装箱和拆箱和ValueOf以及XXXValue -- Java语法糖(Syntax sugar)

自动装箱拆箱发生在基本类型和其包装型互操作的时候。

以前一直不知的拆箱和装箱是valueOf和xxValue的别名。是语法糖的一种

 

 

public static void main(String[] args) {
        Integer i = 10;
        Integer j = 10;
        System.out.println(j == i); //(1) true

        i = 10000;
        j = 10000;
        System.out.println(j == i); //(2) false

        i = new Integer(10000);
        System.out.println(i == 10000); //(3) true
        System.out.println(10000 == i); //(4)  true
        System.out.println(i == 10000L);//(5) true

    }

 

 

(1)处的结果是 true。

(2)处的结果是false。

(3)处的结果是true，true，true

 

(1),(2)处的结果不同让人惊讶。i = 10；和i=10000;发生了自动装箱，编译器自动编译成 Integer.valueOf(10)和Integer.valueOf(10000)。看看这个方法的实现。

 

public static Integer valueOf(int i) {
        if(i >= -128 && i <= IntegerCache.high)
            return IntegerCache.cache[i + 128];
        else
            return new Integer(i);
    }

 

看到某些Integer对象是有缓存的，范围是-128，到high。

再看看IntegerCachestatic 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++);
}

    high默认是127，可以通过integerCacheHighPropValue

调整，看看这个属性的说明

 

/**
    * 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. During VM initialization the
    * getAndRemoveCacheProperties method may be used to get and remove any system
    * properites that configure the cache size. At this time, the size of the
    * cache may be controlled by the vm option -XX:AutoBoxCacheMax=<size>.
    */

   // value of java.lang.Integer.IntegerCache.high property (obtained during VM init)
   private static String integerCacheHighPropValue;

 可以通过该参数调整缓存的范围  -XX:AutoBoxCacheMax=<size>

 

3，4，5处发生拆箱，编译之后其实调用了intValue，或者longValue。获得的是基本类型。

 

 

总结： 两个包装类型的值比较，最好不要用 ==

http://jilen.iteye.com/blog/1189353