java關於Integer設置-128到127的靜態緩存

　　今天在一個java群里，看到有個群友問到如下為什么第一個為true，第二個為false。　　

　　System.out.println(Integer.valueOf("50")==Integer.valueOf("50"));　　　//true
　　System.out.println(Integer.valueOf("200")==Integer.valueOf("200"));　　//false

　　由於一開始他問的第二句，我還想當然的以為是new的對象，肯定不一樣，但是為什么第一句為true呢，后來通過查找資料發現

　　1、https://www.zhihu.com/question/29879295/answer/102396251　　

　　2、http://blog.csdn.net/chengzhezhijian/article/details/9628251

    /**
     * Returns a <tt>Integer</tt> instance representing the specified
     * <tt>int</tt> value.
     * If a new <tt>Integer</tt> 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.
     *
     * @param  i an <code>int</code> value.
     * @return a <tt>Integer</tt> instance representing <tt>i</tt>.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        if(i >= -128 && i <= IntegerCache.high)
            return IntegerCache.cache[i + 128];
        else
            return new Integer(i);
    }

　　

 private static class IntegerCache {
        static final int high;
        static final Integer cache[];

        static {
            final int low = -128;

            // high value may be configured by property
            int h = 127;
            if (integerCacheHighPropValue != null) {
                // Use Long.decode here to avoid invoking methods that
                // require Integer's autoboxing cache to be initialized
                int i = Long.decode(integerCacheHighPropValue).intValue();
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - -low);
            }
            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
        }

        private IntegerCache() {}
    }

　　valueOf會將常用的值（-128 to 127）cache起來。當i值在這個范圍時，會比用構造方法Integer(int)效率和空間上更好。