);
}
} /**
* Handles (rare) calls of indexOf with a supplementary character.
*/
private int indexOfSupplementary(int ch, int fromIndex) {
if (Character.isValidCodePoint(ch)) {
final char[] value = this.value;
final char hi = Character.highSurrogate(ch);
final char lo = Character.lowSurrogate(ch);
final int max = value.length - 1;
for (int i = fromIndex; i < max; i++) {
if (value[i] == hi && value[i + 1] == lo) {
return i;
}
}
}
return -1;
} /**
* Returns the index within this string of the last occurrence of
* the specified character. For values of {@code ch} in the
* range from 0 to 0xFFFF (inclusive), the index (in Unicode code
* units) returned is the largest value <i>k</i> such that:
* <blockquote><pre>
* this.charAt(<i>k</i>) == ch
* </pre></blockquote>
* is true. For other values of {@code ch}, it is the
* largest value <i>k</i> such that:
* <blockquote><pre>
* this.codePointAt(<i>k</i>) == ch
* </pre></blockquote>
* is true. In either case, if no such character occurs in this
* string, then {@code -1} is returned. The
* {@code String} is searched backwards starting at the last
* character.
*
* @param ch a character (Unicode code point).
* @return the index of the last occurrence of the character in the
* character sequence represented by this object, or
* {@code -1} if the character does not occur.
*/
public int lastIndexOf(int ch) {
return lastIndexOf(ch, value.length - 1);
} /**
* Returns the index within this string of the last occurrence of
* the specified character, searching backward starting at the
* specified index. For values of {@code ch} in the range
* from 0 to 0xFFFF (inclusive), the index returned is the largest
* value <i>k</i> such that:
* <blockquote><pre>
* (this.charAt(<i>k</i>) == ch) {@code &&} (<i>k</i> <= fromIndex)
* </pre></blockquote>
* is true. For other values of {@code ch}, it is the
* largest value <i>k</i> such that:
* <blockquote><pre>
* (this.codePointAt(<i>k</i>) == ch) {@code &&} (<i>k</i> <= fromIndex)
* </pre></blockquote>
* is true. In either case, if no such character occurs in this
* string at or before position {@code fromIndex}, then
* {@code -1} is returned.
*
* <p>All indices are specified in {@code char} values
* (Unicode code units).
*
* @param ch a character (Unicode code point).
* @param fromIndex the index to start the search from. There is no
* restriction on the value of {@code fromIndex}. If it is
* greater than or equal to the length of this string, it has
* the same effect as if it were equal to one less than the
* length of this string: this entire string may be searched.
* If it is negative, it has the same effect as if it were -1:
* -1 is returned.
* @return the index of the last occurrence of the character in the
* character sequence represented by this object that is less
* than or equal to {@code fromIndex}, or {@code -1}
* if the character does not occur before that point.
*/
public int lastIndexOf(int ch, int fromIndex) {
if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
// handle most cases here (ch is a BMP code point or a
// negative value (invalid code point))
final char[] value = this.value;
int i = Math.min(fromIndex, value.length - 1);
for (; i >= 0; i--) {
if (value[i] == ch) {
return i;
}
}
return -1;
} else {
return lastIndexOfSupplementary(ch, fromIndex);
}
} /**
* Handles (rare) calls of lastIndexOf with a supplementary character.
*/
private int lastIndexOfSupplementary(int ch, int fromIndex) {
if (Character.isValidCodePoint(ch)) {
final char[] value = this.value;
char hi = Character.highSurrogate(ch);
char lo = Character.lowSurrogate(ch);
int i = Math.min(fromIndex, value.length - 2);
for (; i >= 0; i--) {
if (value[i] == hi && value[i + 1] == lo) {
return i;
}
}
}
return -1;
} /**
* Returns the index within this string of the first occurrence of the
* specified substring.
*
* <p>The returned index is the smallest value <i>k</i> for which:
* <blockquote><pre>
* this.startsWith(str, <i>k</i>)
* </pre></blockquote>
* If no such value of <i>k</i> exists, then {@code -1} is returned.
*
* @param str the substring to search for.
* @return the index of the first occurrence of the specified substring,
* or {@code -1} if there is no such occurrence.
*/
public int indexOf(String str) {
return indexOf(str, 0);
} /**
* Returns the index within this string of the first occurrence of the
* specified substring, starting at the specified index.
*
* <p>The returned index is the smallest value <i>k</i> for which:
* <blockquote><pre>
* <i>k</i> >= fromIndex {@code &&} this.startsWith(str, <i>k</i>)
* </pre></blockquote>
* If no such value of <i>k</i> exists, then {@code -1} is returned.
*
* @param str the substring to search for.
* @param fromIndex the index from which to start the search.
* @return the index of the first occurrence of the specified substring,
* starting at the specified index,
* or {@code -1} if there is no such occurrence.
*/
public int indexOf(String str, int fromIndex) {
return indexOf(value, 0, value.length,
str.value, 0, str.value.length, fromIndex);
} /**
* Code shared by String and AbstractStringBuilder to do searches. The
* source is the character array being searched, and the target
* is the string being searched for.
*
* @param source the characters being searched.
* @param sourceOffset offset of the source string.
* @param sourceCount count of the source string.
* @param target the characters being searched for.
* @param fromIndex the index to begin searching from.
*/
static int indexOf(char[] source, int sourceOffset, int sourceCount,
String target, int fromIndex) {
return indexOf(source, sourceOffset, sourceCount,
target.value, 0, target.value.length,
fromIndex);
} /**
* Code shared by String and StringBuffer to do searches. The
* source is the character array being searched, and the target
* is the string being searched for.
*
* @param source the characters being searched.
* @param sourceOffset offset of the source string.
* @param sourceCount count of the source string.
* @param target the characters being searched for.
* @param targetOffset offset of the target string.
* @param targetCount count of the target string.
