HashMap putAll(Map) in Java

putAll(Map): This method is available in java.util.HashMap class of Java.

Syntax:

void java.util.HashMap.putAll(Map<? extends K, ? extends V> m)

This method takes one argument. This method copies all of the mappings from the specified map to this map.

Note: These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

Parameters: One parameter is required for this method.

m: mappings to be stored in this map.

Throws:

NullPointerException -  if the specified map is null

Approach 1: When no exception

Java

import java.util.HashMap;

public class HashMapputAll {
    public static void main(String[] args) {
        HashMap<String, Integer> hashMap = 
                 new HashMap<String, Integer>();

        hashMap.put("Hello", 1);
        hashMap.put("World", 2);

        HashMap<String, Integer> hashMap2 = 
                  new HashMap<String, Integer>();

        hashMap2.putAll(hashMap);

        System.out.println(hashMap2);

    }
}

Output:

{Hello=1, World=2}

Approach 2: NullPointerException

Java

import java.util.HashMap;

public class HashMapputAll {
    public static void main(String[] args) {
        HashMap<String, Integer> hashMap = null;

        HashMap<String, Integer> hashMap2 = 
                   new HashMap<String, Integer>();

        hashMap2.putAll(hashMap);

        System.out.println(hashMap2);

    }
}

Output:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Map.size()" because "m" is null
	at java.base/java.util.HashMap.putMapEntries(HashMap.java:497)
	at java.base/java.util.HashMap.putAll(HashMap.java:785)