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Java Map computeIfAbsent() Example

By Arvind Rai, February 19, 2020
The computeIfAbsent is the default method of java.util.Map and has been introduced in Java 8. The computeIfAbsent method works when the value associated with specified key is not available or null and in this case the computeIfAbsent method put the new value for that key computed by given mapping function. Find the method declaration from Java doc.
default V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) 
The key is the key for which we need to associate a value.
The mappingFunction is the java.util.function.Function type that compute a value for mapping.
The computeIfAbsent returns existing or new value computed by given mapping function.

The computeIfAbsent method works as following.
1. If the specified key is not already associated with a value and new value calculated by mapping function is not null, in this case the computeIfAbsent method will put the new value for the specified key.
2. If the specified key is not already associated with a value and new value calculated by mapping function is null, in this case the computeIfAbsent method will not put the new value for the specified key.
3. If the specified key is already associated with a value and new value calculated by mapping function is not null, in this case the computeIfAbsent method will not put the new value for the specified key.
4. If the specified key is associated with a null value and new value calculated by mapping function is not null, in this case the computeIfAbsent method will put the new value for the specified key.

Example-1

Specified key is not associated with a value
and new value calculated by mapping function is not null.
In this case the computeIfAbsent method will put the new value for the specified key.
ComputeIfAbsent1.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfAbsent1 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
     
     String value = cityMap.computeIfAbsent(103, k -> "Noida");
     
     System.out.println(value);
     System.out.println(cityMap);
  }
} 
Output
Noida
{101=Varanasi, 102=Prayag, 103=Noida} 
The same can be achieved without using computeIfAbsent method as following.
ComputeIfAbsent11.java
import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;

public class ComputeIfAbsent11 {
  public static void main(String[] args) {
	Map<Integer, String> cityMap = new HashMap<>();
	cityMap.put(101, "Varanasi");
	cityMap.put(102, "Prayag");

	Function<Integer, String> mappingFunction = k -> "Noida";
	int key = 103;
	String value = mappingFunction.apply(key);	
	if (cityMap.get(key) == null) {
	  if (value != null)
		cityMap.put(key, value);
	}

	System.out.println(value);
	System.out.println(cityMap);
  }
} 
Output
Noida
{101=Varanasi, 102=Prayag, 103=Noida} 
Find one more example of computeIfAbsent method.
ComputeIfAbsent12.java
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ComputeIfAbsent12 {
  public static void main(String[] args) {
     Map<String, List<String>> map = new HashMap<>();
     
     List<String> countries = new ArrayList<>();
     countries.add("Bharat");
     
     map.put("countries", countries);
     
     map.computeIfAbsent("capitals", k -> new ArrayList<>()).add("Delhi");
     
     System.out.println(map);
  }
} 
Output
{capitals=[Delhi], countries=[Bharat]} 

Example-2

Specified key is not associated with a value
and new value calculated by mapping function is null.
In this case the computeIfAbsent method will not put the new value for the specified key.
ComputeIfAbsent2.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfAbsent2 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
     
     cityMap.computeIfAbsent(103, k -> null);
     
     System.out.println(cityMap);
  }
} 
Output
{101=Varanasi, 102=Prayag} 

Example-3

Specified key is associated with a value
and new value calculated by mapping function is not null.
In this case the computeIfAbsent method will not put the new value for the specified key. There will be no change in Map.
ComputeIfAbsent3.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfAbsent3 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
     
     cityMap.computeIfAbsent(102, k -> "Prayagraj");
     
     System.out.println(cityMap);
  }
} 
Output
{101=Varanasi, 102=Prayag} 

Example-4

Specified key is associated with a null value
and new value calculated by mapping function is not null.
In this case the computeIfAbsent method will put the new value for the specified key.
ComputeIfAbsent4.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfAbsent4 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, null);
     
     cityMap.computeIfAbsent(102, k -> "Prayagraj");
     
     System.out.println(cityMap);
  }
} 
Output
{101=Varanasi, 102=Prayagraj} 

Reference

Java doc: Map
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ARVIND RAI
ARVIND RAI
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