* @param fromIndex the index to begin searching from.
*/
static int indexOf(char[] source, int sourceOffset, int sourceCount,
char[] target, int targetOffset, int targetCount,
int fromIndex) {
if (fromIndex >= sourceCount) {
return (targetCount == 0 ? sourceCount : -1);
}
if (fromIndex < 0) {
fromIndex = 0;
}
if (targetCount == 0) {
return fromIndex;
} char first = target[targetOffset];
int max = sourceOffset + (sourceCount - targetCount); for (int i = sourceOffset + fromIndex; i <= max; i++) {
/* Look for first character. */
if (source[i] != first) {
while (++i <= max && source[i] != first);
} /* Found first character, now look at the rest of v2 */
if (i <= max) {
int j = i + 1;
int end = j + targetCount - 1;
for (int k = targetOffset + 1; j < end && source[j]
== target[k]; j++, k++); if (j == end) {
/* Found whole string. */
return i - sourceOffset;
}
}
}
return -1;
} /**
* Returns the index within this string of the last occurrence of the
* specified substring. The last occurrence of the empty string ""
* is considered to occur at the index value {@code this.length()}.
*
* <p>The returned index is the largest value <i>k</i> for which:
* <blockquote><pre>
* this.startsWith(str, <i>k</i>)
* </pre></blockquote>
* If no such value of <i>k</i> exists, then {@code -1} is returned.
*
* @param str the substring to search for.
* @return the index of the last occurrence of the specified substring,
* or {@code -1} if there is no such occurrence.
*/
public int lastIndexOf(String str) {
return lastIndexOf(str, value.length);
} /**
* Returns the index within this string of the last occurrence of the
* specified substring, searching backward starting at the specified index.
*
* <p>The returned index is the largest value <i>k</i> for which:
* <blockquote><pre>
* <i>k</i> {@code <=} fromIndex {@code &&} this.startsWith(str, <i>k</i>)
* </pre></blockquote>
* If no such value of <i>k</i> exists, then {@code -1} is returned.
*
* @param str the substring to search for.
* @param fromIndex the index to start the search from.
* @return the index of the last occurrence of the specified substring,
* searching backward from the specified index,
* or {@code -1} if there is no such occurrence.
*/
public int lastIndexOf(String str, int fromIndex) {
return lastIndexOf(value, 0, value.length,
str.value, 0, str.value.length, fromIndex);
} /**
* Code shared by String and AbstractStringBuilder to do searches. The
* source is the character array being searched, and the target
* is the string being searched for.
*
* @param source the characters being searched.
* @param sourceOffset offset of the source string.
* @param sourceCount count of the source string.
* @param target the characters being searched for.
* @param fromIndex the index to begin searching from.
*/
static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
String target, int fromIndex) {
return lastIndexOf(source, sourceOffset, sourceCount,
target.value, 0, target.value.length,
fromIndex);
} /**
* Code shared by String and StringBuffer to do searches. The
* source is the character array being searched, and the target
* is the string being searched for.
*
* @param source the characters being searched.
* @param sourceOffset offset of the source string.
* @param sourceCount count of the source string.
* @param target the characters being searched for.
* @param targetOffset offset of the target string.
* @param targetCount count of the target string.
* @param fromIndex the index to begin searching from.
*/
static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
char[] target, int targetOffset, int targetCount,
int fromIndex) {
/*
* Check arguments; return immediately where possible. For
* consistency, don't check for null str.
*/
int rightIndex = sourceCount - targetCount;
if (fromIndex < 0) {
return -1;
}
if (fromIndex > rightIndex) {
fromIndex = rightIndex;
}
/* Empty string always matches. */
if (targetCount == 0) {
return fromIndex;
} int strLastIndex = targetOffset + targetCount - 1;
char strLastChar = target[strLastIndex];
int min = sourceOffset + targetCount - 1;
int i = min + fromIndex; startSearchForLastChar:
while (true) {
while (i >= min && source[i] != strLastChar) {
i--;
}
if (i < min) {
return -1;
}
int j = i - 1;
int start = j - (targetCount - 1);
int k = strLastIndex - 1; while (j > start) {
if (source[j--] != target[k--]) {
i--;
continue startSearchForLastChar;
}
}
return start - sourceOffset + 1;
}
}indexOf
string 的操作并返回new String
/**
* Returns a string that is a substring of this string. The
* substring begins with the character at the specified index and
* extends to the end of this string. <p>
* Examples:
* <blockquote><pre>
* "unhappy".substring(2) returns "happy"
* "Harbison".substring(3) returns "bison"
* "emptiness".substring(9) returns "" (an empty string)
* </pre></blockquote>
*
* @param beginIndex the beginning index, inclusive.
* @return the specified substring.
* @exception IndexOutOfBoundsException if
* {@code beginIndex} is negative or larger than the
* length of this {@code String} object.
*/
public String substring(int beginIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
int subLen = value.length - beginIndex;
if (subLen < 0) {
throw new StringIndexOutOfBoundsException(subLen);
}
return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
} /**
* Returns a string that is a substring of this string. The
* substring begins at the specified {@code beginIndex} and
* extends to the character at index {@code endIndex - 1}.
* Thus the length of the substring is {@code endIndex-beginIndex}.
* <p>
* Examples:
* <blockquote><pre>
* "hamburger".substring(4, 8) returns "urge"
* "smiles".substring(1, 5) returns "mile"
* </pre></blockquote>
*
* @param beginIndex the beginning index, inclusive.
* @param endIndex the ending index, exclusive.
* @return the specified substring.
* @exception IndexOutOfBoundsException if the
* {@code beginIndex} is negative, or
* {@code endIndex} is larger than the length of
* this {@code String} object, or
* {@code beginIndex} is larger than
* {@code endIndex}.
