java新特性：自动装箱与拆箱带来的问题

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//  Date   : 2009-7-17

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package org.zdxue.javase.lang;

 

/**

 * Java自动装箱与拆箱

 *

 * @version           1.0 - 2009-7-17

 * @author            

 */

public class Boxing_Unboxing

{

    public static void main(String[] args)

    {

        Integer a = 1024; //自动装箱(编辑器会隐式处理为：Integer a = new Integer(1024); )

        Integer b = 1024; //自动装箱(编辑器会隐式处理为：Integer b = new Integer(1024); )

        int c = 1024; //基本数据类型

        int d = a; //自动拆箱(编辑器会隐式处理为：int d = a.intValue(); )

        double e = a;//自动拆箱(编辑器会隐式处理为：int d = a.doubleValue(); )

        short f = a.shortValue(); //不可以写成：short f = a; 因为隐式转换会丢失精度，必须向上（高精度）转型

        boolean test = a < b || a == b || a > b; //对象运算

        System.out.println(test);

        System.out.println(a == c); //自动将a拆箱后，与c进行计算

        System.out.println(d);

        System.out.println(e);

        System.out.println(d == e);

        System.out.println(f);

 

        //---------------------------------------------

        /**

         * int值在-128~127之间，装箱时，在内存堆中只new一次，则h1,h2指向同一个对象，因此地址== true

         * 但是不在上述的范围时（-128~127），会在内存堆中各个new一次，也就是说，引用地址是不同的，因此== false

         */

        Integer h1 = 127;

        Integer h2 = 127;

        System.out.println("h1=h2 ->" + (h1 == h2)); //在范围内，true

 

        Integer h3 = 128;

        Integer h4 = 128;

        System.out.println("h3=h4 ->" + (h3 == h4)); //不在范围内，false

 

        //--------------------------------------

        Long l1 = 128L;

        Long l3 = 128L;

        long l2 = 100;

 

        Long l5 = 127L;

        Long l4 = 127L;

 

        System.out.println("l1==l2 -> " + (l1 == l2));

        System.out.println("l1==l3 -> " + (l1 == l3)); //不在范围内，false

        System.out.println("l4==l5 -> " + (l4 == l5)); //在范围内(-128~127)，true

 

        //---------------- 注意 ------------------------------------

        Integer i = null;

        //int j = i.intValue();

        int j = i; //编译上没有问题，但是已经隐含NullPointerException, 编译器会处理为：ing j = i.intValue();

        System.out.println(j);

        //---------------------------------------------------------

 

        int d0 = 100;

        Double d1 = (double)d0; //注意与下面的区别

        double d2 = d0;

    }

}

 

转自：http://xuezhongde.iteye.com/blog/427745