HashTable forEach() method in Java with Examples

The forEach(BiConsumer) method of the Hashtable class is used to perform a specified action on each key-value mapping in the hashtable. The action is represented by a BiConsumer, which receives the key and value of each entry.

• Hashtable does not allow null keys or null values.
• The order in which entries are processed is not guaranteed.

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);
        table.put("Booklet", 2500);

        // add 100 in each value using forEach()
        table.forEach((k, v) -> {

            v = v + 100;
            table.replace(k, v);
        });

        // print new mapping using forEach()
        table.forEach(
            (k, v) -> System.out.println("Key : " + k + ", Value : " + v));
    }
}

Key : Booklet, Value : 2600
Key : Clothes, Value : 500
Key : Mobile, Value : 5100
Key : Pen, Value : 110
Key : Book, Value : 600

Explanation: The first forEach() operation visits every key-value mapping. For each entry, the value is increased by 100, and replace() updates the corresponding value in the Hashtable.

Note: The output order of a Hashtable is not guaranteed and may differ between executions.

Syntax

public void forEach(BiConsumer<? super K, ? super V> action)

• Parameters: action –> a BiConsumer representing the operation to be performed for each key-value mapping.
• Return Value: This method does not return any value.
• Exception: NullPointerException –> thrown if the specified action is null.

Program: To Show NullPointerException 

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");

        try {

            // add 100 in each value using forEach()
            table.forEach((k, v) -> {

                v = v + 100;
                table.put(null, v);
            });
        }
        catch (Exception e) {

            System.out.println("Exception: " + e);
        }
    }
}

Exception: java.lang.NullPointerException

Explanation: The forEach() method requires a valid BiConsumer action. Passing null as the action causes a NullPointerException.

Difference Between forEach() and entrySet() Loop

Advantages of forEach()

• Provides short and concise code.
• Makes simple iteration easier to read.
• Works naturally with lambda expressions.
• Directly provides both key and value to the operation.
• Useful for performing the same operation on every entry.