how to compare type of an object's instance to a generic type?

Question

how can i write this code in java?

public class ComponentsManager 
    {
        private List<IComponent> list = new ArrayList<IComponent>();

        public <U extends IComponent> U GetComponent() {

            for (IComponent component : list) {

                if(component instanceof U)
                {
                    return component;
                }
            }
        }
}

but i cant perform instanceof on generic types. how should i do it? thanks.

Answer 1

Basically you can't do that due to type erasure. The normal workaround is to pass a Class object as a parameter; e.g.

    public <U extends IComponent> U GetComponent(Class<U> clazz) {
        for (IComponent component : list) {
            if (clazz.isInstance(component)) {
                return clazz.cast(component);
            }
        }
    }

You could also use if (clazz.equals(component.getClass())) { ... but that does an exact type match ... which is not what the instanceof operator does. The instanceof operator and the Class.instanceOf method both test to see if the value's type is assignment compatible.

Answer 2

You could try adding a couple of 'test' methods:

private static <U extends IComponent> boolean testComponent(U u) {
  return true;
}

private static boolean testComponent(Object o) {
  return false;
}

Then, use testComponent(component) instead of component instanceof U.

Example code:

import java.util.*;

class IComponent {
}

class T1 extends IComponent {
}

public class Test {

  public static <U extends IComponent> boolean testComponent(U u) {
    return true;
  }

  public static boolean testComponent(Object o) {
    return false;
  }

  public static void main(String[] args) {
    T1 t = new T1();
    System.out.println("hm? " + (testComponent(t) ? "true" : "false"));
  }
}

Output:

hm? true