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.

在參數傳遞和返回值中使用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);
????}
} 下面程序注意點： 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，所以首先將object類型轉換為Integer，然后傳給freq
??????m.put(args[i],(freq==null? one:(freq+1)));?//freq自動解包成int值
????}
????System.out.println(m.size() + "distinct word detected");
????
????System.out.println(m);
???? }

} 執行結果:

#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]); //減少 m.get(args[i])的強制類型轉換
??????m.put(args[i],(freq==null? one:(freq+1)));
????}
????System.out.println(m.size() + "distinct word detected");
????
????System.out.println(m);
???? }

}

轉載于:https://blog.51cto.com/vicky001/403771

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