How to use java empty HashSet in if-statement?

Question

public static void main(String[] args) {
    Map<String, HashSet<String>> test = new HashMap<String, HashSet<String>>();

    test.put("1", new HashSet<String>());
    System.out.println(test);

    System.out.println(test.get("1"));

    if(test.get("1") == null){
        System.out.println("Hello world");
    }
}

The first println gets me {1=[]} The second one gets me []
I am trying to print out "Hello world" but the if statement isn't going through.
Is the empty HashSet, [] not equal to null?

How do I use the empty HashSet in this if statement?

Answer 1

There is a difference between null, which means "nothing at all," and an empty HashSet. An empty HashSet is an actual HashSet, but one that just coincidentally happens to not have any elements in it. This is similar to how null is not the same as the empty string "", which is a string that has no characters in it.

To check if the HashSet is empty, use the isEmpty method:

if(test.get("1").isEmpty()){
    System.out.println("Hello world");
}

Hope this helps!