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Java中ConcurrentHashMap的10个示例

在不浪费您更多时间的情况下，以下是Java中ConcurrentHashMap的10个有用示例。通过这些示例，您将学习如何在Jav a中使用ConcurrentHashMap，例如创建地图，插入键值对，更新键值对，删除映射，检查键或值是否存在于地图中，进行迭代。键或值，等等。

1.如何创建具有默认容量的ConcurrentHashMap 首先，让我们学习如何在Java中创建并发哈希图。这是一个使用默认容量创建空的ConcurrentHashMap的示例。

ConcurrentHashMap programmingLanguages = new ConcurrentHashMap();
System.out.println("Empty ConcurrentHashMap : " + programmingLanguages);

2.如何将对象添加到ConcurrentHashMap 创建ConcurrentHashMap之后，就该添加一些映射了。让我们将一些键和值存储到Java中的ConcurrentHashMap中。如果您看下面的代码，与我们之前看到的添加映射的HashMa示例没有什么不同，唯一的区别是它的线程安全。

programmingLanguages.put("Java", Integer.valueOf(18));
programmingLanguages.put("Scala", Integer.valueOf(10));
programmingLanguages.put("C++", Integer.valueOf(31));
programmingLanguages.put("C", Integer.valueOf(41));
System.out .println("ConcurrentHashMap with four mappings : " + programmingLanguages);

3.如何检查键是否存在于ConcurrentHashMap中 现在，您已经添加了映射，现在检查其时间是否是ConcurrentHashMap中是否存在键。这次，我们将使用Map接口中的containsKey（）方法，由于CHM实现了Map接口，因此CHM上也可以使用containsKey（）方法。

boolean isJavaExist = programmingLanguages.containsKey("Java");
boolean isPythonExist = programmingLanguages.containsKey("Python");
System.out.printf("Does Programming language Map has %s? %b %n", "Java",
                          isJavaExist);

System.out.printf("Does Programming language Map contains %s? %b %n", "Python",
  isPythonExist);

4.如何从Java中的ConcurrentHashMap检索值 这是从Java中的ConcurrentHashMap检索值的示例。这个示例与其他任何映射（例如HashMap或HashTable）非常相似，因为我们使用相同的get（）方法从Java中的ConcurrentHashMap检索值。

int howOldIsJava = programmingLanguages.get("Java");
int howOldIsC = programmingLanguages.get("C");
System.out.printf("How old is Java programming langugae? %d years %n", howOldIsJava);
System.out.printf("How old is C langugae? %d years %n", howOldIsC);

5.如何检查ConcurrentHashMap中是否存在值 这是检查ConcurrentHashMap中是否存在值的示例。同样，此示例与我们之前看到的HashMap containsValue（）示例非常相似。

boolean is41Present = programmingLanguages.containsValue(Integer.valueOf(41));
boolean is31Present = programmingLanguages.containsValue(Integer.valueOf(31));
System.out.printf("Does value 41 is present in ConcurrentHashMap? %b %n", is41Present);
System.out.printf("Does value 31 is present in ConcurrentHashMap? %b %n", is31Present);

6.如何在Java中查找ConcurrentHashMap的大小 您可以使用size（）方法来查找ConcurrentHashMap中存在多少个键值对。size（）方法返回映射总数。

int numberOfMappings = programmingLanguages.size();
System.out.printf("ConcurrentHashMap %s, contains %d mappings %n",
                programmingLanguages, numberOfMappings);

7.如何在Java中循环遍历ConcurrentHashMap？ 有多种方法可以循环遍历Java中的ConcurrentHashMap。实际上，您还可以使用所有四种方式通过ConcurrentHashMap遍历Map。最终，它还实现了java.utill.Map接口，因此它遵守Map的约定

Set> entrySet = programmingLanguages.entrySet();
for (Map.Entry mapping : entrySet) {
   System.out.printf("Key : %s, Value: %s %n", mapping.getKey(), mapping.getValue());
}

