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Java ConcurrentHashMap put()用法及代碼示例


Java中的ConcurrentHashmap類的put()方法用於將映射插入此映射中。它將參數作為(鍵，值)對。如果現有鍵傳遞了值，則此方法將更新該鍵的值。否則，如果通過了新的配對，則該配對將作為一個整體插入。

用法:

public V put(K key, V value)

參數：此方法接受兩個強製性參數：


  • key：–與指定值關聯的鍵
  • value：–與指定鍵關聯的值

返回值：此方法返回與key關聯的先前值；如果沒有key映射，則返回null。

異常：如果指定的鍵或值為null，則此方法引發NullPointerException

下麵是說明put()方法的示例：

示例1:當鍵值傳遞時是新的。

// Java program to demonstrate 
// put() method 
  
import java.util.concurrent.ConcurrentHashMap; 
import java.util.*; 
  
public class ConcurrentHashMapExample { 
  
    public static void main(String[] args) 
    { 
        // Creating ConcurrentHashMap 
        Map<String, String> 
            my_cmmap = new ConcurrentHashMap<String, String>(); 
  
        // Adding elements to the map 
        // using put() method 
        my_cmmap.put("1", "1"); 
        my_cmmap.put("2", "1"); 
        my_cmmap.put("3", "1"); 
        my_cmmap.put("4", "1"); 
        my_cmmap.put("5", "1"); 
        my_cmmap.put("6", "1"); 
  
        // Printing the map 
        System.out.print(my_cmmap); 
    } 
}
輸出： 
{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

示例2:當現有鍵值被傳遞時。

// Java program to demonstrate 
// put() method 
  
import java.util.concurrent.ConcurrentHashMap; 
import java.util.*; 
  
public class ConcurrentHashMapExample { 
  
    public static void main(String[] args) 
    { 
        // Creating ConcurrentHashMap 
        Map<String, String> 
            my_map = new ConcurrentHashMap<String, String>(); 
  
        // Variable to get the returned value by put() 
        String returnedValue; 
  
        // Adding elements to the map 
        // using put() method 
        // and printing the returned value each time 
        returnedValue = my_map.put("Geek", "100"); 
        System.out.println("Map: " + my_map); 
        System.out.println("Returned Value: " + returnedValue); 
        System.out.println(); 
  
        returnedValue = my_map.put("Geek", "200"); 
        System.out.println("Map: " + my_map); 
        System.out.println("Returned Value: " + returnedValue); 
        System.out.println(); 
  
        returnedValue = my_map.put("Geek", "300"); 
        System.out.println("Map: " + my_map); 
        System.out.println("Returned Value: " + returnedValue); 
        System.out.println(); 
  
        returnedValue = my_map.put("Geek", "400"); 
        System.out.println("Map: " + my_map); 
        System.out.println("Returned Value: " + returnedValue); 
        System.out.println(); 
  
        returnedValue = my_map.put("Geek", "500"); 
        System.out.println("Map: " + my_map); 
        System.out.println("Returned Value: " + returnedValue); 
        System.out.println(); 
  
        System.out.print(my_map); 
    } 
}
輸出： 
Map: {Geek=100}
Returned Value: null

Map: {Geek=200}
Returned Value: 100

Map: {Geek=300}
Returned Value: 200

Map: {Geek=400}
Returned Value: 300

Map: {Geek=500}
Returned Value: 400

{Geek=500}

示例3:演示NullPointerException

// Java program to demonstrate 
// put() method 
  
import java.util.concurrent.ConcurrentHashMap; 
import java.util.*; 
  
public class ConcurrentHashMapExample { 
  
    public static void main(String[] args) 
    { 
        // Creating ConcurrentHashMap 
        Map<String, String> 
            my_cmmap = new ConcurrentHashMap<String, String>(); 
  
        // Adding elements to the map 
        // using put() method 
        my_cmmap.put("1", "1"); 
        my_cmmap.put("2", "1"); 
        my_cmmap.put("3", "1"); 
        my_cmmap.put("4", "1"); 
        my_cmmap.put("5", "1"); 
        my_cmmap.put("6", "1"); 
  
        // Printing the map 
        System.out.println(my_cmmap + "\n"); 
  
        // When null key is passed 
        try { 
            System.out.println("When (null, \"10\") "
                               + "is passed as parameter:"); 
            my_cmmap.put(null, "10"); 
        } 
        catch (Exception e) { 
            System.out.println("Exception: " + e + "\n"); 
        } 
  
        // When null value is passed 
        try { 
            System.out.println("When (\"10\", null) "
                               + "is passed as parameter:"); 
            my_cmmap.put("10", null); 
        } 
        catch (Exception e) { 
            System.out.println("Exception: " + e + "\n"); 
        } 
    } 
}
輸出： 
{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

When (null, "10") is passed as parameter:
Exception: java.lang.NullPointerException

When ("10", null) is passed as parameter:
Exception: java.lang.NullPointerException


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注：本文由純淨天空篩選整理自barykrg大神的英文原創作品 ConcurrentHashMap put() method in Java。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。