Java Integer -128~127

浅浅的花香味﹌ 2022-01-23 08:07 206阅读 0赞

今天刷到了一道题，为什么第一个为true，第二个为false。

System.out.println(Integer.valueOf("100")==Integer.valueOf("100"));　　//true
    System.out.println(Integer.valueOf("200")==Integer.valueOf("200"));　　//false

研究源码发现

/**
         * 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会将常用的值（-128 to 127）cache起来。当i值在这个范围时，会比用构造方法Integer(int)效率和空间上更好。