JPanel components won't display

Question

I try to add some components to an JPanel. paintComponent() is called, but the components won't display. revalidate() didn't work.

The structure looks like the following:

public class AbstractView extends JPanel implements ViewInterface {
}

public class GraphView extends AbstractView {
    public void doSomeStuff(){
        this.add(new ElementView());
        this.revalidate();
    }    
}

public class ElementView extends AbstractView {
    @Override
    public void paintComponent(Graphics g) {
        // this method is called but has no effect
        super.paintComponent(g);
        g.fillRect(0,0,100,100);
        this.repaint();
    }
}

I think that's the relevant code. Methods like getPreferredSize() and even getX(),getY(),getWidth(),getHeight() are implemented and seem to work like expected. I searched Google for hours, but nothing seems to help. Am i searching the bug at the right location?

edit:

Here is some more of the Code:

1:

public class AbstractView extends JPanel implements ViewInterface {
@Override
public void update() {}
@Override
public boolean containsPoint(Point p) {
    return false;
}
@Override
public ApplicationView topContainer() {
    return null;
}
@Override
public Dimension getPreferredSize() {
    return new Dimension(this.getBounds().getSize());
}
@Override Rectangle getBounds(){
    return Rectangle(0,0,0,0);
}
@Override
public int getX() {
   return this.getBounds().x;
}
@Override
public int getY() {
    return this.getBounds().y;
}
@Override
public int getWidth() {
    return this.getBounds().width;
}
@Override
public int getHeight() {
    return this.getBounds().height;
}

}

2:

class BodyView extends AbstractView {

Body body;

BodyView(Body b, AbstractView c){
    this.body = b;
    this.setLayout(null);
}

private int radius(){
    return (int) body.radius();
}

@Override
public void paintComponent(Graphics g) {
    // called but without effect
    super.paintComponent(g);
    g.setColor(this.color);
    int x = (int) body.position.at(0);
    int y = (int) body.position.at(1);
    int radius = radius();
    g.fillOval(x-radius, y-radius, 2*radius, 2*radius);
}

@Override
public Rectangle getBounds(){
    int radius = this.radius();
    return new Rectangle(getX() - radius - 2, getY() - radius - 2, 2*radius + 2, 2*radius + 2);
}

@Override
public int getX() {
    return (int) body.position.at(0);
}
@Override
public int getY() {
    return (int) body.position.at(1);
}
}

3:

class SpaceView extends AbstractView {

Space space;
Set<BodyView> bodyViews;

SpaceView(Space s, AbstractView c){
    this.space = s;
    this.initializeBodyViews();
    this.setLayout(null);
}

private void initializeBodyViews(){
    bodyViews = new HashSet<BodyView>();
    for(Body body : space.bodies){
        BodyView view = new BodyView(body, this);
        // new components are added here 
        addBodyView(view);
    }
}

public void addBodyView(BodyView view){
    bodyViews.add(view);
    this.add(view);
}
}

If this isn't enough to determine the problem i would like to refer to this Github.