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【第13章：Java类集】_Map接口使用的注意事项

发布时间：2021-01-03 15:51:05编辑：雪饮阅读（）

 map接口可以通过entrySet方法结合Iterator进行迭代

import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Iterator;
public class TestJava{
       public static void main(String args[]){
              Map<String,String> map=new HashMap<String,String>();
              map.put("kasumi","霞");
              map.put("ayane","凌音");
              map.put("momiji","红叶");
              map.put("snowDrink","雪饮");
              Set<Map.Entry<String,String>> allset=map.entrySet();
              Iterator<Map.Entry<String,String>> iter=allset.iterator();
              while(iter.hasNext()){
                     Map.Entry<String,String> me=iter.next();
                     System.out.println(me.getKey()+" --> "+me.getValue());
              }
       }
}

基于此又可以foreach输出
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Iterator;
public class TestJava{
       public static void main(String args[]){
              Map<String,String> map=new HashMap<String,String>();
              map.put("kasumi","霞");
              map.put("ayane","凌音");
              map.put("momiji","红叶");
              map.put("snowDrink","雪饮");
              for(Map.Entry<String,String> me:map.entrySet()){
                     System.out.println(me.getKey()+" --> "+me.getValue());
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
snowDrink --> 雪饮
momiji --> 红叶
ayane --> 凌音
kasumi --> 霞
Map接口应用在自定义类中
import java.util.HashMap;
import java.util.Map;
class Person{
       private String name;
       private int age;
       public Person(String name,int age){
              this.name=name;
              this.age=age;
       }
       public String toString(){
              return "姓名:"+this.name+" 年龄:"+this.age;
       }
}
public class TestJava{
       public static void main(String args[]){
              Map<String,Person> map=new HashMap<String,Person>();
              map.put("kasumi",new Person("霞",24));
              System.out.println(map.get("kasumi"));
       }
}
D:\>javac TestJava.java
D:\>java TestJava
姓名:霞 年龄:24
但是自定义类型若做为key的参数传值则会有问题，如
import java.util.HashMap;
import java.util.Map;
class Person{
       private String name;
       private int age;
       public Person(String name,int age){
              this.name=name;
              this.age=age;
       }
       public String toString(){
              return "姓名:"+this.name+" 年龄:"+this.age;
       }
}
public class TestJava{
       public static void main(String args[]){
              Map<Person,String> map=new HashMap<Person,String>();
              map.put(new Person("霞",24),"kasumi");
              System.out.println(map.get(new Person("霞",24)));
       }
}
D:\>javac TestJava.java
D:\>java TestJava
null
其实这里很显而易见就能看出问题所在，首先每次new肯定就是开辟一个新的堆内存地址，那么肯定都是唯一的，除非只new一次，然后第二次查找的时候直接用第一次new的引用，如：
import java.util.HashMap;
import java.util.Map;
class Person{
       private String name;
       private int age;
       public Person(String name,int age){
              this.name=name;
              this.age=age;
       }
       public String toString(){
              return "姓名:"+this.name+" 年龄:"+this.age;
       }
}
public class TestJava{
       public static void main(String args[]){
              Map<Person,String> map=new HashMap<Person,String>();
              Person kasumi=new Person("霞",24);
              map.put(kasumi,"kasumi");
              System.out.println(map.get(kasumi));
       }
}
D:\>javac TestJava.java
D:\>java TestJava
kasumi
这样虽然能实现自定义类的对象做为key传递，但是如果我非要使用匿名对象来进行key传递呢？其实也是有办法的，只需要在该匿名对象所属类中实现equals和hashCode这两个方法的覆写即可。如：
import java.util.HashMap;
import java.util.Map;
class Person{
       private String name;
       private int age;
       public Person(String name,int age){
              this.name=name;
              this.age=age;
       }
       public String toString(){
              return "姓名:"+this.name+" 年龄:"+this.age;
       }
       public boolean equals(Object obj){
              if(obj==this){
                     return true;
              }
              if(!(obj instanceof Person)){
                     return false;
              }
              Person p=(Person)obj;
              if(p.name.equals(this.name) && p.age==this.age){
                     return true;
              }
              return false;
       }
       public int hashCode(){
              return this.name.hashCode()*this.age;
       }
}
public class TestJava{
       public static void main(String args[]){
              Map<Person,String> map=new HashMap<Person,String>();
              map.put(new Person("霞",24),"kasumi");
              System.out.println(map.get(new Person("霞",24)));
       }
}
D:\>javac TestJava.java
D:\>java TestJava
kasumi