Java 中的 ConcurrentHashMap put()方法
原文:https://www . geeksforgeeks . org/concurrenthashmap-put-method-in-Java/
Java 中类的 put() 方法 ConcurrentHashmap用来在这个映射中插入一个映射。它将参数作为(键、值)对。如果现有的键被传递了一个值，那么这个方法更新该键的值。否则，如果传递了一个新的对，那么这个对就会作为一个整体被插入。
语法:
public V put(K key, V value)
参数:该方法接受两个强制参数:
- 键:–与指定值相关联的键
- 值:–与指定键相关联的值
返回值:此方法返回与键关联的前一个值，如果没有键映射，则返回空值。
异常:如果指定的键或值为空，该方法将抛出空指针异常
下面是举例说明 put()方法:
程序 1: 当键时，传递的值是新的。
// 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);
}
}
Output:
{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}
程序 2: 当一个现有的键，值被传递。
// 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);
}
}
Output:
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}
程序 3: 演示空指针异常
// 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");
}
}
}
Output:
{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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