Java中Integer自动装箱误区

  偶然间接到一段代码：

public class TestInteger {public static void main(String[] args) {Integer i = 100, j = 100;if (i == j) {System.out.println("i == j");} else {System.out.println("i != j");}}}

结果可想而知：输出的是i == j; But 请看下文！

public class TestInteger {public static void main(String[] args) {Integer i = 200, j = 200;if (i == j) {System.out.println("i == j");} else {System.out.println("i != j");}}}

这只是单纯的增大了i和j的值，结果却不一样。输出的是 i != j , 回想一下自动装箱，== 运算符可以判断基本类型是否相等，我开始怀疑是否当i = 200， j = 200，时是否产生了不同的对象。查阅jdk的源代码真的有所发现：

/**     * 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);    }

  综合代码和注释：这个方法会缓存 -128 到 127的值，但是超过这个范围会返回一个 Integer 对象。 还有一个疑问的是 这个IntegerCache，看名字是说整数类缓存，调用里面的静态数组然后返回。再次找到代码：

 /**     * 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() {}    }

这就不难理解了， Java原来是会自动缓存 -128 - 127 的数值之内的数组，方便快速取值。但是这个最大的值在不同平台上不一样。

综上所述： 当数值在-128和127之间时，产生的值是获得内部的缓存。当数值不在之类的时候，返回的是一个类，而且还不会自动拆箱。