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Java Hashtable put() Method - GeeksforGeeks

Last Updated : 22 May, 2025

The Hashtable.put() method is a part of java.util package. This method is used to insert a mapping into a table. This means we can insert a specific key and the value it maps to into a particular table. If an existing key is passed, then the previous value gets replaced by the new value. If a new pair is passed, then the pair gets inserted as a whole.

Below is the syntax of put() method.

Hash_Table.put(key, value)

Parameters:

This method takes two parameters, both of the Object type of the Hashtable.

• key: This refers to the key element that needs to be inserted into the table for mapping.
• value: This refers to the value that the above key will map to.

Return Value: If an existing key is passed, the previous value gets returned. If a new pair is passed, then null is returned.

Example 1: In this example, we are going to use the put() method when an existing key is passed.

// Java program to demonstrate Hashtable.put() method
import java.util.*;

public class Geeks {
    public static void main(String[] args) {
        
        // Create an empty Hashtable
        Hashtable<Integer, String> ht = new Hashtable<Integer, String>();
        
        // Insert values into the Hashtable
        ht.put(10, "Geeks");
        ht.put(15, "4");
        ht.put(20, "Geeks");
        ht.put(25, "Welcomes");
        ht.put(30, "You");
        
        System.out.println("Initial table is: " + ht);
        
        // Insert existing key along with a new value
        String returned = ht.put(20, "All");
        
        // Verify the returned value
        System.out.println("Returned value is: " + returned);
        
        System.out.println("New table is: " + ht);
    }
}

Example 2: In this example, let's use the put() method when a new key is passed and we will see what it prints.

// Java program to demonstrate Hashtable.put() method
import java.util.*;

public class Geeks {
    public static void main(String[] args) {
        
        // Create an empty Hashtable
        Hashtable<Integer, String> ht = new Hashtable<Integer, String>();
        
        // Insert values into the Hashtable
        ht.put(10, "Geeks");
        ht.put(15, "4");
        ht.put(20, "Geeks");
        ht.put(25, "Welcomes");
        ht.put(30, "You");
        
        System.out.println("Initial table is: " + ht);
        
        // Insert new key along with a value
        String returned = ht.put(50, "All");
        
        // Verify the returned value
        System.out.println("Returned value is: " + returned);

        System.out.println("New table is: " + ht);
    }
}

Important Points:

• The put() method replaces the value if the key is already exists.
• The method returns the previous value associated with the key or it returns null if the key was not present.
• This method works with any combination of data types as the key and value.

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