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【第13章：Java类集】_Map接口

发布时间：2021-01-03 12:35:44编辑：雪饮阅读（）

HashMap的put与get操作

HashMap就是键值对的形式存在的
put则就是接收一对键值对，get则是根据key获取对应val
import java.util.HashMap;
import java.util.Map;
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");
              String val=map.get("momiji");
              System.out.println("根据key‘momiji’取出的值："+val);
       }
}

当对应key不存在时候，则get取出的就是null
import java.util.HashMap;
import java.util.Map;
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");
              String val=map.get("snowDrink");
              System.out.println("根据key‘snowDrink’取出的值："+val);
       }
}
D:\>javac TestJava.java
D:\>java TestJava
根据key‘snowDrink’取出的值：null
containsKey可以判断当前map里面是否存在某key
containsValue可以判断当前map里面是否存在某val
import java.util.HashMap;
import java.util.Map;
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");
              if(map.containsKey("snowDrink")){
                     System.out.println("搜索的key存在！");
              }
              else{
                     System.out.println("搜索的key不存在!");
              }
              if(map.containsValue("红叶")){
                     System.out.println("搜索的value存在!");
              }
              else{
                     System.out.println("搜索的value不存在!");
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
搜索的key不存在!
搜索的value存在!
keySet可以获取map中的所有key并且可以利用Set接口进行Iterator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<String> keys=map.keySet();
              Iterator<String> iter=keys.iterator();
              while(iter.hasNext()){
                     String str=iter.next();
                     System.out.println(str);
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
snowDrink
momiji
ayane
kasumi
values可以获取map中的所有val，并可以利用Collection接口进行Iterator迭代
import java.util.HashMap;
import java.util.Map;
import java.util.Iterator ;
import java.util.Collection;
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");
              Collection<String> keys=map.values();
              Iterator<String> iter=keys.iterator();
              while(iter.hasNext()){
                     String str=iter.next();
                     System.out.println(str);
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
雪饮
红叶
凌音
霞
那么就可以一次性获取全部的key和val
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Iterator ;
import java.util.Collection;
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");
              System.out.println("----------全部key--------");
              Set<String> keys=map.keySet();
              Iterator<String> iterk=keys.iterator();
              while(iterk.hasNext()){
                     String str=iterk.next();
                     System.out.println(str);
              }
             
              System.out.println("----------全部val--------");
              Collection<String> values=map.values();
              Iterator<String> iterv=values.iterator();
              while(iterv.hasNext()){
                     String str=iterv.next();
                     System.out.println(str);
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
----------全部key--------
snowDrink
momiji
ayane
kasumi
----------全部val--------
雪饮
红叶
凌音
霞
用TreeMap为Map实例化则map中所有key会被自动排序
import java.util.TreeMap;
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 TreeMap<String,String>();
              map.put("kasumi","霞");
              map.put("ayane","凌音");
              map.put("momiji","红叶");
              map.put("snowDrink","雪饮");
              Set<String> keys=map.keySet();
              Iterator<String> iterk=keys.iterator();
              while(iterk.hasNext()){
                     String str=iterk.next();
              System.out.println(str+"-->"+map.get(str));
              }
       }
}
D:\>javac TestJava.java
D:\>java TestJava
ayane-->凌音
kasumi-->霞
momiji-->红叶
snowDrink-->雪饮
用WeakHashMap为Map实例化则map中的键值对会在JVM回收后就丢失
为了验证这个，所以我们就要建立一大堆的堆内存咯，做为key与val。
import java.util.WeakHashMap;
import java.util.Map;
public class TestJava{
       public static void main(String args[]){
              Map<String,String> map=new WeakHashMap<String,String>();
              map.put(new String("kasumi"),new String("霞"));
              map.put(new String("ayane"),new String("凌音"));
              map.put(new String("momiji"),new String("红叶"));
              map.put(new String("snowDrink"),new String("雪饮"));
              System.gc();
              map.put(new String("lisa"),new String("丽莎"));
              System.out.println(map);
       }
}
D:\>javac TestJava.java
D:\>java TestJava
{lisa=丽莎}
这种情况被称为弱引用