Casting with type parameters in java

Question

Say I have a class like this:

class ParameterizedType<T>{
    public boolean equals(Object o){
        if (ParameterizedType<T>.class.isInstance(o)){
            ParameterizedType<T> other = (ParameterizedType<T>) o;
            return this.toString() == other.toString();
        }
        return false;
    }
}

I can get two different warnings from eclipse in such a method.

ParameterizedType<T>.class is not syntactically correct
(ParameterizedType<T>) o is an unchecked cast

How could one get around this?

@kocko has the right information. To get around this you need to use @SuppressWarnings("unchecked"). Eclipse gives helpful hints when you hover over the warning. By putting this in you are telling yourself, and other developers, that yes I acknowledge that this is unchecked but I need to do it. — markbernard
– markbernard, Commented Jul 20, 2015 at 14:26
Are you trying to implement equals by comparing string references? It's bad practice to calculate equals by referring to the toString method anyway, but comparing strings with == is a serious mistake. — RealSkeptic
– RealSkeptic, Commented Jul 20, 2015 at 14:26

Community · Accepted Answer · 2017-05-23 11:51:11Z

6

ParameterizedType<T>.class is not syntactically correct

The reason for this is that with .class you're referring the .class value at Runtime and since Generics is a compile-time feature of Java and the type parameters get erased and replaced with actual type(s), then the statement doesn't make sense at Runtime and the <T> is completely redundant.

Note that the same rule applies for the instanceof operator (i.e. you can't do if (something instanceof SomeGenericClass<T>))

(ParameterizedType<T>) o is an unchecked cast

There's no way to compare an Object to ParameterizedType unless downcasting. The IDE correctly suggests that the cast is unchecked, but in order to compare the instances, you have to downcast at some point. In this case, adding a @SuppressWarnings("unchecked") above the method would be fine.

Please also note that Strings in Java should not be compared with ==

edited May 23, 2017 at 11:51

CommunityBot

11 silver badge

answered Jul 20, 2015 at 14:21

Konstantin Yovkov

63k9 gold badges107 silver badges152 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Casting with type parameters in java

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related