HashTable compute() method in Java with Examples

The compute(Key, BiFunction) method of Hashtable class allows to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping is found).

• If the remapping function passed in compute() of Hashtable returns null as a return value then the mapping is removed from Hashtable(or remains absent if initially absent).
• If the remapping function throws an exception, the exception is rethrown, and the current mapping is left unchanged.
• During computation, modification this map using this method is not allowed.
• The compute() method can be used to update an existing value inside Hashtable.
  For example, This mapping append string value of mapping:

  Hashtable.compute(key, (k, v) -> v.append("strValue"))
  

• This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.

Syntax:

public V 
       compute(K key,
             BiFunction<? super K, ? super V, ? 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 new value associated with the specified key, or null if none.

Exception: This method throws:

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

Below programs illustrate the compute(Key, BiFunction) method:

Program 1:

// Java program to demonstrate
// compute(Key, BiFunction) 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());

        // remap the values of hashTable
        // using compute method
        table.compute("Pen", (key, val)
                                 -> val + 15);
        table.compute("Clothes", (key, val)
                                     -> val - 120);

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

Output:

hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
new hashTable: {Book=500, Mobile=5000, Pen=25, Clothes=280}

Output:

hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
new hashTable: {Book=500, Mobile=5000, Pen=25, Clothes=280}

Program 2:

// Java program to demonstrate
// compute(Key, BiFunction) 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());

        // remap the values of hashTable
        // using compute method
        table.compute(3, (key, val)
                             -> val.substring(0, 4) + "00RS");
        table.compute(2, (key, val)
                             -> val.substring(0, 2) + "$");

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

Output:

hashTable: {3=1000RS, 2=500RS, 1=100RS}
new hashTable: {3=100000RS, 2=50$, 1=100RS}

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