Java 中的 ConcurrentHashMap remove()方法
原文:https://www . geesforgeks . org/concurrenthashmap-remove-method-in-Java/
A .移除(对象键)
Java 中类的移除(对象键)方法用于从地图中移除映射。如果该键在地图中不存在，则该函数不执行任何操作。
语法:
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.
下面是说明 remove()方法的程序:
程序一:
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<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("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: 用空键
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<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("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
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