Java method returning object or directly manipulate it?

Question

I have come upon this (to me weird) piece of code. I have actually never seen this used and never used it myself, so it was something quite confusing... which goes along the lines of

• using hashmap as an example here, but other objects would behave the same

public static void fillData(HashMap<Object, Object> dataMap){
    dataMap.put("key","value");
}

Now this was very confusing, as the way I learned you do it is more like this

public static HashMap<Object, Object> fillData(){
    HashMap<Object, Object> dataMap = new HashMap<>();
    dataMap.put("key","value");
    return dataMap;
}

Now is there a time when I should use one way or the other? Im still pretty new to programming, but I havent found much of anything about this type of structure.

I also have experimented around and found this only works with objects, and not primitives...

Answer 1

Well, think about the case where the method manipulates an existing non-empty map. In that case the first example makes perfect sense.

Answer 2

I was googling this same subject in the morning and came across this discussion, which states that the first form of

public static void fillData(HashMap<Object, Object> dataMap){
    dataMap.put("key","value");
}

is considered "a bad practice, vestigial or pre-OOP times."