Cast an object to class type passed as parameter

Question

I have a parent class and 2 child classes. I am trying to implement a function that takes the type of the child and which child as parameters.

When I use child.newInstance(), I want to store it in a variable of the type that is passed and call a function from the second parameter.

Below are the classes

public class Parent {
    public void test() {
        System.out.println("Test from parent");
    }
}

public class ChildA extends Parent {
    public void testChildA() {
        System.out.println("Test from child a");
    }
}

public class ChildB extends Parent {
    public void testChildB() {
        System.out.println("Test from child b");
    }
}

and here is the method I'm trying to implement

public class Driver {
    Parent func(Class child, String whichChild) throws Exception {
        // whichChild: "ChildA" or "ChildB"

        Object obj = child.newInstance();
        // cast obj to type of child and call the method "test" and "test" + whichChild
    }
}

Can it be done what I am trying to do? If yes, how can I cast this object to the type that is passed?

Solution 1

If you add a constraint to child, you don't need a cast at all to get a Parent:

Parent func(Class<? extends Parent> child, String whichChild) throws Exception {
    // whichChild: "ChildA" or "ChildB"

    Parent obj = child.newInstance();
    //...
}

However, you still can't call the testChildA etc method, since all you have is an instance of Parent. You'd need to use reflection to get the method:

Method method = obj.getClass().getMethod().getMethod("test" + whichChild);
method.invoke(obj);

It would be better to have a method on the interface of Parent which you can invoke, and is overridden in the subclasses.

public abstract class Parent {
  public void test() {
    System.out.println("Test from parent");
  }

  public abstract void testChild();
}

then simply call:

obj.testChild();

or, as Emanuele Ivaldi points out, just override test in ChildA and ChildB and invoke that directly.

Solution 2

Not sure exactly what you're doing but you can use Class.cast(...).

Eg

public <T> T getInstance(Class<T> type) {
    Object o = type.newInstance();
    T t = type.cast(o);
    return t;
}

Comments

[hakuna matata]

With reflection I can call the method testChildA even if obj is Parent in Parent obj = child.newInstance();?

[Andy Turner]

Sure - you can call it - but it doesn't mean that it is guaranteed to work. Method is decoupled from the instance that invoke it on. For instance, you could call method.invoke("string") - which will result in a runtime exception. It's up to you to ensure that you only call it in a way that makes sense.

[Andy Turner]

Why bother passing child as a parameter then? It helps if you highlight what you have actually changed, rather than just saying "try this block of code without explanation".

[Andy Turner]

Class<? extends Parent> constraints it.

[Andy Turner]

return type.newInstance() would be simpler.

[Discern]

I agree with that sir. Will edit my answer, thanks :)

[Emanuele Ivaldi]

The alternative would be to check the class of the object and cast it to the correct implementation and call the proper method, it's not flexible,it sucks but it avoids using reflection, did I say it sucks?

[lance-java]

Agreed, Im just trying to show the usage of Class.cast(Object)