遍历其父类java时获取一个子类

Question

I am looping through my Media arraylist ArrayList that has some child classes (Audio,Video,Picture) And when I loop through my Media arraylist

public double getTotalVideoRunTime()
    {
        int total = 0;
        for(Media v : mList)
        {
            if(v.isVideo())
            {

            }
        }
        return total;
    }

I'm not sure how to get 'runtime' which is a variable from the video class, do I have to loop differently? I am not sure how I can get variables from child classes, I assume its possible(I hope!)

Answer 1

You have to use instanceof and cast it to the correct class.

eg

if(v instanceof Video){
Video a = (Video) v;
a.childMethodOfVideo();
}

if(v instanceof Audio){
Audio a = (Audio) v;
a.childMethodOfAudio();
}

Answer 2

Not completely sure what you are trying to achieve, but if you want the variables a certain class has, you can use reflection API for it:

Field[] fields = v.getClass().getFields();
for (Field f : fields) System.out.println(f.getName());

Similarly if you want the methods:

Method[] methods = v.getClass().getMethods()();
for (Method m : methods) m.invoke(arguments...);

I must warn you that using reflection has a lot of cons (performance, safety) - and many times if you need to use it, it indicates a code smell .

---

If you are trying to do some specific behavior for a certain class, the correct way to do it is implement it as abstract method, and call it - the dynamic type will be chosen, and the desired method will be called:

public abstract static class Media { 
    abstract public void foo();
}
public static class Video extends Media{
    @Override
    public void foo() {
        System.out.println("video");

    }
}
public static class Image extends Media{
    @Override
    public void foo() {
        System.out.println("image");

    }
}

public static void main(String ar[]) {
    Media m = new Video();
    m.foo();
}