*/
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > value.length) {
throw new StringIndexOutOfBoundsException(endIndex);
}
int subLen = endIndex - beginIndex;
if (subLen < 0) {
throw new StringIndexOutOfBoundsException(subLen);
}
return ((beginIndex == 0) && (endIndex == value.length)) ? this
: new String(value, beginIndex, subLen);
} /**
* Returns a character sequence that is a subsequence of this sequence.
*
* <p> An invocation of this method of the form
*
* <blockquote><pre>
* str.subSequence(begin, end)</pre></blockquote>
*
* behaves in exactly the same way as the invocation
*
* <blockquote><pre>
* str.substring(begin, end)</pre></blockquote>
*
* @apiNote
* This method is defined so that the {@code String} class can implement
* the {@link CharSequence} interface.
*
* @param beginIndex the begin index, inclusive.
* @param endIndex the end index, exclusive.
* @return the specified subsequence.
*
* @throws IndexOutOfBoundsException
* if {@code beginIndex} or {@code endIndex} is negative,
* if {@code endIndex} is greater than {@code length()},
* or if {@code beginIndex} is greater than {@code endIndex}
*
* @since 1.4
* @spec JSR-51
*/
public CharSequence subSequence(int beginIndex, int endIndex) {
return this.substring(beginIndex, endIndex);
} /**
* Concatenates the specified string to the end of this string.
* <p>
* If the length of the argument string is {@code 0}, then this
* {@code String} object is returned. Otherwise, a
* {@code String} object is returned that represents a character
* sequence that is the concatenation of the character sequence
* represented by this {@code String} object and the character
* sequence represented by the argument string.<p>
* Examples:
* <blockquote><pre>
* "cares".concat("s") returns "caress"
* "to".concat("get").concat("her") returns "together"
* </pre></blockquote>
*
* @param str the {@code String} that is concatenated to the end
* of this {@code String}.
* @return a string that represents the concatenation of this object's
* characters followed by the string argument's characters.
*/
public String concat(String str) {
int otherLen = str.length();
if (otherLen == 0) {
return this;
}
int len = value.length;
char buf[] = Arrays.copyOf(value, len + otherLen);
str.getChars(buf, len);
return new String(buf, true);
} /**
* Returns a string resulting from replacing all occurrences of
* {@code oldChar} in this string with {@code newChar}.
* <p>
* If the character {@code oldChar} does not occur in the
* character sequence represented by this {@code String} object,
* then a reference to this {@code String} object is returned.
* Otherwise, a {@code String} object is returned that
* represents a character sequence identical to the character sequence
* represented by this {@code String} object, except that every
* occurrence of {@code oldChar} is replaced by an occurrence
* of {@code newChar}.
* <p>
* Examples:
* <blockquote><pre>
* "mesquite in your cellar".replace('e', 'o')
* returns "mosquito in your collar"
* "the war of baronets".replace('r', 'y')
* returns "the way of bayonets"
* "sparring with a purple porpoise".replace('p', 't')
* returns "starring with a turtle tortoise"
* "JonL".replace('q', 'x') returns "JonL" (no change)
* </pre></blockquote>
*
* @param oldChar the old character.
* @param newChar the new character.
* @return a string derived from this string by replacing every
* occurrence of {@code oldChar} with {@code newChar}.
*/
public String replace(char oldChar, char newChar) {
if (oldChar != newChar) {
int len = value.length;
int i = -1;
char[] val = value; /* avoid getfield opcode */ while (++i < len) {
if (val[i] == oldChar) {
break;
}
}
if (i < len) {
char buf[] = new char[len];
for (int j = 0; j < i; j++) {
buf[j] = val[j];
}
while (i < len) {
char c = val[i];
buf[i] = (c == oldChar) ? newChar : c;
i++;
}
return new String(buf, true);
}
}
return this;
} /**
* Tells whether or not this string matches the given <a
* href="../util/regex/Pattern.html#sum">regular expression</a>.
*
* <p> An invocation of this method of the form
* <i>str</i>{@code .matches(}<i>regex</i>{@code )} yields exactly the
* same result as the expression
*
* <blockquote>
* {@link java.util.regex.Pattern}.{@link java.util.regex.Pattern#matches(String,CharSequence)
* matches(<i>regex</i>, <i>str</i>)}
* </blockquote>
*
* @param regex
* the regular expression to which this string is to be matched
*
* @return {@code true} if, and only if, this string matches the
* given regular expression
*
* @throws PatternSyntaxException
* if the regular expression's syntax is invalid
*
* @see java.util.regex.Pattern
*
* @since 1.4
* @spec JSR-51
*/
public boolean matches(String regex) {
return Pattern.matches(regex, this);
} /**
* Returns true if and only if this string contains the specified
* sequence of char values.
*
* @param s the sequence to search for
* @return true if this string contains {@code s}, false otherwise
* @since 1.5
*/
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1;
} /**
* Replaces the first substring of this string that matches the given <a
* href="../util/regex/Pattern.html#sum">regular expression</a> with the
* given replacement.
*
* <p> An invocation of this method of the form
* <i>str</i>{@code .replaceFirst(}<i>regex</i>{@code ,} <i>repl</i>{@code )}
* yields exactly the same result as the expression
*
* <blockquote>
* <code>
* {@link java.util.regex.Pattern}.{@link
* java.util.regex.Pattern#compile compile}(<i>regex</i>).{@link
* java.util.regex.Pattern#matcher(java.lang.CharSequence) matcher}(<i>str</i>).{@link
* java.util.regex.Matcher#replaceFirst replaceFirst}(<i>repl</i>)
* </code>
* </blockquote>
*
*<p>
* Note that backslashes ({@code \}) and dollar signs ({@code $}) in the
* replacement string may cause the results to be different than if it were
* being treated as a literal replacement string; see
* {@link java.util.regex.Matcher#replaceFirst}.
* Use {@link java.util.regex.Matcher#quoteReplacement} to suppress the special
* meaning of these characters, if desired.
