Removing all Mapping From HashMap in Java
Last Updated : 17 Dec, 2020
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