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 :
- clear() method
- remove through iterator
- 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