*
* @param regex
* the regular expression to which this string is to be matched
* @param replacement
* the string to be substituted for the first match
*
* @return The resulting {@code String}
*
* @throws PatternSyntaxException
* if the regular expression's syntax is invalid
*
* @see java.util.regex.Pattern
*
* @since 1.4
* @spec JSR-51
*/
public String replaceFirst(String regex, String replacement) {
return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
} /**
* Replaces each substring of this string that matches the given <a
* href="../util/regex/Pattern.html#sum">regular expression</a> with the
* given replacement.
*
* <p> An invocation of this method of the form
* <i>str</i>{@code .replaceAll(}<i>regex</i>{@code ,} <i>repl</i>{@code )}
* yields exactly the same result as the expression
*
* <blockquote>
* <code>
* {@link java.util.regex.Pattern}.{@link
* java.util.regex.Pattern#compile compile}(<i>regex</i>).{@link
* java.util.regex.Pattern#matcher(java.lang.CharSequence) matcher}(<i>str</i>).{@link
* java.util.regex.Matcher#replaceAll replaceAll}(<i>repl</i>)
* </code>
* </blockquote>
*
*<p>
* Note that backslashes ({@code \}) and dollar signs ({@code $}) in the
* replacement string may cause the results to be different than if it were
* being treated as a literal replacement string; see
* {@link java.util.regex.Matcher#replaceAll Matcher.replaceAll}.
* Use {@link java.util.regex.Matcher#quoteReplacement} to suppress the special
* meaning of these characters, if desired.
*
* @param regex
* the regular expression to which this string is to be matched
* @param replacement
* the string to be substituted for each match
*
* @return The resulting {@code String}
*
* @throws PatternSyntaxException
* if the regular expression's syntax is invalid
*
* @see java.util.regex.Pattern
*
* @since 1.4
* @spec JSR-51
*/
public String replaceAll(String regex, String replacement) {
return Pattern.compile(regex).matcher(this).replaceAll(replacement);
} /**
* Replaces each substring of this string that matches the literal target
* sequence with the specified literal replacement sequence. The
* replacement proceeds from the beginning of the string to the end, for
* example, replacing "aa" with "b" in the string "aaa" will result in
* "ba" rather than "ab".
*
* @param target The sequence of char values to be replaced
* @param replacement The replacement sequence of char values
* @return The resulting string
* @since 1.5
*/
public String replace(CharSequence target, CharSequence replacement) {
return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}string operate
split分割方法解析
/**
* Splits this string around matches of the given
* <a href="../util/regex/Pattern.html#sum">regular expression</a>.
*
* <p> The array returned by this method contains each substring of this
* string that is terminated by another substring that matches the given
* expression or is terminated by the end of the string. The substrings in
* the array are in the order in which they occur in this string. If the
* expression does not match any part of the input then the resulting array
* has just one element, namely this string.
*
* <p> When there is a positive-width match at the beginning of this
* string then an empty leading substring is included at the beginning
* of the resulting array. A zero-width match at the beginning however
* never produces such empty leading substring.
*
* <p> The {@code limit} parameter controls the number of times the
* pattern is applied and therefore affects the length of the resulting
* array. If the limit <i>n</i> is greater than zero then the pattern
* will be applied at most <i>n</i> - 1 times, the array's
* length will be no greater than <i>n</i>, and the array's last entry
* will contain all input beyond the last matched delimiter. If <i>n</i>
* is non-positive then the pattern will be applied as many times as
* possible and the array can have any length. If <i>n</i> is zero then
* the pattern will be applied as many times as possible, the array can
* have any length, and trailing empty strings will be discarded.
*
* <p> The string {@code "boo:and:foo"}, for example, yields the
* following results with these parameters:
*
* <blockquote><table cellpadding=1 cellspacing=0 summary="Split example showing regex, limit, and result">
* <tr>
* <th>Regex</th>
* <th>Limit</th>
* <th>Result</th>
* </tr>
* <tr><td align=center>:</td>
* <td align=center>2</td>
* <td>{@code { "boo", "and:foo" }}</td></tr>
* <tr><td align=center>:</td>
* <td align=center>5</td>
* <td>{@code { "boo", "and", "foo" }}</td></tr>
* <tr><td align=center>:</td>
* <td align=center>-2</td>
* <td>{@code { "boo", "and", "foo" }}</td></tr>
* <tr><td align=center>o</td>
* <td align=center>5</td>
* <td>{@code { "b", "", ":and:f", "", "" }}</td></tr>
* <tr><td align=center>o</td>
* <td align=center>-2</td>
* <td>{@code { "b", "", ":and:f", "", "" }}</td></tr>
* <tr><td align=center>o</td>
* <td align=center>0</td>
* <td>{@code { "b", "", ":and:f" }}</td></tr>
* </table></blockquote>
*
* <p> An invocation of this method of the form
* <i>str.</i>{@code split(}<i>regex</i>{@code ,} <i>n</i>{@code )}
* yields the same result as the expression
*
* <blockquote>
* <code>
* {@link java.util.regex.Pattern}.{@link
* java.util.regex.Pattern#compile compile}(<i>regex</i>).{@link
* java.util.regex.Pattern#split(java.lang.CharSequence,int) split}(<i>str</i>, <i>n</i>)
* </code>
* </blockquote>
*
*
* @param regex
* the delimiting regular expression
*
* @param limit
* the result threshold, as described above
*
* @return the array of strings computed by splitting this string
* around matches of the given regular expression
*
* @throws PatternSyntaxException
* if the regular expression's syntax is invalid
*
* @see java.util.regex.Pattern
*
* @since 1.4
* @spec JSR-51
*/
public String[] split(String regex, int limit) {
/* fastpath if the regex is a
(1)one-char String and this character is not one of the
RegEx's meta characters ".$|()[{^?*+\\", or
(2)two-char String and the first char is the backslash and
the second is not the ascii digit or ascii letter.
