import java.util.*;

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

        Map<String, String>
                toEmails = Collections
                .singletonMap("key", "Value");

        Map<String, String> userEmails = new HashMap<>();
        userEmails.put("abc@gmail.com", "abc");

        toEmails.putAll(userEmails);

    }
}

when i’m trying to run the above code, I’m encountering the following exception

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.AbstractMap.put(AbstractMap.java:209)
    at java.util.AbstractMap.putAll(AbstractMap.java:281)
    at Hash_Map_Demo.main(Hash_Map_Demo.java:13)

I understand that here I’m trying to add a Hashmap to Collections.singletonmap
But aren’t they same and what is happening behind.

>Solution :

Collections.singletonMap() returns an immutable map. Use some other Map implementation, for example:

HashMap<String, String> toEmails = new HashMap();
toEmails.put("key", "Value")

Map<String, String> userEmails = new HashMap<>();
userEmails.put("abc@gmail.com", "abc");

toEmails.putAll(userEmails);