Premature optimization in Java: when to use “x = foo.getX()” vs simply “foo.getX()”

Question

When I find myself calling the same getter method multiple times, should this be considered a problem? Is it better to [always] assign to a local variable and call only once?

Answer 1

That depends on what getX() actually does. Consider this class:

public class Foo {
    private X x;

    public X getX() { return x; }
}

In this case, when you make a call to foo.getX(), JVM will optimize it all the way down to foo.x (as in direct reference to foo's private field, basically a memory pointer). However, if the class looks like this:

public class Foo {
    private X x;

    public X getX() { return cleanUpValue(x); }

    private X cleanUpValue(X x) {
        /* some modifications/sanitization to x such as null safety checks */
    }
}

the JVM can't actually inline it as efficiently anymore since by Foo's constructional contract, it has to sanitize x before handing it out.

To summarize, if getX() doesn't really do anything beyond returning a field, then there's no difference after initial optimization runs to the bytecode in whether you call the method just once or multiple times.