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Java ConcurrentHashMap: computeIfAbsent() Example

By Arvind Rai, February 12, 2022
On this page we will learn computeIfAbsent() method of Java ConcurrentHashMap class.
Find the Java doc of computeIfAbsent method.
V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) 
1. If the specified key is absent, the mappingFunction computes a value and adds this new key/value in the map.
2. If the specified key is already present, the mappingFunction is not computed and there is no change in the map.
3. If the specified key is absent and the mappingFunction computes a value as null then this new key/value is not added to map.
4. The entire method invocation is performed atomically.
5. The mapping function must not modify this map during computation.

6. Parameters :
key - key to associate a value.
mappingFunction - the function to compute a value.

7. Returns : New value after computation or null.

8. Throws :
NullPointerException: If specified key or provided mappingFunction is null.
IllegalStateException: If computation is recursive.
RuntimeException: If mappingFunction throws RuntimeException.



Example-1

ComputeIfAbsent1.java
package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;

public class ComputeIfAbsent1 {
  public static void main(String[] args) {
	ConcurrentHashMap<Integer, String> conMap = new ConcurrentHashMap<>();
	conMap.put(10, "Varanasi");
	conMap.put(20, "Prayag");
	conMap.put(30, "Ayodhya");
	
	System.out.println("--- key is present ---");
	String v1 = conMap.computeIfAbsent(20, k -> k + " - Kanyakumari");
	System.out.println("v1: " + v1);
	System.out.println(conMap);	// No change	

	System.out.println("--- key is not present but value is null ---");
	String v2 = conMap.computeIfAbsent(40, k -> null);
	System.out.println("v2: " + v2);
	System.out.println(conMap); // No change
	
	System.out.println("--- key is not present ---");
	String v3 = conMap.computeIfAbsent(50, k -> k + " - Kanyakumari");
	System.out.println("v3: " + v3);
	System.out.println(conMap); // New entry added
  }
} 
Output
--- key is present ---
v1: Prayag
{20=Prayag, 10=Varanasi, 30=Ayodhya}
--- key is not present but value is null ---
v2: null
{20=Prayag, 10=Varanasi, 30=Ayodhya}
--- key is not present ---
v3: 50 - Kanyakumari
{50=50 - Kanyakumari, 20=Prayag, 10=Varanasi, 30=Ayodhya} 

Example-2

ComputeIfAbsent2.java
package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;

public class ComputeIfAbsent2 {
  public static void main(String[] args) {
	ConcurrentHashMap<Integer, Integer> conMap = new ConcurrentHashMap<>();
	conMap.put(10, 100);
	conMap.put(20, 200);
	conMap.put(30, 300);
	
	System.out.println("--- key is present ---");
	Integer v1 = conMap.computeIfAbsent(20, k -> k * 10);
	System.out.println("v1: " + v1);
	System.out.println(conMap);	// No change	

	System.out.println("--- key is not present but value is null ---");
	Integer v2 = conMap.computeIfAbsent(40, k -> null);
	System.out.println("v2: " + v2);
	System.out.println(conMap); // No change
	
	System.out.println("--- key is not present ---");
	Integer v3 = conMap.computeIfAbsent(50, k -> k * 10);
	System.out.println("v3: " + v3);
	System.out.println(conMap); // New entry added
  }
} 
Output
--- key is present ---
v1: 200
{20=200, 10=100, 30=300}
--- key is not present but value is null ---
v2: null
{20=200, 10=100, 30=300}
--- key is not present ---
v3: 500
{50=500, 20=200, 10=100, 30=300} 

Reference

Java ConcurrentHashMap
POSTED BY
ARVIND RAI
ARVIND RAI










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