Java return the Object/modify the object (coding guidelines)

Question

If a method populates/modifies an object, would it be preferable to return the object or to keep the return type as void and the method would modify the Object through its reference?

public Obj populate(Obj o)
{
....
return o;
}

public void populate(Obj o)
{
....
}

I know this is a trivial question, but which one is the most preferred?

Answer 1

I depends on your style, but one advantage of returning: you could call populate(o).doSomethingElse();, i.e. you can chain method calls.

Have a look at how StringBuilder does that for example, which allows things like this new StringBuilder().append("a").append("b")....

Answer 2

I would go with Command-Query separation generally.

That is to say, if the method mutates the argument, it should not also return the argument.

There are however (as noted in the wikipedia article above) situations where violating this general principle is appropriate.

Answer 3

I would say the "first" option, to return the object. This has no disadvantages, and leave you "room" to make a change of implementation in the future (for example, returning a "deep copy" instead of the same object) without modifying the signature.

In short, i see it more flexible.