看看下面這段程式碼跟你想的結果一樣嗎？

        Integer a =127;
        Integer b = 127;
        System.out.println(a==b);//true

        Integer a1 = 128;
        Integer b1 = 128;
        System.out.println(a1==b1);//false

為什麼會是這樣的結果，我們反編譯.class檔案看看

    Integer a = Integer.valueOf(127);
    Integer b = Integer.valueOf(127
 
);
    System.out.println(a == b);

    Integer a1 = Integer.valueOf(128);
    Integer b1 = Integer.valueOf(128);
    System.out.println(a1 == b1);

其實是Integer作者在寫這個類時，為了避免重複建立物件，對一定範圍的Integer做了快取，如果該值沒有在該範圍內則new一個新物件返回，從原始碼中可以看到如下程式碼：

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return
 
 IntegerCache.cache[i + (-IntegerCache.low)]; //i如果在該範圍內直接返回快取值
        return new Integer(i);
    }

IntegerCache 是Integer 類的靜態內部類，再來看看IntegerCache的實現

 private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static
 
 {//當類被載入到java虛擬機器是進行快取建立
            // 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() {}
    }