Java 中关于Integer的陷阱

在Java中，八大基本类型都有属于他们的包装类，但是int的包装类Integer就比较特殊

我们先看一段程序

		Integer num1 = 128;
        Integer num2 = 128;
        Integer num3 = 12;
        Integer num4 = 12;
        System.out.println(num1 == num2);
        System.out.println(num3 == num4);

从结果我们可以看到，明明 num1 和 num2都是128，但是却不相等；而都等于12时，确实相等的

这是Integer的源码

	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() {}
    }

    /**
     * Returns an {@code Integer} instance representing the specified
     * {@code int} value.  If a new {@code Integer} instance is not
     * required, this method should generally be used in preference to
     * the constructor {@link #Integer(int)}, as this method is likely
     * to yield significantly better space and time performance by
     * caching frequently requested values.
     *
     * This method will always cache values in the range -128 to 127,
     * inclusive, and may cache other values outside of this range.
     *
     * @param  i an {@code int} value.
     * @return an {@code Integer} instance representing {@code i}.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

这是因为在Integer中，内有提供的数缓存只有-128 ~ 127；如果超过了这个范围，则将创建新的空间来存储这个数，而Java里面 ” ==“ 号的机制就是判断两个类型的地址，既然重新开辟了空间，当面两个128 当然就不想等了