Hashtable computeIfAbsent() method in Java with Examples

Last Updated : 30 Oct, 2018

The computeIfAbsent(Key, Function) method of Hashtable class which allows you to compute value of a mapping for specified key if key is not already associated with a value (or is mapped to null).

If mapping function of this method returns null, then no mapping is recorded.
If the remapping function throws an exception, the exception is rethrown, and the no mapping is recorded.
During computation, modification this map using this method is not allowed.
This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.

Syntax:

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

Parameters: This method accepts two parameters:

key: key with which the value is to be associated.
remappingFunction: function to do the operation on value.

Returns: This method returns current (existing or computed) value associated with the specified key, or null if mapping returns null. Exception: This method throws:

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

Below programs illustrate the computeIfAbsent(Key, Function) method: Program 1:

Java

// Java program to demonstrate
// computeIfAbsent(Key, Function) method.

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // create a table and add some values
        Map<String, Integer> table = new Hashtable<>();
        table.put("Pen", 10);
        table.put("Book", 500);
        table.put("Clothes", 400);
        table.put("Mobile", 5000);

        // print map details
        System.out.println("hashTable: "
                           + table.toString());

        // provide value for new key which is absent
        // using computeIfAbsent method
        table.computeIfAbsent("newPen", k -> 600);
        table.computeIfAbsent("newBook", k -> 800);

        // print new mapping
        System.out.println("new hashTable: "
                           + table);
    }
}

Output:

hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
new hashTable: {newPen=600, Book=500, newBook=800, Mobile=5000, Pen=10, Clothes=400}

Program 2:

Java

// Java program to demonstrate
// computeIfAbsent(Key, Function) method.

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // create a table and add some values
        Map<Integer, String> table = new Hashtable<>();
        table.put(1, "100RS");
        table.put(2, "500RS");
        table.put(3, "1000RS");

        // print map details
        System.out.println("hashTable: "
                           + table.toString());

        // provide value for new key which is absent
        // using computeIfAbsent method
        table.computeIfAbsent(4, k -> "600RS");

        // this will not effect anything
        // because key 1 is present
        table.computeIfAbsent(1, k -> "800RS");

        // print new mapping
        System.out.println("new hashTable: "
                           + table);
    }
}

Output:

hashTable: {3=1000RS, 2=500RS, 1=100RS}
new hashTable: {4=600RS, 3=1000RS, 2=500RS, 1=100RS}

References: https://docs.oracle.com/javase/10/docs/api/java/util/Hashtable.html#computeIfAbsent(K, java.util.function.Function)

HashTable putIfAbsent() method in Java with Examples

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Hashtable computeIfAbsent() method in Java with Examples

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