When to use primitive and when reference types in Java

Question

In which case should you use primitive types(int) or reference types (Integer)?

This question sparked my curiosity.

Answer 1

My rule of thumb is: use boxed primitives only when it's necessary to get the code to compile. The only places in your code where the names of the primitive wrapper classes should appear is in generic type parameters and static method calls:

List intList = new ArrayList();

int n = Integer.parseInt("123");

That's the advice I would give to new Java programmers. As they learn more, they'll run into situations where they have to be more discerning, like when dealing with Maps or databases, but by then they should also have a better understanding of the difference between primitives and boxed primitives.

Autoboxing tempts us to believe int and Integer (for example) are interchangeable, but it's a trap. If you mix the two kinds of value indiscriminately, you can end up comparing two Integer values with == or trying to unbox a null without realizing it. The resulting bugs can be intermittent and difficult to track down.

It doesn't help that comparing boxed primitives with == sometimes works as if it were doing a value comparison. It's an illusion caused by the fact that values within a certain range are automatically cached in the process of autoboxing. It's the same problem we've always had with String values: comparing them with == sometimes "works" because you're actually comparing two references to the same, cached object.

When dealing with strings we can just tell the n00bs never to compare them with ==, as we've been doing all along. But comparing primitives with == is perfectly valid; the trick (thanks to autoboxing) is being sure the values really are primitives. The compiler will now let us declare a variable as an Integer and use it as if it were an int; that means we have to exercise a greater level of discipline and treat it as an error when someone does so without good reason.