*/
char ch = 0;
if (((regex.value.length == 1 &&
".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) ||
(regex.length() == 2 &&
regex.charAt(0) == '\\' &&
(((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 &&
((ch-'a')|('z'-ch)) < 0 &&
((ch-'A')|('Z'-ch)) < 0)) &&
(ch < Character.MIN_HIGH_SURROGATE ||
ch > Character.MAX_LOW_SURROGATE))
{
int off = 0;
int next = 0;
boolean limited = limit > 0;
ArrayList<String> list = new ArrayList<>();
while ((next = indexOf(ch, off)) != -1) {
if (!limited || list.size() < limit - 1) {
list.add(substring(off, next));
off = next + 1;
} else { // last one
//assert (list.size() == limit - 1);
list.add(substring(off, value.length));
off = value.length;
break;
}
}
// If no match was found, return this
if (off == 0)
return new String[]{this}; // Add remaining segment
if (!limited || list.size() < limit)
list.add(substring(off, value.length)); // Construct result
int resultSize = list.size();
if (limit == 0) {
while (resultSize > 0 && list.get(resultSize - 1).length() == 0) {
resultSize--;
}
}
String[] result = new String[resultSize];
return list.subList(0, resultSize).toArray(result);
}
return Pattern.compile(regex).split(this, limit);
} /**
* Splits this string around matches of the given <a
* href="../util/regex/Pattern.html#sum">regular expression</a>.
*
* <p> This method works as if by invoking the two-argument {@link
* #split(String, int) split} method with the given expression and a limit
* argument of zero. Trailing empty strings are therefore not included in
* the resulting array.
*
* <p> The string {@code "boo:and:foo"}, for example, yields the following
* results with these expressions:
*
* <blockquote><table cellpadding=1 cellspacing=0 summary="Split examples showing regex and result">
* <tr>
* <th>Regex</th>
* <th>Result</th>
* </tr>
* <tr><td align=center>:</td>
* <td>{@code { "boo", "and", "foo" }}</td></tr>
* <tr><td align=center>o</td>
* <td>{@code { "b", "", ":and:f" }}</td></tr>
* </table></blockquote>
*
*
* @param regex
* the delimiting regular expression
*
* @return the array of strings computed by splitting this string
* around matches of the given regular expression
*
* @throws PatternSyntaxException
* if the regular expression's syntax is invalid
*
* @see java.util.regex.Pattern
*
* @since 1.4
* @spec JSR-51
*/
public String[] split(String regex) {
return split(regex, 0);
}split
在分割方法里面，可以看到有两种方法进行分割的，一种是遍历char数组，用List保存分割结果，另一种则是直接用Pattern器的分割方法。
join连接方法
/**
* Returns a new String composed of copies of the
* {@code CharSequence elements} joined together with a copy of
* the specified {@code delimiter}.
*
* <blockquote>For example,
* <pre>{@code
* String message = String.join("-", "Java", "is", "cool");
* // message returned is: "Java-is-cool"
* }</pre></blockquote>
*
* Note that if an element is null, then {@code "null"} is added.
*
* @param delimiter the delimiter that separates each element
* @param elements the elements to join together.
*
* @return a new {@code String} that is composed of the {@code elements}
* separated by the {@code delimiter}
*
* @throws NullPointerException If {@code delimiter} or {@code elements}
* is {@code null}
*
* @see java.util.StringJoiner
* @since 1.8
*/
public static String join(CharSequence delimiter, CharSequence... elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
// Number of elements not likely worth Arrays.stream overhead.
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
} /**
* Returns a new {@code String} composed of copies of the
* {@code CharSequence elements} joined together with a copy of the
* specified {@code delimiter}.
*
* <blockquote>For example,
* <pre>{@code
* List<String> strings = new LinkedList<>();
* strings.add("Java");strings.add("is");
* strings.add("cool");
* String message = String.join(" ", strings);
* //message returned is: "Java is cool"
*
* Set<String> strings = new LinkedHashSet<>();
* strings.add("Java"); strings.add("is");
* strings.add("very"); strings.add("cool");
* String message = String.join("-", strings);
* //message returned is: "Java-is-very-cool"
* }</pre></blockquote>
*
* Note that if an individual element is {@code null}, then {@code "null"} is added.
*
* @param delimiter a sequence of characters that is used to separate each
* of the {@code elements} in the resulting {@code String}
* @param elements an {@code Iterable} that will have its {@code elements}
* joined together.
*
* @return a new {@code String} that is composed from the {@code elements}
* argument
*
* @throws NullPointerException If {@code delimiter} or {@code elements}
* is {@code null}
*
* @see #join(CharSequence,CharSequence...)
* @see java.util.StringJoiner
* @since 1.8
*/
public static String join(CharSequence delimiter,
Iterable<? extends CharSequence> elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}join method
join实现原理是用StringJoiner，StringJoiner则是封装了StringBuilder 进行实现的。
大小写和去除空格方法
/**
* Converts all of the characters in this {@code String} to lower
* case using the rules of the given {@code Locale}. Case mapping is based
* on the Unicode Standard version specified by the {@link java.lang.Character Character}
* class. Since case mappings are not always 1:1 char mappings, the resulting
* {@code String} may be a different length than the original {@code String}.
