JAVA Inheritance Generics and Casting.

Question

Hello,

I have two classes which both extends Example.

public class ClassA extends Example {

    public ClassA() {
        super("a", "class");
    }
    ....

}

public class ClassB extends Example {

    public ClassB() {
        super("b", "class");
    }

    ....

}

public class Example () {

    public String get(String x, String y) {
        return "Hello";
    }

}

So thats all very well. So suppose we have another class called ExampleManager. With example manager I want to use a generic type and consequently return that generic type. e.g.

public class ExampleManager<T extends Example> {

    public T getExample() {

        return new T("example","example"); // So what exactly goes here?

    }

}

So where I am returning my generic type how do i get this to actually work correctly and cast Example as either classA or classB?

Many Thanks

Answer 1

You can't use a generic type to instantiate new object (i.e. you can't do new T(params)).

Answer 2

When you are creating a concrete instance of object (that is, you use new), you have know the actual implementing class, you can't use a generic type.

What are you actually trying to achieve? How do you decide whether you want to create ClassA or ClassB?

Try this:

public class ExampleManager {
    public ClassA createClassA() {
        return new ClassA("example","example");
    }

    public ClassB createClassB() {
        return new ClassB("example","example");
    }
}

or this:

public class ExampleManager {
    public Example createExample() {
        if(a == b) {
           return new ClassB("example","example"); 
        }
        return new ClassB("example","example"); 
    }
}

Answer 3

Invalid answer.

Answer 4

As others have said, you can't use new to create a new instance of an unknown type. Without using reflection, you could make ExampleManager an abstract superclass of factories.

public abstract class ExampleManager<T extends Example> {
     public abstract T getExample(String x, String y);
}

public class ClassAManager extends ExampleManager<ClassA> {
     public ClassA getExample(String x, String y) {
          return new ClassA(x, y);
     }
}

public class ClassBManager extends ExampleManager<ClassB> {
     public ClassB getExample(String x, String y) {
          return new ClassB(x, y);
     }
}