这个 put（） 课堂教学法 Java中的ConcurrentHashmap 用于将贴图插入此贴图中。它将参数作为（键、值）对。如果向现有密钥传递值，则此方法会更新该密钥的值。否则，如果通过一个新的对，则该对作为一个整体插入。
null
语法：
public V put(K key, V value)
参数： 此方法接受两个强制参数：
- 钥匙 ：–与指定值关联的键
- 价值 ：–与指定键关联的值
返回值： 此方法返回与key关联的前一个值，如果key没有映射，则返回null。
例外情况： 这个方法抛出 空指针异常 如果指定的键或值为空
下面是说明put（）方法的示例：
项目1： 当输入键时，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); } } |
输出：
{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}
项目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); } } |
输出：
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}
方案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 + "" ); // 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 + "" ); } // 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 + "" ); } } } |
输出：
{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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