* <p>
* Examples of lowercase mappings are in the following table:
* <table border="1" summary="Lowercase mapping examples showing language code of locale, upper case, lower case, and description">
* <tr>
* <th>Language Code of Locale</th>
* <th>Upper Case</th>
* <th>Lower Case</th>
* <th>Description</th>
* </tr>
* <tr>
* <td>tr (Turkish)</td>
* <td>\u0130</td>
* <td>\u0069</td>
* <td>capital letter I with dot above -> small letter i</td>
* </tr>
* <tr>
* <td>tr (Turkish)</td>
* <td>\u0049</td>
* <td>\u0131</td>
* <td>capital letter I -> small letter dotless i </td>
* </tr>
* <tr>
* <td>(all)</td>
* <td>French Fries</td>
* <td>french fries</td>
* <td>lowercased all chars in String</td>
* </tr>
* <tr>
* <td>(all)</td>
* <td><img src="doc-files/capiota.gif" alt="capiota"><img src="doc-files/capchi.gif" alt="capchi">
* <img src="doc-files/captheta.gif" alt="captheta"><img src="doc-files/capupsil.gif" alt="capupsil">
* <img src="doc-files/capsigma.gif" alt="capsigma"></td>
* <td><img src="doc-files/iota.gif" alt="iota"><img src="doc-files/chi.gif" alt="chi">
* <img src="doc-files/theta.gif" alt="theta"><img src="doc-files/upsilon.gif" alt="upsilon">
* <img src="doc-files/sigma1.gif" alt="sigma"></td>
* <td>lowercased all chars in String</td>
* </tr>
* </table>
*
* @param locale use the case transformation rules for this locale
* @return the {@code String}, converted to lowercase.
* @see java.lang.String#toLowerCase()
* @see java.lang.String#toUpperCase()
* @see java.lang.String#toUpperCase(Locale)
* @since 1.1
*/
public String toLowerCase(Locale locale) {
if (locale == null) {
throw new NullPointerException();
} int firstUpper;
final int len = value.length; /* Now check if there are any characters that need to be changed. */
scan: {
for (firstUpper = 0 ; firstUpper < len; ) {
char c = value[firstUpper];
if ((c >= Character.MIN_HIGH_SURROGATE)
&& (c <= Character.MAX_HIGH_SURROGATE)) {
int supplChar = codePointAt(firstUpper);
if (supplChar != Character.toLowerCase(supplChar)) {
break scan;
}
firstUpper += Character.charCount(supplChar);
} else {
if (c != Character.toLowerCase(c)) {
break scan;
}
firstUpper++;
}
}
return this;
} char[] result = new char[len];
int resultOffset = 0; /* result may grow, so i+resultOffset
* is the write location in result */ /* Just copy the first few lowerCase characters. */
System.arraycopy(value, 0, result, 0, firstUpper); String lang = locale.getLanguage();
boolean localeDependent =
(lang == "tr" || lang == "az" || lang == "lt");
char[] lowerCharArray;
int lowerChar;
int srcChar;
int srcCount;
for (int i = firstUpper; i < len; i += srcCount) {
srcChar = (int)value[i];
if ((char)srcChar >= Character.MIN_HIGH_SURROGATE
&& (char)srcChar <= Character.MAX_HIGH_SURROGATE) {
srcChar = codePointAt(i);
srcCount = Character.charCount(srcChar);
} else {
srcCount = 1;
}
if (localeDependent ||
srcChar == '\u03A3' || // GREEK CAPITAL LETTER SIGMA
srcChar == '\u0130') { // LATIN CAPITAL LETTER I WITH DOT ABOVE
lowerChar = ConditionalSpecialCasing.toLowerCaseEx(this, i, locale);
} else {
lowerChar = Character.toLowerCase(srcChar);
}
if ((lowerChar == Character.ERROR)
|| (lowerChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
if (lowerChar == Character.ERROR) {
lowerCharArray =
ConditionalSpecialCasing.toLowerCaseCharArray(this, i, locale);
} else if (srcCount == 2) {
resultOffset += Character.toChars(lowerChar, result, i + resultOffset) - srcCount;
continue;
} else {
lowerCharArray = Character.toChars(lowerChar);
} /* Grow result if needed */
int mapLen = lowerCharArray.length;
if (mapLen > srcCount) {
char[] result2 = new char[result.length + mapLen - srcCount];
System.arraycopy(result, 0, result2, 0, i + resultOffset);
result = result2;
}
for (int x = 0; x < mapLen; ++x) {
result[i + resultOffset + x] = lowerCharArray[x];
}
resultOffset += (mapLen - srcCount);
} else {
result[i + resultOffset] = (char)lowerChar;
}
}
return new String(result, 0, len + resultOffset);
} /**
* Converts all of the characters in this {@code String} to lower
* case using the rules of the default locale. This is equivalent to calling
* {@code toLowerCase(Locale.getDefault())}.
* <p>
* <b>Note:</b> This method is locale sensitive, and may produce unexpected
* results if used for strings that are intended to be interpreted locale
* independently.
* Examples are programming language identifiers, protocol keys, and HTML
* tags.
* For instance, {@code "TITLE".toLowerCase()} in a Turkish locale
* returns {@code "t\u005Cu0131tle"}, where '\u005Cu0131' is the
* LATIN SMALL LETTER DOTLESS I character.
* To obtain correct results for locale insensitive strings, use
* {@code toLowerCase(Locale.ROOT)}.
* <p>
* @return the {@code String}, converted to lowercase.
* @see java.lang.String#toLowerCase(Locale)
*/
public String toLowerCase() {
return toLowerCase(Locale.getDefault());
} /**
* Converts all of the characters in this {@code String} to upper
* case using the rules of the given {@code Locale}. Case mapping is based
* on the Unicode Standard version specified by the {@link java.lang.Character Character}
* class. Since case mappings are not always 1:1 char mappings, the resulting
* {@code String} may be a different length than the original {@code String}.
* <p>
* Examples of locale-sensitive and 1:M case mappings are in the following table.
