Java Hashtable computeIfAbsent() Method

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Description

The Java Hashtable 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 hashtable unless null.

Declaration

Following is the declaration for java.util.Hashtable.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 hashtable.

Computing a Mapping if Absent for Specific Key in a HashTable of Integer, Integer Pair Example

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

package com.tutorialspoint;

import java.util.Hashtable;

public class HashtableDemo {
   public static void main(String args[]) {
      
      // create two hash tables
      Hashtable<Integer, Integer> hashtable = new Hashtable<>();

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

      System.out.println("Updated Hashtable: " + hashtable);   
   }    
}

Output

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

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

Computing a Mapping if Absent for Specific Key in a HashTable of Integer, String Pair Example

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

package com.tutorialspoint;

import java.util.Hashtable;

public class HashtableDemo {
   public static void main(String args[]) {
      
      // create two hash tables
      Hashtable<Integer, String> hashtable = new Hashtable<>();

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

      System.out.println("Updated Hashtable: " + hashtable);   
   }    
}

Output

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

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

Computing a Mapping if Absent for Specific Key in a HashTable of Integer, Object Pair Example

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

package com.tutorialspoint;

import java.util.Hashtable;

public class HashtableDemo {
   public static void main(String args[]) {
      
      // create two hash tables
      Hashtable<Integer, Student> hashtable = new Hashtable<>();

      // populate hashtable
      hashtable.put(1, new Student(1, "Julie"));      
      hashtable.put(3, new Student(3, "Adam"));
     
      // print the hashtable
      System.out.println("Hashtable: " + hashtable);
 
      // update the values of the hashtable
      hashtable.computeIfAbsent(2, (key) -> {return new Student(2, "Robert");});
      
      System.out.println("Updated Hashtable: " + hashtable);   
   }    
}
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 + " ]";
   }
}

Output

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

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