java box & unboxing

http://www.java2s.com/Tutorial/Java/0040__Data-Type/BoxingandUnboxing.htm    Boxing and Unboxing

1、Boxing refers to the conversion of a primitive to a corresponding wrapper instance, such as from an int to  a java.lang.Integer. 2、Unboxing is the conversion of a wrapper instance to a primitive type, such as from Byte to byte.

在参数传递和返回值中使用autoBox

class AutoBox2 {
     static int m(Integer v) {
         return v; // auto-unbox to int
    }

     public static void main(String args[]) {
        Integer iOb = m(100);

        System.out.println(iOb);
    }
}

 

下面程序注意点：

1、map的get  ，put方法

2、auto-unboxing

import java.util.*;

public class TestArgsWords{
     private static final int one =1;
        
     public static void main(String args[]){
        
    Map m = new HashMap();
     for( int i=0;i<args.length;i++){
    
      Integer freq= (Integer)m.get(args[i]);

        //map的get方法返回值是object，所以首先将object类型转换为Integer，然后传给freq
      m.put(args[i],(freq== null? one:(freq+1)));  //freq自动解包成int值
    }
    System.out.println(m.size() + "distinct word detected");
    
    System.out.println(m);
     }

}

 

执行结果:

#java TestArgsWords aa    aa    bb cc ab    bb

#4distinct word detected
{aa=2, ab=1, bb=2, cc=1}

 

3、利用泛型Generic改良上述程序

import java.util.*;

public class TestArgsWords{
     private static final int one =1;
        
     public static void main(String args[]){
        
    Map<String,Integer> m = new HashMap<String,Integer> ();

//map 泛型
     for( int i=0;i<args.length;i++){
    
      Integer freq= m.get(args[i]); //减少 m.get(args[i])的强制类型转换
      m.put(args[i],(freq== null? one:(freq+1)));
    }
    System.out.println(m.size() + "distinct word detected");
    
    System.out.println(m);
     }

}