java generic type parameter with newInstance

Question

I am not sure why Java is requiring me to cast the return of the makeInstance method to T? What am I doing wrong? Is there a better way to accomplish this?

public class Scratch {

    public <T extends Base> T freshen(T instance) {
        //why do I need to cast this to T
        return makeInstance(instance.getClass());
    }

    public <T extends Base> T makeInstance(Class<T> type) {
        try {
            return type.newInstance();
        } catch (InstantiationException ex) {
            Logger.getLogger(Scratch.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            Logger.getLogger(Scratch.class.getName()).log(Level.SEVERE, null, ex);
        }
        return null;
    }

    public static final class Base {

    }

}

Sergey Kalinichenko · Accepted Answer · 2015-02-14 23:40:58Z

2

The reason Java wants you to cast the result is that T returned from makeInstance method is not guaranteed to be the same as T of freshen method. The reason for it is that getClass() does not return Class<T>. Instead, it returns Class<? extends X>, where X is the erasure of the static type of instance, i.e. Object. That's where the compiler's chain of inference stops: it cannot infer from that call that the return type would be T, requiring you to do the cast.

An alternative to casting the instance would be casting the class, like this:

return makeInstance((Class<T>)instance.getClass());

edited Feb 14, 2015 at 23:40

answered Feb 13, 2015 at 14:59

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

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

1 Comment

newacct Over a year ago

"getClass() returns Class<?>" getClass() actually returns Class<? extends |X|>, where |X| is the erasure of the static type of instance, which in this case is Object.

Itay Maman · Accepted Answer · 2015-02-13 15:07:54Z

1

The root cause for your problem is "type erasure": when the Java compiler compiles the body of the freshen() method it essentially replaces the T type with its upper bound, namely: Base.

So here's the reasoning the compiler is doing: - instance is of type Base (as I said, T was replaced with Base) - .getClass() is invoked on a variable of type Base so the return value of this call is Class<? extends Base> - the parameter that is passed to makeInstance() is therefore, from the compiler's point of view, of type Class<? extends Base>. Consequently the T inside makeInstance() is also Base => the method returns Base

answered Feb 13, 2015 at 15:07

Itay Maman

30.8k12 gold badges93 silver badges124 bronze badges

2 Comments

cyberoblivion Over a year ago

Very good explanation. I only picked other because he gave me a good solution. Thanks

Itay Maman Over a year ago

@cyberoblivion you can up-vote it to indicate that it is valuable to future readers of your question

Tanmay Patil · Accepted Answer · 2015-02-13 15:02:39Z

0

You can solve this problem using following.

public class Scratch<T extends Base> {

    public T freshen(T instance) {
        // No need to cast this to T
        return makeInstance(instance.getClass());
    }

    public T makeInstance(Class<T> type) {
        try {
            return type.newInstance();
        } catch (InstantiationException ex) {
            Logger.getLogger(Scratch.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            Logger.getLogger(Scratch.class.getName()).log(Level.SEVERE, null, ex);
        }
        return null;
    }

    public static final class Base {

    }

}

Hope this helps.

answered Feb 13, 2015 at 15:02

Tanmay Patil

7,0772 gold badges27 silver badges45 bronze badges

Collectives™ on Stack Overflow

java generic type parameter with newInstance

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related