面试----integet c=128； integer d=128；a==b(false)

    @Test
    public void  text(){
     Integer a=128;
     Integer b=128;
     System.out.println(a==b);

     String c="abc";
     String d="abc";
     System.out.println(c==d);



    }

底层实现:

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() {}
    }

底层的意思是： 如果定义的变量值 在IntegerCache.low与IntegerCache.high之间，那就看看前面缓存中cache中有没有这个值，如果有这个值的话就直接返回，否则就返回一个新的Integer实例。IntegerCache.low的默认值是-128，IntegerCache.high的默认值是127。