8. PutIfAbsent示例-仅当ConcurrentHashMap中不存在键时才添加键吗？ 这是一个有用的方法，仅当地图或字典中尚不存在元素时，才可以使用它来插入元素。这也是常见的ConcurrentHashMap面试问题，经常问初学者。

System.out.printf("Before : %s %n", programmingLanguages);
programmingLanguages.putIfAbsent("Java", 22); 
System.out.printf("After : %s %n", programmingLanguages);
programmingLanguages.put("Python", 23);  
System.out.printf("After : %s %n", programmingLanguages);

9.如何替换ConcurrentHashMap中的映射？ 您可以使用replace方法来更新ConcurrentHashMap中的键的值。此方法同时使用键和值，并用新值更新旧值，如下所示：

programmingLanguages.replace("Java", 20);
System.out.println("ConcurrentHashMap After replace : " + programmingLanguages);

10.如何从Java中的ConcurrentHashMap中删除键值？ 您可以使用ConcurrentHashMap的remove（）方法从Map中删除映射。此方法将删除键和值，并且ConcurrentHashMap的大小将减小一，如以下示例所示：

programmingLanguages.remove("C++");
System.out.println("ConcurrentHashMap After remove : " + programmingLanguages)

运行此代码后，将删除“ C ++”键的映射。

11.如何在迭代ConcurrentHashMap时删除键 这是在Java中遍历ConcurrentHashMap时删除键的代码示例。同样，这与从HashMap中删除键没有什么不同，因为我们从Map接口中使用了相同的remove（）方法，该方法也由Java中的ConcurrentHashMap类继承。

Iterator keys = programmingLanguages.keySet().iterator();
while (keys.hasNext()) {
    System.out.printf("Removing key %s from ConcurrentHashMap %n", keys.next());
    keys.remove();
}

remove（）方法从ConcurrentHashMap中删除当前的Key，就像Iterator对List，Set和Map所做的一样。

12.如何在Java中检查ConcurrentHashMap是否为空？ 您可以使用ConcurrentHashMap的isEmpty（）方法检查给定的Map是否为空。如果ConcurrentHashMap没有任何映射，则此方法将返回true，如以下示例所示：

boolean isEmpty = programmingLanguages.isEmpty();
System.out.printf("Is ConcurrentHashMap %s is empty? %b ", programmingLanguages, isEmpty);

ConcurrentHashMap示例Java 这是完整的Java程序，您可以在Eclipse中 复制粘贴或从命令行运行它以进行操作：

import java.util.Iterator; 
import java.util.Map; 
import java.util.Set; 
import java.util.concurrent.ConcurrentHashMap;   
/** * Java program to demonstrate how to use Concurrent HashMap in Java by simple examples. * * @author javin */   
public class ConcurrentHashMapExamples{   
  public static void main(String args[]) {   
    // Creates a ConcurrentHashMap with default capacity 
    ConcurrentHashMap programmingLanguages = new ConcurrentHashMap();
    System.out.println("Empty ConcurrentHashMap : " + programmingLanguages);       // Adding objects into ConcurrentHashMap 
    programmingLanguages.put("Java", Integer.valueOf(18));
    programmingLanguages.put("Scala", Integer.valueOf(10));
    programmingLanguages.put("C++", Integer.valueOf(31));
    programmingLanguages.put("C", Integer.valueOf(41));

    System.out.println("ConcurrentHashMap with four mappings : " +
                       programmingLanguages);       

    // Checking if a key exists in ConcurrentHashMap or not 
    boolean isJavaExist = programmingLanguages.containsKey("Java"); 
    boolean isPythonExist = programmingLanguages.containsKey("Python");
    System.out.printf("Does Programming language Map has %s? %b %n", "Java",
                      isJavaExist); 
    System.out.printf("Does Programming language Map contains %s? %b %n",
                      "Python", isPythonExist);       

    // Retrieving values from ConcurrentHashMap in Java 
    int howOldIsJava = programmingLanguages.get("Java"); 
    int howOldIsC = programmingLanguages.get("C"); 

