Integer使用雙等號比較會發生什麼

話很少說，根據如下程序運行，打印的結果爲何不一樣?

Integer a = 100;
Integer b = 100;

System.out.println(a == b);//print : true


Integer a = 200;
Integer b = 200;

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

以上兩段代碼在執行後爲何會出現這樣神奇的效果，糾察緣由在Integer的源碼中一段特殊的代碼，在代碼中會在虛擬機加載時將-128至127的數據提早緩存到內存中，因此會致使雙等號比較會出現不一致的問題（內存引用地址已緩存）。

在開發中對象類型的比較必定記得使用eques進行內容比較。

/**
     * 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() {}
    }