What type of Exception should I throw if the wrong type of object is passed?

Question

What type of Exception should I throw if the wrong type of object is passed into my compareTo method?

ClassCastException?

Answer 1

It would be IllegalArgumentException in a general sense when the passed in value is not the right one.

However, as @Tom's answer below suggests, it could also be a ClassCastException for incorrect types. However, I am yet to encounter user code that does this.

But more fundamentally, if you're using the compareTo with generics, it will be a compile time error.

Consider this:

public class Person implements Comparable<Person> {
    private String name;
    private int age;

    @Override
    public int compareTo(Person o) {
       return this.name.compareTo(o.name);
    }
}

Where do you see the possibility of a wrong type being passed in the above example?

Answer 2

Unsurprisingly, the API docs specify the exception to be thrown in this case.

ClassCastException - if the specified object's type prevents it from being compared to this object.

Assuming you are using generics, you will automatically get this exception if someone attempts to call your methods using raw types, reflections or some other unsafe technique.