HashMap putAll() Method in Java

The java.util.HashMap.putAll() is an inbuilt method of HashMap class that is used for the copy operation. The method copies all of the elements i.e., the mappings, from one map into another.

Syntax:

new_hash_map.putAll(exist_hash_map)

Parameters: The method takes one parameter exist_hash_map that refers to the existing map we want to copy from.



Return Value: The method does not return any values.

Exception: The method throws NullPointerException if the map we want to copy from is NULL.

Below programs illustrates the working of java.util.HashMap.putAll() method:
Program 1: Mapping String Values to Integer Keys.

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// Java code to illustrate the putAll() method
import java.util.*;
  
public class Hash_Map_Demo {
public static void main(String[] args) {
      
    // Creating an empty HashMap
    HashMap<Integer, String> hash_map = new HashMap<Integer, String>();
  
    // Mapping string values to int keys 
    hash_map.put(10, "Geeks");
    hash_map.put(15, "4");
    hash_map.put(20, "Geeks");
    hash_map.put(25, "Welcomes");
    hash_map.put(30, "You");
  
    // Displaying the HashMap
    System.out.println("Initial Mappings are: " + hash_map);
  
    // Creating a new hash map and copying
    HashMap<Integer, String> new_hash_map = new HashMap<Integer, String>();
    new_hash_map.putAll(hash_map);
  
    // Displaying the final HashMap
    System.out.println("The new map looks like this: " + new_hash_map);
}
}

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Output:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
The new map looks like this: {25=Welcomes, 10=Geeks, 20=Geeks, 30=You, 15=4}

Program 2: Mapping Integer Values to String Keys.

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// Java code to illustrate the putAll() method
import java.util.*;
  
public class Hash_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty HashMap
        HashMap<String, Integer> hash_map = new HashMap<String, Integer>();
  
        // Mapping int values to string keys
        hash_map.put("Geeks", 10);
        hash_map.put("4", 15);
        hash_map.put("Geeks", 20);
        hash_map.put("Welcomes", 25);
        hash_map.put("You", 30);
  
        // Displaying the HashMap
        System.out.println("Initial Mappings are: " + hash_map);
  
        // Creating a new hash map and copying
        HashMap<String, Integer> new_hash_map = new HashMap<String, Integer>();
        new_hash_map.putAll(hash_map);
  
        // Displaying the final HashMap
        System.out.println("The new map looks like this: " + new_hash_map);
    }
}

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Output:

Initial Mappings are: {4=15, Geeks=20, You=30, Welcomes=25}
The new map looks like this: {Geeks=20, 4=15, You=30, Welcomes=25}

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



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