Java中的 ConcurrentHashMap remove() 方法

A. 移除（对象键）

Java中 ConcurrentHashmap 类的remove(Object key)方法用于从映射中移除映射。如果映射中不存在该键，则此函数不执行任何操作。

句法：

public V remove(Object key)

参数：

This method accepts a mandatory parameter key which is the key that needs to be removed

返回值：

This method returns the previous value associated with key, 
or null if there was no mapping for key.

例外：

This method throws NullPointerException if the specified key is null.

以下是说明 remove() 方法的程序：

方案一：

Java

// Java program to demonstrate remove() method
 
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
 
public class ConcurrentHashMapExample {
 
    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map my_cmmap
            = new ConcurrentHashMap();
 
        // 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("Map: " + my_cmmap);
        System.out.println();
 
        // Removing the mapping
        // with existing key 6
        // using remove() method
        String valueRemoved = my_cmmap.remove("6");
 
        // Printing the map after remove()
        System.out.println(
            "After removing mapping with key 6:");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Value removed: "
                           + valueRemoved);
        System.out.println();
 
        // Removing the mapping
        // with non-existing key 10
        // using remove() method
        valueRemoved = my_cmmap.remove("10");
 
        // Printing the map after remove()
        System.out.println(
            "After removing mapping with key 10:");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Value removed: "
                           + valueRemoved);
    }
}

输出

Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

After removing mapping with key 6:
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Value removed: 1

After removing mapping with key 10:
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Value removed: null

程序 2：用 null 键演示 NullPointerException

Java

// Java program to demonstrate remove() method
 
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
 
public class ConcurrentHashMapExample {
 
    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map my_cmmap
            = new ConcurrentHashMap();
 
        // 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("Map: " + my_cmmap);
        System.out.println();
 
        try {
            // Removing the mapping with null key
            // using remove() method
            my_cmmap.remove(null);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出

Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

Exception: java.lang.NullPointerException

B.remove(对象键，对象值)

Java中ConcurrentHashmap类的remove(Object key, Object value)方法用于从map中移除映射。在映射中搜索具有指定（键，值）对的映射，如果找到则删除，并返回true。如果映射中不存在该键，则此函数不执行任何操作并返回 false。

句法：

public boolean remove(Object key, Object value)

参数：

This method accepts two parameters  
key - key with which the specified value is associated.
value - value expected to be associated with the specified key.

返回值：

This method returns true if the value was removed. Else it returns false.

例外：

This method throws NullPointerException if the specified key is null.

以下是说明 remove() 方法的程序：

方案一：

Java

// Java program to demonstrate remove() method
 
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
 
public class ConcurrentHashMapExample {
 
    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map my_cmmap
            = new ConcurrentHashMap();
 
        // 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("Map: " + my_cmmap);
        System.out.println();
 
        // Removing the mapping
        // with existing pair 6, 1
        // using remove() method
        boolean valueRemoved = my_cmmap.remove("6", "1");
 
        // Printing the map after remove()
        System.out.println(
            "After removing mapping with pair (6, 1):");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Was value removed: "
                           + valueRemoved);
        System.out.println();
 
        // Removing the mapping
        // with non-existing pair 1, 2
        // using remove() method
        valueRemoved = my_cmmap.remove("1", "2");
 
        // Printing the map after remove()
        System.out.println(
            "After removing mapping with pair (1, 2):");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Was value removed: "
                           + valueRemoved);
    }
}

输出

Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

After removing mapping with pair (6, 1):
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Was value removed: true

After removing mapping with pair (1, 2):
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Was value removed: false

程序 2：用 null 键演示 NullPointerException

Java

// Java program to demonstrate
// remove(key, value) null key
 
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
 
public class ConcurrentHashMapExample {
 
    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map my_cmmap
            = new ConcurrentHashMap();
 
        // 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("Map: " + my_cmmap);
        System.out.println();
 
        try {
            // Removing the mapping
            // with null key
            // using remove() method
            my_cmmap.remove(null, "1");
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出

Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

Exception: java.lang.NullPointerException

程序 3 ：用 null 值演示 NullPointerException

Java

// Java program to demonstrate
// remove(key, value) null value
 
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
 
public class ConcurrentHashMapExample {
 
    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map my_cmmap
            = new ConcurrentHashMap();
 
        // 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("Map: " + my_cmmap);
 
        try {
            // Removing the mapping with correct key and
            // null value using remove() method
            my_cmmap.remove("1", null);
            my_cmmap.remove("7", null);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出

Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}