ConcurrentMap in Java

In this article, we will read ConcurrentMap in Java.

Java ConcurrentMap interface

ConcurrentMap interface in Java is a synchronized map that allows more than one thread to access the map. It is thread-safe and doe not affect the consistency of the map elements. It is part of the java.util.concurrent package and is available from JDK 1.5 onwards. The ConcurrentMap interface extends the Map interface.

Classes that implement ConcurrentMap

ConcurrentMap in Java is an interface and hence requires classes to implement its methods. For this, we can either use the ConcurrentSkipListMap or the ConcurrentHashMap.

Methods of ConcurrentMap in Java

Below are the methods present in the ConcurrentMap interface in Java. It also imports all the main methods that are present in the Map interface.

Example: Insert elements in ConcurrentMap in Java

We can insert elements into a Java ConcurrentMap using the put() method. We can add the entries as a key-value pair. This example shows how to insert an Integer-String as key-value pair. The putIfAbsent() method inserts the entry if there is no value mapping for the specified key.

import java.util.concurrent.*;

public class ConcurrentMapDemo {

  public static void main(String[] args) {
    ConcurrentMap<Integer,String> cm = new ConcurrentHashMap<Integer,String>();
    cm.put(111, "Aarthi");
    cm.put(222, "Banu");
    cm.put(333, "Chetan");
    
    cm.putIfAbsent(444, "Devi");
    
    System.out.println(cm);
  }

}

{444=Devi, 333=Chetan, 222=Banu, 111=Aarthi}

Example: Remove elements

The below example shows how to remove elements from the ConcurrentMap in Java. For this, we can use the remove() method either by specifying only key or both key-value. If we pass both key-value in the remove() method, the specific key-value mapping is not present, it does not remove the entry with the specified key.

import java.util.concurrent.*;

public class ConcurrentMapDemo {

  public static void main(String[] args) {
    ConcurrentMap<Integer,String> cm = new ConcurrentHashMap<Integer,String>();
    System.out.println("Insert elements...");
    cm.put(111, "Aarthi");
    cm.put(222, "Banu");
    cm.put(333, "Chetan");
    
    cm.putIfAbsent(444, "Devi");
    
    System.out.println(cm);
    
    System.out.println("Removing elements...");
    cm.remove(222);
    cm.remove(333, "Chetan");
    System.out.println(cm);
  }

}

Insert elements...
{444=Devi, 333=Chetan, 222=Banu, 111=Aarthi}
Removing elements...
{444=Devi, 111=Aarthi}

Example: Access elements

The below example shows how to access different elements from the ConcurrentMap. The get() method returns the value of the specified key. The getOrDefault() method returns the value of the specified key if present else returns the default value. We can also check if the specified key is present using the containsKey() and specified value using the containsValue() method. It returns true if present else returns false.

import java.util.concurrent.*;

public class ConcurrentMapDemo {

  public static void main(String[] args) {
    ConcurrentMap<Integer,String> cm = new ConcurrentHashMap<Integer,String>();
    System.out.println("Insert elements...");
    cm.put(111, "Aarthi");
    cm.put(222, "Banu");
    cm.put(333, "Chetan");
    
    cm.putIfAbsent(444, "Devi");
    
    System.out.println(cm);
    
    System.out.println(cm.get(111));
    System.out.println(cm.getOrDefault(555, "Default"));
    System.out.println(cm.getOrDefault(333, "Default"));
    System.out.println(cm.containsKey(222));
    System.out.println(cm.containsKey(555));
    System.out.println(cm.containsValue("Banu"));
    System.out.println(cm.containsValue("Eesha"));
    
  }

}

Insert elements...
{444=Devi, 333=Chetan, 222=Banu, 111=Aarthi}
Aarthi
Default
Chetan
true
false
true
false

Example: Iterate elements

We can iterate through all elements in the ConcurrentMap by using an iterator over the entrySet. Using the getKey() method, we can retrieve the key and using the getValue() method, we can retrieve the value. We can also retrieve the keys alone using the keySet() and values using the values() method.

import java.util.Iterator;
import java.util.concurrent.*;

public class ConcurrentMapDemo {

  public static void main(String[] args) {
    ConcurrentMap<Integer,String> cm = new ConcurrentHashMap<Integer,String>();
    cm.put(111, "Aarthi");
    cm.put(222, "Banu");
    cm.put(333, "Chetan");
    
    cm.putIfAbsent(444, "Devi");
    
    Iterator<ConcurrentHashMap.Entry<Integer,String>> it = cm.entrySet().iterator();
    while(it.hasNext()) {
      ConcurrentHashMap.Entry<Integer,String> e = it.next();
      System.out.println("Key:" + e.getKey() + " " + "Value: " + e.getValue());
    }

    System.out.println("Key Set: " + cm.keySet());
    System.out.println("Values: " + cm.values());
      
  }

}

Key:444 Value: Devi
Key:333 Value: Chetan
Key:222 Value: Banu
Key:111 Value: Aarthi
Key Set: [444, 333, 222, 111]
Values: [Devi, Chetan, Banu, Aarthi]

Example: Replace elements

This example shows to replace elements using the replace() method. It replaces the old value with the specified new value for the mentioned key.

import java.util.Iterator;
import java.util.concurrent.*;

public class ConcurrentMapDemo {

  public static void main(String[] args) {
    ConcurrentMap<Integer,String> cm = new ConcurrentHashMap<Integer,String>();
    System.out.println("Insert elements...");
    cm.put(111, "Aarthi");
    cm.put(222, "Banu");
    cm.put(333, "Chetan");
    
    cm.putIfAbsent(444, "Devi");
    System.out.println(cm);
    
    cm.replace(222, "Bharati");
    cm.replace(333, "Chetan", "Chitra");
    System.out.println("Elements after replace operation:");
    System.out.println(cm);
    
  }

}

Insert elements...
{444=Devi, 333=Chetan, 222=Banu, 111=Aarthi}
Elements after replace operation:
{444=Devi, 333=Chitra, 222=Bharati, 111=Aarthi}

Reference Reference2