int num = 3;

 num = num + 4;
 //这样写在JDK1.5中是没有问题的

 Integer x = 3;

 x = x + 4;

 Integer a = new Integer(127);

 Integer b = new Integer(127);

 System.out.println(a==b);//false

 System.out.println(a.equals(b));//true

 Integer c = 127;//JDK1.5以后，自动装箱如果装箱的是一个字节那么该数据会被共享  不会重新开辟空间

 Integer d = 127;

 System.out.println(c==d);//true

 System.out.println(c.equals(d));//true

     /**

      * Cache to support the object identity semantics of autoboxing for values between

      * -128 and 127 (inclusive) as required by JLS.

      *

      * The cache is initialized on first usage.  The size of the cache

      * may be controlled by the -XX:AutoBoxCacheMax=<size> option.

      * During VM initialization, java.lang.Integer.IntegerCache.high property

      * may be set and saved in the private system properties in the

      * sun.misc.VM class.

      */

     private static class IntegerCache {

         static final int low = -128;

         static final int high;

         static final Integer cache[];

         static {

             // high value may be configured by property

             int h = 127;

             String integerCacheHighPropValue =

                 sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");

             if (integerCacheHighPropValue != null) {

                 int i = parseInt(integerCacheHighPropValue);

                 i = Math.max(i, 127);

                 // Maximum array size is Integer.MAX_VALUE

                 h = Math.min(i, Integer.MAX_VALUE - (-low) -1);

             }

             high = h;

             cache = new Integer[(high - low) + 1];

             int j = low;

             for(int k = 0; k < cache.length; k++)

                 cache[k] = new Integer(j++);

         }

         private IntegerCache() {}

     }

     /**

      * Returns an {@code Integer} instance representing the specified

      * {@code int} value.  If a new {@code Integer} instance is not

      * required, this method should generally be used in preference to

      * the constructor {@link #Integer(int)}, as this method is likely

      * to yield significantly better space and time performance by

      * caching frequently requested values.

      *

      * This method will always cache values in the range -128 to 127,

      * inclusive, and may cache other values outside of this range.

      *

      * @param  i an {@code int} value.

      * @return an {@code Integer} instance representing {@code i}.

      * @since  1.5

      */

     public static Integer valueOf(int i) {

         assert IntegerCache.high >= 127;

         if (i >= IntegerCache.low && i <= IntegerCache.high)

             return IntegerCache.cache[i + (-IntegerCache.low)];

         return new Integer(i);

     }