Hashtable put() Method in Java

Last Updated : 28 Jun, 2018

The java.util.Hashtable.put() method of Hashtable is used to insert a mapping into a table. This means we can insert a specific key and the value it is mapping 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.

Syntax:

Hash_Table.put(key, value)

Parameters: The method takes two parameters, both are 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 would map into.

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

Below programs are used to illustrate the working of java.util.Hashtable.put() Method:
Program 1: When passing an existing key.

// Java code to illustrate the put() method 
import java.util.*; 
  
public class Hash_Table_Demo { 
    public static void main(String[] args) 
    { 
  
        // Creating an empty Hashtable 
        Hashtable<Integer, String> hash_table =  
                           new Hashtable<Integer, String>(); 
  
        // Inserting values into the table 
        hash_table.put(10, "Geeks"); 
        hash_table.put(15, "4"); 
        hash_table.put(20, "Geeks"); 
        hash_table.put(25, "Welcomes"); 
        hash_table.put(30, "You"); 
  
        // Displaying the Hashtable 
        System.out.println("Initial table is: " + hash_table); 
  
        // Inserting existing key along with new value 
        String returned_value = (String)hash_table.put(20, "All"); 
  
        // Verifying the returned value 
        System.out.println("Returned value is: " + returned_value); 
  
        // Displaying the new table 
        System.out.println("New table is: " + hash_table); 
    } 
} 

Output:

Initial table is: {10=Geeks, 20=Geeks, 30=You, 15=4, 25=Welcomes}
Returned value is: Geeks
New table is: {10=Geeks, 20=All, 30=You, 15=4, 25=Welcomes}

Program 2: When passing a new key.

// Java code to illustrate the put() method 
import java.util.*; 
  
public class Hash_Table_Demo { 
    public static void main(String[] args) 
    { 
  
        // Creating an empty Hashtable 
        Hashtable<Integer, String> hash_table =  
                           new Hashtable<Integer, String>(); 
  
        // Inserting values into the table 
        hash_table.put(10, "Geeks"); 
        hash_table.put(15, "4"); 
        hash_table.put(20, "Geeks"); 
        hash_table.put(25, "Welcomes"); 
        hash_table.put(30, "You"); 
  
        // Displaying the Hashtable 
        System.out.println("Initial table is: " + hash_table); 
  
        // Inserting existing key along with new value 
        String returned_value = (String)hash_table.put(50, "All"); 
  
        // Verifying the returned value 
        System.out.println("Returned value is: " + returned_value); 
  
        // Displaying the new table 
        System.out.println("New table is: " + hash_table); 
    } 
} 

Output:

Initial table is: {10=Geeks, 20=Geeks, 30=You, 15=4, 25=Welcomes}
Returned value is: null
New table is: {10=Geeks, 20=Geeks, 30=You, 50=All, 15=4, 25=Welcomes}

Note: The same operation can be performed with any type of variation and combination of different data types.

Hashtable get() Method in Java

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