How to Remove all Mappings from HashMap in Java with Example

In this tutorial, I will be sharing how to remove all the key-value mappings from the HashMap. In the last tutorial, we learned how to remove an entry object or specific mapping from HashMap based on key. There are 3 ways to achieve our goal:

1. Using clear() method [Easiest]

2. Using removeIf() method

3. Removing through iterator

Read Also  :  How to check size of HashMap in Java

Let's dive deep into the topic:

Remove All Mappings from HashMap

1. Using clear() method

Syntax :

 public void clear()

According to Oracle docs, clear() method removes all of the mappings from the map.

Program to Remove all Mappings from HashMap

 import java.util.*;

public class HashMapRemoveAllMapping {
  public static void main(String[] args) {
      
    // Creating a HashMap object
    HashMap<Integer,String> map = new HashMap<>();
    
    // Integer Keys and String Values are added to the HashMap object
    map.put(50000, "Lawyer");
    map.put(100000, "Engineer");
    map.put(200000, "Doctor");
    
    // Showing HashMap object elements
    System.out.println("Original HashMap: "+ map);
    
    // Removing all the mappings from the HashMap object
    map.clear();
    
    // Showing the updated HashMap object
    System.out.println("Updated HashMap: "+ map);
    
  }
}

Output:
Original HashMap: {50000=Lawyer, 100000=Engineer, 200000=Doctor}
Updated HashMap: {}

2. Using removeIf() method

Syntax:

 default boolean removeIf(Predicate<? super E> filter)

According to Oracle docs, removeIf() method returns true if we are able to remove all of the elements of the given collection satisfying the given predicate.

 import java.util.*;

public class HashMapRemoveAllMapping2 {
    
  public static void main(String[] args) {
      
    // Initializing a HashMap object
    HashMap<String,String> map2 = new HashMap<>();
    
    // String Keys and String Values are added to the HashMap object
    map2.put("Profession1", "Doctor");
    map2.put("Profession2", "Engineer");
    map2.put("Profession3", "Lawyer");
    
    // Printing HashMap object elements
    System.out.println("Original HashMap elements are: "+ map2);
    
    // Fetching all the keys of the HashMap object
    Set<String> keys = map2.keySet();
    
    // entrySet() method is used to iterate the Set
    map2.entrySet().removeIf(entry -> keys.contains(entry.getKey()));
    
    // Printing the transformed HashMap object
    System.out.println("Updated HashMap is: " + map2);
  }
}

Output:
Original HashMap elements are: {Profession2=Engineer, Profession3=Lawyer, Profession1=Doctor}
Updated HashMap is: {}

3. Removing through iterator

In this third approach, you have to traverse through the map using iterator and then call the remove() method on the iterator.

 import java.util.*;

public class HashMapRemoveAllMapping3 {
    
  public static void main(String[] args) {
      
    // Creating a HashMap object
    HashMap<String,Integer> map3 = new HashMap<>();
    
    // String Keys and Integer Values are added to the HashMap object
    map3.put("Salary1", 1000);
    map3.put("Salary2", 2000);
    map3.put("Salary3", 3000);
    
    // Displaying HashMap object elements
    System.out.println("Original HashMap: "+ map3);
    
    // Getting all the keys of the HashMap object
    Set<String> keys = map3.keySet();
    
    // Iterating through the HashMap
    Iterator it = keys.iterator();
    while (it.hasNext())
    {
        it.next();
        it.remove();
    }
    
    // Displaying the transformed HashMap object
    System.out.println("Updated HashMap: "+ map3);
  }
}

Output:
Original HashMap: {Salary3=3000, Salary2=2000, Salary1=1000}
Updated HashMap: {}

That's all for today. Please mention in the comments if you have any questions related to how to remove all mappings from HashMap in Java with examples.