Hashtable computeIfAbsent() method in Java with Examples

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 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 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)