    System.out.printf("How old is Java programming langugae? %d years %n",
                      howOldIsJava); 
    System.out.printf("How old is C langugae? %d years %n", howOldIsC);       

    // Checking if a value exists in ConcurrentHashMap 
    boolean is41Present = programmingLanguages.containsValue(
      Integer.valueOf(41)); 
    boolean is31Present = programmingLanguages.containsValue(
      Integer.valueOf(31)); 

    System.out.printf("Does value 41 is present in ConcurrentHashMap? %b %n",
                      is41Present); 
    System.out.printf("Does value 31 is present in ConcurrentHashMap? %b %n",
                      is31Present);       


    // Finding Size of ConcurrentHashMap 
    int numberOfMappings = programmingLanguages.size();
    System.out.printf("ConcurrentHashMap %s, contains %d mappings %n",
                      programmingLanguages, numberOfMappings);       


    // Loop over ConcurrentHashMap in Java 
    Set> entrySet = programmingLanguages.entrySet(); 
    for (Map.Entry mapping : entrySet) { 
      System.out.printf("Key : %s, Value: %s %n", mapping.getKey(),
                        mapping.getValue()); 
    }       

    //PutIfAbsent Example - Adding keys only if its not present in 
    //ConcurrentHashMap
    System.out.printf("Before : %s %n", programmingLanguages);  
    programmingLanguages.putIfAbsent("Java", 22); // Already exists

    System.out.printf("After : %s %n", programmingLanguages);  
    programmingLanguages.put("Python", 23); // Added 


    System.out.printf("After : %s %n", programmingLanguages);       
    // Replacing a Mapping in ConcurrentHashMap
    programmingLanguages.replace("Java", 20);
    System.out.println("ConcurrentHashMap After replace : " +
                       programmingLanguages);       



    // Removing key values from ConcurrentHashMap
    programmingLanguages.remove("C++"); 
    System.out.println("ConcurrentHashMap After remove : " +
                       programmingLanguages);       

    // Removing Keys, while Iterating over ConcurrentHashMap 
    Iterator keys = programmingLanguages.keySet().iterator(); 
    while (keys.hasNext()) { 
      System.out.printf("Removing key %s from ConcurrentHashMap %n",
                        keys.next()); 
      keys.remove();   
    }       

    // How to check if ConcurrentHashMap is empty 
    boolean isEmpty = programmingLanguages.isEmpty(); 
    System.out.printf("Is ConcurrentHashMap %s is empty? %b ",
                      programmingLanguages, isEmpty);   
  }       
}

输出：

Empty ConcurrentHashMap : {}
ConcurrentHashMap with four mappings : {C=41, Scala=10, Java=18, C++=31}
Does Programming language Map has Java? true
Does the Programming language Map contain Python? false
How old is Java programming language? 18 years
How old is C language? 41 years
Does value 41 is present in ConcurrentHashMap? true
Does value 31 is present in ConcurrentHashMap? true
ConcurrentHashMap {C=41, Scala=10, Java=18, C++=31}, contains 4 mappings
Key: C, Value: 41
Key: Scala, Value: 10
Key: Java, Value: 18
Key : C++, Value: 31
Before : {C=41, Scala=10, Java=18, C++=31}
After : {C=41, Scala=10, Java=18, C++=31}
After : {C=41, Python=23, Scala=10, Java=18, C++=31}
ConcurrentHashMap After replace : {C=41, Python=23, Scala=10, Java=20, C++=31}
ConcurrentHashMap After remove : {C=41, Python=23, Scala=10, Java=20}
Removing key C from ConcurrentHashMap
Removing key Python from ConcurrentHashMap
Removing key Scala from ConcurrentHashMap
Removing key Java from ConcurrentHashMap
Is ConcurrentHashMap {} is empty? true

这就是Java中的ConcurrentHashMap示例。正如我所说的，在完成这些示例之后，您将对ConcurrentHashMap的工作原理以及如何正确使用它有更好的了解。现在，您对如何在Java中的ConcurrentHashMap上创建，添加，更新，搜索和删除条目有了一个很好的了解。

原文链接：http://codingdict.com

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