Can AnyOne Please Explain this HashMap Behaviour

Question

My Program is public class Demo {

public static void main(String[] args) {
    List<String> arr = new ArrayList<>();
    arr.add("a");
    arr.add("b");
    Map<List<String>, String> map = new HashMap<>();
    map.put(arr, "Ravinda");
    System.out.println(map.get(arr));
    arr.add("c");
    System.out.println(map.get(arr));
}

}

Output is: Ravindra and null

I am not able to get why the output of second System.out.println is null.

Can anyone please explain.

Answer 1

When you call: map.put(arr, "Ravinda"); you are setting the key of the "Ravinda" value to be a List containing two strings.

By calling arr.add("c"); you are modifying List used earlier to index the "Ravinda" value in your hashmap.

Since the arr List has been changed, it no longer matches the key specified when you called: map.put(arr, "Ravinda");

This is why the hashmap is returning a null value when you try to access it for the second time.

The hashmap still contains the 'Ravinda' value, but this value is indexed against the list containing only the two values.