Java AutoBoxing揭秘-优快云博客

csdn回复的一个帖子。

当时看到这段代码感觉很荒谬。

1.Integer i1=100; //怎么可以这样初始化呢?

2.i1==i2 //2个不一样的对象比较怎么得出来得结果是true?

不过后来看到有人回复说是jdk1.5的新特性。

查了下jdk1.5的java language features

对于为什么Integer i1=100可以这样写有一点明白了

As any Java programmer knows, you can’t put an int (or other primitive value) into a collection. Collections can only hold object references, so you have to box primitive values into the appropriate wrapper class (which is Integer in the case of int). When you take the object out of the collection, you get the Integer that you put in; if you need an int, you must unbox the Integer using the intValue method. All of this boxing and unboxing is a pain, and clutters up your code. The autoboxing and unboxing feature automates the process, eliminating the pain and the clutter.

还附带了一个例子

public class Frequency {
public static void main(String[] args) {
Map<String, Integer> m = new TreeMap<String, Integer>();
for (String word : args) {
Integer freq = m.get(word);
m.put(word, (freq == null ? 1 : freq + 1));
}
System.out.println(m);
}
}

但可以对于为什么i1==i2打印出来时true还是想不明白。网上找资料。终于找到一篇比较靠谱的文章，提示说用dj反编译下看看。我试了下.

java 代码

public static void main(String[] args) {
Integer i=200;
Integer ii=200;
System.out.println(ii==i);
Integer iii=100;
Integer iiii=100;
System.out.println(iii==iiii);
Map<String, Integer> m = new TreeMap<String, Integer>();
for (String word : args) {
Integer freq = m.get(word);
m.put(word, (freq == null ? 1 : freq + 1));
}
System.out.println(m);
}

反编译为

public static void main(String args[]) { Integer integer = Integer.valueOf(200); Integer integer1 = Integer.valueOf(200); System.out.println(integer1 == integer); Integer integer2 = Integer.valueOf(100); Integer integer3 = Integer.valueOf(100); System.out.println(integer2 == integer3); TreeMap treemap = new TreeMap(); String args1[] = args; int i = args1.length; for(int j = 0; j < i; j++) { String s = args1[j]; Integer integer4 = (Integer)treemap.get(s); treemap.put(s, Integer.valueOf(integer4 != null ? integer4.intValue() + 1 : 1)); } System.out.println(treemap); }

发觉原来和我想的不一样，我本来以为AutoBoxing是通过new一个wrapper class实现的结果却是用valueOf()。但是就算这样，那还是2个对象阿。

于是去看了下Integer.valueOf的源码。

public static Integer valueOf(int i) { final int offset = 128; if (i >= -128 && i <= 127) { // must cache return IntegerCache.cache[i + offset]; } return new Integer(i); }

说明对于一个byte范围大小的参数有特别的处理。于是看了下IntegerCache的源码

这是Integer的一个内部类

private static class IntegerCache { private IntegerCache(){} static final Integer cache[] = new Integer[-(-128) + 127 + 1]; static { for(int i = 0; i < cache.length; i++) cache[i] = new Integer(i - 128); } }