Removing all Mapping From HashMap in Java

Maps are used for when you want to associate a key with a value and Lists are an ordered collection. Map is an interface in the Java Collection Framework and a HashMap is one implementation of the Map interface. HashMap is efficient for locating a value based on a key and inserting and deleting values based on a key.

For removing all mappings from a HashMap in java we can use various approaches :

1. clear() method
2. remove through iterator
3. removeIf() method

Method 1: Using clear() method

The java.util.HashMap.clear() method in Java is used to clear and remove all the elements or mappings from a specified HashMap.

Syntax:

Hash_Map.clear()

Java

// Java program to remove mappings from
// HashMap using clear() method

import java.io.*;

import java.util.*;
class GFG {
    public static void main(String[] args)
    {

        HashMap<String, Integer> gfg = new HashMap<>();

        // adding values in hashMap 1
        gfg.put("DSA", 100);
        gfg.put("Problem Solving", 100);
        gfg.put("Development", 99);
        gfg.put("Interviews", 99);
        gfg.put("Competitive Programming", 97);
        gfg.put("FANG", 99);

        // printing the size and elements
        System.out.println("-------before removing------");
        System.out.println(gfg);
        System.out.println(gfg.size());

        // clear() method
        gfg.clear();

        System.out.println("--------After removing-------");
        System.out.println(gfg);
        System.out.println(gfg.size());
    }
}

Output

-------before removing------
{DSA=100, FANG=99, Competitive Programming=97, Problem Solving=100, Development=99, Interviews=99}
6
--------After removing-------
{}
0

Method 2: remove through the iterator

• In this approach, you will iterate over the Map using iterator and then call remove on the iterator.
• In this case, we are iterating over keys obtained from the calling keySet() method.
• hasNext() is used to check whether there is a next element present inside the collection or not.

Java

// Java program to remove each key value pair
// by iterating over the Hashmap

import java.io.*;
import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        HashMap<String, Integer> gfg = new HashMap<>();

        // adding values in hashMap 1
        gfg.put("DSA", 100);
        gfg.put("Problem Solving", 100);
        gfg.put("Development", 99);
        gfg.put("Interviews", 99);
        gfg.put("Competitive Programming", 97);
        gfg.put("FANG", 99);

        // printing the size and elements
        System.out.println("-------before removing------");
        System.out.println(gfg);
        System.out.println(gfg.size());

        // getting all keys of map using keySet()

        Set keyset = gfg.keySet();

        // iterating over the keys and removing
        // hasNext() method is used to check whether next
        // element present inside the collection or not

        Iterator itr = keyset.iterator();
        while (itr.hasNext()) {
            itr.next();
            itr.remove();
        }

        System.out.println("--------After removing-------");
        System.out.println(gfg);
        System.out.println(gfg.size());
    }
}

Output

-------before removing------
{DSA=100, FANG=99, Competitive Programming=97, Problem Solving=100, Development=99, Interviews=99}
6
--------After removing-------
{}
0

Method 3: Using removeIf() method

Syntax:

public boolean removeIf(Predicate filter)

Parameter: This method takes a parameter filter which represents a predicate which returns true for elements to be removed.

Returns: This method returns True if the predicate returns true, and we are able to remove elements.

Exception: This method throws NullPointerException if the specified filter is null.

• In this method, you need to specify the condition on which the element will be removed but for our case, we need to remove all elements.
• Hence, we need to iterate on every key in entryset. We will check the set of keys obtained using the keySet() method.
• We are removing every key of entrySet which is present in keySet. Every entry object, 1 entry=1key+1 value.
• keySet() gives all keys present inside the Map.

Java

// Java program to remove the hashings
// from HashMap using removeIf()

import java.io.*;

import java.util.*;

public class GFG {
    public static void main(String args[])
    {

        HashMap<String, Integer> gfg = new HashMap<>();

        // adding values in hashMap 1
        gfg.put("DSA", 100);
        gfg.put("Problem Solving", 100);
        gfg.put("Development", 99);
        gfg.put("Interviews", 99);
        gfg.put("Competitive Programming", 97);
        gfg.put("FANG", 99);

        System.out.println("-------before removing------");
        System.out.println(gfg);
        System.out.println(gfg.size());

        // getting all the keys of map

        Set<String> keySet = gfg.keySet();

        // checking the entry set
        // key in keySet and remove
        // it one by one
        gfg.entrySet().removeIf(
            entry -> keySet.contains(entry.getKey()));

        System.out.println("--------After removing-------");
        System.out.println(gfg);
        System.out.println(gfg.size());
    }
}

Output

-------before removing------
{DSA=100, FANG=99, Competitive Programming=97, Problem Solving=100, Development=99, Interviews=99}
6
--------After removing-------
{}
0