Java Integer之“==”问题

有趣的一个Java基础类的问题。

Integer a = 100;
Integer b = 100;
System.out.println(a == b); 

Integer c = 1000;
Integer d = 1000;
System.out.println(c == d);

输出分别为true,false.
“==”我们都知道是引用目标地址的判断是否相等。因此上述应该都是false才对的。但是为什么会出现这种情况了，去Integer.java源码看下就知道了。

在Integer.java中，发现了其有一个内部类IntegerCache，这个类的作用就是用来缓存-127~128的整数的，所以任何在-127~128之间的两个数值相等的整数以“==”比较都是true。相当于调用了Integer.valueof(int i)的方法

a = Integer.valueof(100);
b = Integer.valueof(100);

public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

IntegerCache的源码：

 /**
     * 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 {@code -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) {
                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() {}
    }

参考：https://dzone.com/articles/why-1000-1000-retyrns-false-but-100-100-returns-true