CODEWITHSUNDEEP
javaoopgenerics
M T
M T

Reputation: 998

Abstract classes and generics

Suppose I have the following classes:

public abstract class AbstractClass {
   ...
}

public class ConcreteClass extends AbstractClass {
   ...
}

a builder:

public class Builder{
    static public ConcreteClass build(){
        ...
    }
}

and a generic function in some other class:

public <T extends AbstractClass> T myFunction(){
    T a = Builder.build();
    return a;
}

It was my understanding that the compiler should have enough information to allow such assignment, however, it throws an error:

Type mismatch: cannot convert from ConcreteClass to T

Why is that and what are the potential dangers of such assignment?

Upvotes: 0

Views: 68

Answers (2)

Criticize SE actions means ban
Criticize SE actions means ban

Reputation: 58858

Someone could write this:

public class OtherConcreteClass extends AbstractClass {
    ...
}

...

SomeOtherClass soc = SomeOtherClass.<OtherConcreteClass>myFunction();

In that call to myFunction, T is OtherConcreteClass, and you can't cast ConcreteClass to OtherConcreteClass.

Upvotes: 2

Sbodd
Sbodd

Reputation: 11454

In short: T doesn't have to be ConcreteClass, it could also any sibling of ConcreteClass (e.g. if you declared ConcreteClass2 that also extended ConcreteClass).

Upvotes: 0

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