从超类或接口列表中获取子类实例

Question

( This question is similar to mine but I felt that they were different enough that I could create a new one).

Basically in my small 3D game all of my entities are made up of many different components, one for textures, one for movement etc. All of these sub-components inherit from the interface "Component". Up until now I've used methods like this;

public MovementComponent getMovementComponent() {
    for (Component c : components) {
        if (c instanceof MovementComponent)
            return (MovementComponent) c;
    }

    return null;
}

To get an instance of a certain component. But now I wish to change this to a more general method that can get any type of component from my list of components. I tested doing something like this;

public Component getComponentType(Class component) {
    for (Component c : components) {
        if (c.getClass() == component)
            return c.getClass().cast(c);
    }

    return null;
}

But even though I cast the result this always returns an """instance""" of the interface, never an instance of a sub-component, which means I still have to cast it to the appropriate type. Basically I wish for a method that can:

1. Iterate through a collection.
2. Find a specific sub-class.
3. Return the instance of that sub-class.

I feel like I'm on the right track here but I can't get it to work. Thanks for your responses and feedback.

Answer 1

You need something like

public <T extends Component> T getComponentType(Class<T> component) {
    ...
    return (T) c;
}

Method declaration tells that it will return a component of the same type passed in parameters.

Call it like this:

MovingComponent mc = smth.getComponentType(MovingComponent.class);

Answer 2

You need to use a generic parameter, something like this:

public <C extends Component> C getComponentType(Class<C> subclass) {
    for (Component c : components) {
        if (subclass.isInstance(c))
            return subclass.cast(c);
    }
    return null;
}

Thank to isInstance , this also finds any further subclass of the given parameter.