拆箱与装箱的一些注意事项(Integer -128~127 的恩怨)

public class Main {

    public static void main(String[] args) {

 

            Integer a = 100;

            Integer b = 100;

            Integer c = 128;

            Integer d = 128;

 

            System.out.println(a==b);

            System.out.println(c==d);

    }

}

 

输出：　　true

　　　　　false

原因：

public static Integer valueOf(int i) {

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

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

      return new Integer(i);

  }

可以看到当不满足if语句中的条件，就会重新创建一个对象返回，那结果必然不相等。继续打开IntegerCache可以看到

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) {

              try {

                  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);

              } catch( NumberFormatException nfe) {

                  // If the property cannot be parsed into an int, ignore it.

              }

          }

          high = h;

 

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

          int j = low;

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

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

 

          // range [-128, 127] must be interned (JLS7 5.1.7)

          assert IntegerCache.high >= 127;

      }

 

      private IntegerCache() {}

  }

在通过valueOf方法创建Integer对象的时候，如果数值在[-128,127]之间，便返回指向IntegerCache.cache中已经存在的对象的引用；否则创建一个新的Integer对象。所以上面代码中a与b相等，c与d不相等。