Understanding hashmap (with Arraylist as value) put method

Question

I'm having trouble getting my head around something I encountered earlier. I have this class named DictionaryOfTranslations, which has a method called add.

The only instance variable this class has is a private HashMap<String, ArrayList<String>> dictionary;.

This is what the add method looks like:

public void add(String word, String translation) {
    this.dictionary.putIfAbsent(word, new ArrayList<>());
    
    ArrayList<String> completed = this.dictionary.get(word);
    completed.add(translation);
    this.dictionary.put(word, completed);
}

The trouble I'm having is to understand why this last line of code: this.dictionary.put(word, completed); is apparently redundant. If I comment this line out, the outcome is exactly the same. My question here is: Why don't you have to specifically have to call the put() method on this hashmap, to add the updated Arraylist to it?

Answer 1

Objects are not passed by value, their references are.

When you use put(), the reference is stored in the HashMap. Similarly, if you use get, you do not get a copy of the list, you get a reference to the same list object.

If you mutate the Map through the reference, you mutate still the same object that is also stored in the map.

Thus, putting it back is simply not needed - you have already changed that very object.