*
* <table border="1" summary="Examples of locale-sensitive and 1:M case mappings. Shows Language code of locale, lower case, upper case, and description.">
* <tr>
* <th>Language Code of Locale</th>
* <th>Lower Case</th>
* <th>Upper Case</th>
* <th>Description</th>
* </tr>
* <tr>
* <td>tr (Turkish)</td>
* <td>\u0069</td>
* <td>\u0130</td>
* <td>small letter i -> capital letter I with dot above</td>
* </tr>
* <tr>
* <td>tr (Turkish)</td>
* <td>\u0131</td>
* <td>\u0049</td>
* <td>small letter dotless i -> capital letter I</td>
* </tr>
* <tr>
* <td>(all)</td>
* <td>\u00df</td>
* <td>\u0053 \u0053</td>
* <td>small letter sharp s -> two letters: SS</td>
* </tr>
* <tr>
* <td>(all)</td>
* <td>Fahrvergnügen</td>
* <td>FAHRVERGNÜGEN</td>
* <td></td>
* </tr>
* </table>
* @param locale use the case transformation rules for this locale
* @return the {@code String}, converted to uppercase.
* @see java.lang.String#toUpperCase()
* @see java.lang.String#toLowerCase()
* @see java.lang.String#toLowerCase(Locale)
* @since 1.1
*/
public String toUpperCase(Locale locale) {
if (locale == null) {
throw new NullPointerException();
} int firstLower;
final int len = value.length; /* Now check if there are any characters that need to be changed. */
scan: {
for (firstLower = 0 ; firstLower < len; ) {
int c = (int)value[firstLower];
int srcCount;
if ((c >= Character.MIN_HIGH_SURROGATE)
&& (c <= Character.MAX_HIGH_SURROGATE)) {
c = codePointAt(firstLower);
srcCount = Character.charCount(c);
} else {
srcCount = 1;
}
int upperCaseChar = Character.toUpperCaseEx(c);
if ((upperCaseChar == Character.ERROR)
|| (c != upperCaseChar)) {
break scan;
}
firstLower += srcCount;
}
return this;
} /* result may grow, so i+resultOffset is the write location in result */
int resultOffset = 0;
char[] result = new char[len]; /* may grow */ /* Just copy the first few upperCase characters. */
System.arraycopy(value, 0, result, 0, firstLower); String lang = locale.getLanguage();
boolean localeDependent =
(lang == "tr" || lang == "az" || lang == "lt");
char[] upperCharArray;
int upperChar;
int srcChar;
int srcCount;
for (int i = firstLower; i < len; i += srcCount) {
srcChar = (int)value[i];
if ((char)srcChar >= Character.MIN_HIGH_SURROGATE &&
(char)srcChar <= Character.MAX_HIGH_SURROGATE) {
srcChar = codePointAt(i);
srcCount = Character.charCount(srcChar);
} else {
srcCount = 1;
}
if (localeDependent) {
upperChar = ConditionalSpecialCasing.toUpperCaseEx(this, i, locale);
} else {
upperChar = Character.toUpperCaseEx(srcChar);
}
if ((upperChar == Character.ERROR)
|| (upperChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
if (upperChar == Character.ERROR) {
if (localeDependent) {
upperCharArray =
ConditionalSpecialCasing.toUpperCaseCharArray(this, i, locale);
} else {
upperCharArray = Character.toUpperCaseCharArray(srcChar);
}
} else if (srcCount == 2) {
resultOffset += Character.toChars(upperChar, result, i + resultOffset) - srcCount;
continue;
} else {
upperCharArray = Character.toChars(upperChar);
} /* Grow result if needed */
int mapLen = upperCharArray.length;
if (mapLen > srcCount) {
char[] result2 = new char[result.length + mapLen - srcCount];
System.arraycopy(result, 0, result2, 0, i + resultOffset);
result = result2;
}
for (int x = 0; x < mapLen; ++x) {
result[i + resultOffset + x] = upperCharArray[x];
}
resultOffset += (mapLen - srcCount);
} else {
result[i + resultOffset] = (char)upperChar;
}
}
return new String(result, 0, len + resultOffset);
} /**
* Converts all of the characters in this {@code String} to upper
* case using the rules of the default locale. This method is equivalent to
* {@code toUpperCase(Locale.getDefault())}.
* <p>
* <b>Note:</b> This method is locale sensitive, and may produce unexpected
* results if used for strings that are intended to be interpreted locale
* independently.
* Examples are programming language identifiers, protocol keys, and HTML
* tags.
* For instance, {@code "title".toUpperCase()} in a Turkish locale
* returns {@code "T\u005Cu0130TLE"}, where '\u005Cu0130' is the
* LATIN CAPITAL LETTER I WITH DOT ABOVE character.
* To obtain correct results for locale insensitive strings, use
* {@code toUpperCase(Locale.ROOT)}.
* <p>
* @return the {@code String}, converted to uppercase.
* @see java.lang.String#toUpperCase(Locale)
*/
public String toUpperCase() {
return toUpperCase(Locale.getDefault());
} /**
* Returns a string whose value is this string, with any leading and trailing
* whitespace removed.
* <p>
* If this {@code String} object represents an empty character
* sequence, or the first and last characters of character sequence
* represented by this {@code String} object both have codes
* greater than {@code '\u005Cu0020'} (the space character), then a
* reference to this {@code String} object is returned.
* <p>
* Otherwise, if there is no character with a code greater than
* {@code '\u005Cu0020'} in the string, then a
* {@code String} object representing an empty string is
* returned.
* <p>
* Otherwise, let <i>k</i> be the index of the first character in the
* string whose code is greater than {@code '\u005Cu0020'}, and let
* <i>m</i> be the index of the last character in the string whose code
* is greater than {@code '\u005Cu0020'}. A {@code String}
* object is returned, representing the substring of this string that
* begins with the character at index <i>k</i> and ends with the
* character at index <i>m</i>-that is, the result of
* {@code this.substring(k, m + 1)}.
* <p>
* This method may be used to trim whitespace (as defined above) from
* the beginning and end of a string.
*
* @return A string whose value is this string, with any leading and trailing white
* space removed, or this string if it has no leading or
* trailing white space.
