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?

Answer 1

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;
}

Answer 2

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.