Java ConcurrentHashMap: computeIfAbsent() Example
February 12, 2022
computeIfAbsent() method of Java ConcurrentHashMap class.
Find the Java doc of
computeIfAbsent method.
V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
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
}
}
--- 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
}
}
--- 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}
