Question

I was asked this question in an interview recently:

Can you name any class in the Java API that is final that shouldn’t be or one that isn’t and should be’?

I couldn’t think of any. The question implies that I should know all the API classes like the back of my hand, which I personally wouldn’t expect any Java developer to know.

If anyone knows any such classes, please provide examples.

Answer 1

The first examples that come to mind are some of the non-final Number subclasses, such as BigDecimal and BigInteger, which should probably have been final.

In particular, all of their methods can be overriden. That enables you to create a broken BigDecimal, for example:

public class BrokenBigDecimal extends BigDecimal {
    public BigDecimal add(BigDecimal augend) {
        return BigDecimal.ZERO;
    }
}    

That could create significant issues if you receive BigDecimal from an untrusted code for example.

To paraphrase Effective Java:

• Design and document for inheritance or else prohibit it
• Classes should be immutable unless there’s a very good reason to make them mutable