Java-ArrayList中对象的调用方法

Question

I have a class Kostka , that has its own width (w), height (h), x and y positions for drawing it later on a JPanel using this method

void maluj(Graphics g) {
    g.drawRect(x, y, w, h);
}

Now I need to make more of them and add them in ArrayList .. then call the maluj(g) method for each of the Kostka object stored in the ArrayList

---

So far I've managed to make a method that stores the Kostka objects in ArrayList, but I dont know how to call their methods

class MyPanel extends JPanel {
    ArrayList kos = new ArrayList(5);

    void addKostka() {
        kos.add(new Kostka(20,20,20,20));
    }

    public void paintComponent (Graphics g) {
        super.paintComponent(g);
    }
}

Answer 1

Invoking Methods

That's done the normal way:

// where kostka is an instance of the Kostka type
kostka.whateverMethodYouWant();

However, the way to retrieve the kostka from your list will depend on how you declared the list.

Using the Good Ol' Way (Pre-Java 1.5 Style)

// where index is the position of the the element you want in the list
Kostka kostka = (Kotska) kos.get(index);

Using Generics (The Better Way)

ArrayList<Kostka> kos = new ArrayList<Kostka>(5);

Kostka kostka = kos.get(index);

Answer 2

You can perform a "cast" in order to retrieve the Kostka elements of the ArrayList:

for (int i = 0; i < kos.size(); i++) {
    Kostka kostka = (Kotska)kos.get(i);
    kostka.maluj(g);
}

If you are using a version of Java which supports Generics, the cast is unnecessary. You can do:

ArrayList<Kostka> kos = new ArrayList<Kostka>(5);

for (int i = 0; i < kos.size(); i++) {
    Kostka kostka = kos.get(i);
    kostka.maluj(g);
}