在Java中==的一個坑

 

觀察下面代碼，輸出結果是什么？

 

    public static void main(String[] args) {
        Integer p = 10000;
        Integer q = 10000;
        System.out.println(p == q);
        System.out.println(p.equals(q));
    }

 

運行一次，答案與預想的完全不一樣。在比較數字的時候，一定要用euqals，不能用==c

 

查看Integer的jdk源碼，發現如下片段：

/**
     * 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() {}
    }

這兒的IntegerCache有一個靜態的Integer數組，在類加載時就將-128 到 127 的Integer對象創建了，並保存在cache數組中，一旦程序調用valueOf 方法，如果i的值是在-128 到 127 之間就直接在cache緩存數組中去取Integer對象。

 

如果是-128 到 127 的值，你不管創建多少個這個范圍內的Integer用ValueOf出來的都是同一個對象。