一.代码演示

 Integer i1 = 127;
        Integer i2 = 127;
        System.out.println(i1==i2); //true
        Integer i3 = 128;
        Integer i4 = 128;
        System.out.println(i3==i4);//false

        System.out.println("Byte");
        Byte b1 = 127;
        Byte b2 = 127;
        System.out.println(b1==b2); //true
        Byte b3 = -128;
        Byte b4 = -128;
        System.out.println(b3==b4); //true

        System.out.println("Short");
        Short s1 = 127;
        Short s2 = 127;
        System.out.println(s1==s2); //true
        Short s3 = 128;
        Short s4 = 128;
        System.out.println(s3==s4);//false

        System.out.println("Long");
        Long l1 = Long.valueOf(127);
        Long l2 = Long.valueOf(127);
        System.out.println(l1==l2); //true
        Long l3 = Long.valueOf(128);
        Long l4 = Long.valueOf(128);
        System.out.println(l3==l4);//false

二.结果分析

###1.Integer的装箱缓存

Integer i1 = 127 //实际上 等于 Integer i1 = Integer .valueOf(127) 自动装箱

代码跟进

  public static Integer valueOf(int i) {
  //首先可以看到对参数i进行了范围判断
  		//low = -128; int h = 127
  		//在这个范围内  返回 IntegerCache.cache[i]
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

以上可以知道 当 valueOf(i) i的范围在 -128 ~ 127之间时,返回的是 cache数组的 i + (-IntegerCache.low) 对应下标元素

IntegerCache 是什么呢 根据代码可以知道是内部类

cache 是Integer的数组

 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() {}
    }

看这行代码 for循环给cache动态数组元素赋值

 cache = new Integer[(high - low) + 1];
 			//j = -128
            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;

3.Integer,Byte,Short,Long 四种类型的包装类装箱缓存

再看一个 LongCache 其他都一样

 private static class LongCache {
        private LongCache(){}

        static final Long cache[] = new Long[-(-128) + 127 + 1];

        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Long(i - 128);
        }
    }