Java EnumMap put() 方法

2024年11月8日 | 2 分钟阅读

Java EnumMap 类的 put() 方法用于将给定的值与此 EnumMap 中的给定键关联起来。

语法

参数

key - 要与给定值关联的键

value - 要与给定键关联的值

返回值

此方法返回与指定键关联的旧值。

如果此映射已包含指定键的映射，则此方法返回旧值。
如果没有为指定键找到映射，则此方法返回 null。

Exception

NullPointerException - 如果指定的键为 null。

示例 1

import java.util.*;
public class EnumMapPutExample1 {
     // create an enum  
   public enum Tutorial {
    Java, Python, PHP, Android, Javascript
   };
   public static void main(String[] args) {
    EnumMap< Tutorial, String> map = 
    		new EnumMap< Tutorial, String>( Tutorial.class);

    map.put(Tutorial.Java, "1");
    map.put(Tutorial.Python, "2");
    map.put(Tutorial.PHP, "3");
    map.put(Tutorial.Android, "4");
    //print the map
    System.out.println("Map: "+map);    
    String oldValue1 = map.put(Tutorial.Javascript, "5");
    String oldValue2 = map.put(Tutorial.Java, "20");
    //print the updated map
    System.out.println("Updated Map: " + map);
    System.out.println("First update returns: "+oldValue1);
    System.out.println("Second update returns: "+oldValue2);
  } 
}

输出

Map: {Java=1, Python=2, PHP=3, Android=4}
Updated Map: {Java=20, Python=2, PHP=3, Android=4, Javascript=5}
First update returns: null
Second update returns: 1

示例 2

import java.util.*;
public class EnumMapPutExample2 {
    // create an enum  
     public enum Months {  
     January, February, March, April, May
     };  
      public static void main(String[] args) {  
      //create and populate enum map  
      EnumMap< Months, String> map = 
    		  new EnumMap< Months, String> (Months.class);
	   
      map.put(Months.January,"1");  
      map.put(Months.February,"2");  
      map.put(Months.March,"3");     
      map.put(Months.April,"4");      
      //print the map
      System.out.println("Map: "+map);    
      String oldValue1 = map.put(Months.May, "5");
      String oldValue2 = map.put(Months.March, "15");
      //print the updated map
      System.out.println("Updated Map: "+map);
      System.out.println("First update returns: "+oldValue1);
      System.out.println("Second update returns: "+oldValue2);
    }
}

输出

Map: {January=1, February=2, March=3, April=4}
Updated Map: {January=1, February=2, March=15, April=4, May=5}
First update returns: null
Second update returns: 3

示例 3

import java.util.*;
public class EnumMapPutExample3 {
   // create an enum  
   public enum Days {
    Monday,Tuesday, Wednesday, Thursday
   };
   public static void main(String[] args) {
      EnumMap<Days, String>  map = new EnumMap<Days, String> (Days.class);
	
      map.put(Days.Monday, "1");
      map.put(Days.Tuesday, "2");
      map.put(Days.Wednesday, "3");	      
     //print the map
     System.out.println("Map: "+map);    
     String oldValue1 = map.put(Days.Thursday, "4");
     String oldValue2 = map.put(null, "9");
     //print the updated map
     System.out.println("Updated Map: " + map);
     System.out.println("First update returns: "+oldValue1);
     System.out.println("Second update returns: "+oldValue2);
   }
}

输出

Map: {Monday=1, Tuesday=2, Wednesday=3}
Exception in thread "main" java.lang.NullPointerException
	at java.util.EnumMap.typeCheck(Unknown Source)
	at java.util.EnumMap.put(Unknown Source)
	at EnumMapPutExample3.main(EnumMapPutExample3.java:18)

下一主题Java-enummap-remove-method

Java EnumMap put() 方法

语法

参数

返回值

Exception

示例 1

示例 2

示例 3

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Java EnumMap put() 方法

语法

参数

返回值

Exception

示例 1

示例 2

示例 3

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