Java HashMap computeIfAbsent() Method

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Description

The Java HashMap computeIfAbsent() method is used to compute a mapping for the specified key if the specified key is not already associated with a value (or is mapped to null), using the given mapping function and enters it into this map unless null.

Declaration

Following is the declaration for java.util.HashMap.computeIfAbsent​(K key, Function<? super K,​? extends V> mappingFunction) method.

public V computeIfAbsent​(K key, Function<? super K,​? extends V> mappingFunction)

Parameters

key − key with which the specified value is to be associated

remappingFunction − the remapping function to compute a value

Return Value

The method call returns the new value associated with the specified key, or null if none.

Exception

ConcurrentModificationException − if it is detected that the remapping function modified this map.

Compute Mapping of unmapped key of a HashMap of Integer, Integer Pair Example

The following example shows the usage of Java HashMap computeIfAbsent() method to get updated values of a Map. We've created two Map objects of Integer,Integer pair. Then few entries are added to map, then using computeIfAbsent method, the values are updated and then updated map is printed.

package com.tutorialspoint;

import java.util.HashMap;

public class HashMapDemo {
   public static void main(String args[]) {
      
      // create two hash maps
      HashMap<Integer, Integer> newmap = new HashMap<>();

      // populate map
      newmap.put(1, 1);
      newmap.put(3, 3); 
     
      // print the map
      System.out.println("Map: " + newmap);
 
      // update the values of the map
      newmap.computeIfAbsent(2, (key) -> key);      

      System.out.println("Updated Map: " + newmap);   
   }
}

Let us compile and run the above program, this will produce the following result.

Map: {1=1, 3=3}
Updated Map: {1=1, 2=2, 3=3}

Compute Mapping of unmapped key of a HashMap of Integer, String Pair Example

The following example shows the usage of Java HashMap computeIfAbsent() method to get updated values of a Map. We've created two Map objects of Integer,String pair. Then few entries are added to map, then using computeIfAbsent() method, the values are updated and then updated map is printed.

package com.tutorialspoint;

import java.util.HashMap;

public class HashMapDemo {
   public static void main(String args[]) {
      
      // create two hash maps
      HashMap<Integer, String> newmap = new HashMap<>();

      // populate map
      newmap.put(1, "A");
      newmap.put(3, "C"); 
     
      // print the map
      System.out.println("Map: " + newmap);
 
      // update the values of the map
      newmap.computeIfAbsent(2, (key) -> {return "B";});

      System.out.println("Updated Map: " + newmap);   
   }
}

Let us compile and run the above program, this will produce the following result.

Map: {1=A, 3=C}
Updated Map: {1=A, 2=B, 3=C}

Compute Mapping of unmapped key of a HashMap of Integer, Student Pair Example

The following example shows the usage of Java HashMap computeIfAbsent() method to get a update values of a Map. We've created two Map objects of Integer,Student. Then few entries are added to map, then using computeIfAbsent() method, the values are updated and then updated map is printed.

package com.tutorialspoint;

import java.util.HashMap;

public class HashMapDemo {
   public static void main(String args[]) {
      
      // create two hash maps
      HashMap<Integer, Student> newmap = new HashMap<>();

      // populate map
      newmap.put(1, new Student(1, "Julie"));      
      newmap.put(3, new Student(3, "Adam"));
     
      // print the map
      System.out.println("Map: " + newmap);
 
      // update the values of the map
      newmap.computeIfAbsent(2, (key) -> {return new Student(2, "Robert");});
      
      System.out.println("Updated Map: " + newmap);   
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }
   
   public Student update(String surname) {
	   this.name = this.name.concat(" " + surname);
	   return this;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

Let us compile and run the above program, this will produce the following result.

Map: {1=[ 1, Julie ], 3=[ 3, Adam ]}
Updated Map: {1=[ 1, Julie ], 2=[ 2, Robert ], 3=[ 3, Adam ]}