*/
public String trim() {
int len = value.length;
int st = 0;
char[] val = value; /* avoid getfield opcode */ while ((st < len) && (val[st] <= ' ')) {
st++;
}
while ((st < len) && (val[len - 1] <= ' ')) {
len--;
}
return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}lowupperandtrim
tostring 方法
/**
* This object (which is already a string!) is itself returned.
*
* @return the string itself.
*/
public String toString() {
return this;
} /**
* Converts this string to a new character array.
*
* @return a newly allocated character array whose length is the length
* of this string and whose contents are initialized to contain
* the character sequence represented by this string.
*/
public char[] toCharArray() {
// Cannot use Arrays.copyOf because of class initialization order issues
char result[] = new char[value.length];
System.arraycopy(value, 0, result, 0, value.length);
return result;
}toString
format 格式化，公共静态方法
public static String format(String format, Object... args) {
return new Formatter().format(format, args).toString();
} public static String format(Locale l, String format, Object... args) {
return new Formatter(l).format(format, args).toString();
}format
- valueOf
/**
* Returns the string representation of the {@code Object} argument.
*
* @param obj an {@code Object}.
* @return if the argument is {@code null}, then a string equal to
* {@code "null"}; otherwise, the value of
* {@code obj.toString()} is returned.
* @see java.lang.Object#toString()
*/
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
} /**
* Returns the string representation of the {@code char} array
* argument. The contents of the character array are copied; subsequent
* modification of the character array does not affect the returned
* string.
*
* @param data the character array.
* @return a {@code String} that contains the characters of the
* character array.
*/
public static String valueOf(char data[]) {
return new String(data);
} /**
* Returns the string representation of a specific subarray of the
* {@code char} array argument.
* <p>
* The {@code offset} argument is the index of the first
* character of the subarray. The {@code count} argument
* specifies the length of the subarray. The contents of the subarray
* are copied; subsequent modification of the character array does not
* affect the returned string.
*
* @param data the character array.
* @param offset initial offset of the subarray.
* @param count length of the subarray.
* @return a {@code String} that contains the characters of the
* specified subarray of the character array.
* @exception IndexOutOfBoundsException if {@code offset} is
* negative, or {@code count} is negative, or
* {@code offset+count} is larger than
* {@code data.length}.
*/
public static String valueOf(char data[], int offset, int count) {
return new String(data, offset, count);
} /**
* Equivalent to {@link #valueOf(char[], int, int)}.
*
* @param data the character array.
* @param offset initial offset of the subarray.
* @param count length of the subarray.
* @return a {@code String} that contains the characters of the
* specified subarray of the character array.
* @exception IndexOutOfBoundsException if {@code offset} is
* negative, or {@code count} is negative, or
* {@code offset+count} is larger than
* {@code data.length}.
*/
public static String copyValueOf(char data[], int offset, int count) {
return new String(data, offset, count);
} /**
* Equivalent to {@link #valueOf(char[])}.
*
* @param data the character array.
* @return a {@code String} that contains the characters of the
* character array.
*/
public static String copyValueOf(char data[]) {
return new String(data);
} /**
* Returns the string representation of the {@code boolean} argument.
*
* @param b a {@code boolean}.
* @return if the argument is {@code true}, a string equal to
* {@code "true"} is returned; otherwise, a string equal to
* {@code "false"} is returned.
*/
public static String valueOf(boolean b) {
return b ? "true" : "false";
} /**
* Returns the string representation of the {@code char}
* argument.
*
* @param c a {@code char}.
* @return a string of length {@code 1} containing
* as its single character the argument {@code c}.
*/
public static String valueOf(char c) {
char data[] = {c};
return new String(data, true);
} /**
* Returns the string representation of the {@code int} argument.
* <p>
* The representation is exactly the one returned by the
* {@code Integer.toString} method of one argument.
*
* @param i an {@code int}.
* @return a string representation of the {@code int} argument.
* @see java.lang.Integer#toString(int, int)
*/
public static String valueOf(int i) {
return Integer.toString(i);
} /**
* Returns the string representation of the {@code long} argument.
* <p>
* The representation is exactly the one returned by the
* {@code Long.toString} method of one argument.
*
* @param l a {@code long}.
* @return a string representation of the {@code long} argument.
* @see java.lang.Long#toString(long)
*/
public static String valueOf(long l) {
return Long.toString(l);
} /**
* Returns the string representation of the {@code float} argument.
* <p>
* The representation is exactly the one returned by the
* {@code Float.toString} method of one argument.
*
* @param f a {@code float}.
* @return a string representation of the {@code float} argument.
* @see java.lang.Float#toString(float)
*/
public static String valueOf(float f) {
return Float.toString(f);
} /**
* Returns the string representation of the {@code double} argument.
* <p>
* The representation is exactly the one returned by the
* {@code Double.toString} method of one argument.
*
* @param d a {@code double}.
* @return a string representation of the {@code double} argument.
* @see java.lang.Double#toString(double)
*/
public static String valueOf(double d) {
return Double.toString(d);
}valueOf
在源码提供的所有valueOf方法里面，如果是确定类型的null传入，返回的是字符串“null”，而如果直接传入null，则会发生错误。（“null”是个坑。）
- intern本地方法 ,这个是native 方法，说明是由系统动态库实现的。
/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class {@code String}.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.
* <p>
* It follows that for any two strings {@code s} and {@code t},
* {@code s.intern() == t.intern()} is {@code true}
* if and only if {@code s.equals(t)} is {@code true}.
* <p>
* All literal strings and string-valued constant expressions are
* interned. String literals are defined in section 3.10.5 of the
* <cite>The Java™ Language Specification</cite>.
*
* @return a string that has the same contents as this string, but is
* guaranteed to be from a pool of unique strings.
*/
public native String intern();intern
具体这个方法的原理就跟字符串常量池有关系。
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