Question

I got this as home exercise but I have no clue how to solve it… (Just the access part is kind of difficult to figure out)

So, I have two classes: “Top” and “Main”. The Top class looks like this: (Short explanation: The Top class’ variable “pane” will be used in the class Main as the top panel, Main extends JFrame and has a BorderLayout)

public class Top extends JPanel implements ActionListener {

//public JPanel pane;
public JButton red, green, blue, white, black;

public Top() {
    //pane = new JPanel(); //Useless, as Top is already a JPanel
    this.setLayout(new FlowLayout(FlowLayout.CENTER));
    this.setBackground(new Color(0xc9c9c9)); //A gray background

    red = new JButton("Red");
    red.setBackground(Color.red);
    red.addActionListener(this);
    this.add(red);

    green = new JButton("Green");
    green.setBackground(Color.green);
    green.addActionListener(this);
    this.add(green);

    blue = new JButton("Blue");
    blue.setBackground(Color.blue);
    blue.addActionListener(this);
    this.add(green);

    white = new JButton("White");
    white.setBackground(Color.white);
    white.addActionListener(this);
    this.add(white);

    black = new JButton("Black");
    black.setForeground(Color.white);
    black.setBackground(Color.black);
    black.addActionListener(this);
    this.add(black);
    }

public void actionPerformed(ActionEvent e) {
    Main main = (Main)e.getSource();
    if (e.getSource() == this.red)
        main.setCENTER(1);
    if(e.getSource() == this.green)
        main.setCENTER(2);
    if(e.getSource() == this.blue)
        main.setCENTER(3);
    if(e.getSource() == this.white)
        main.setCENTER(4);
    if(e.getSource() == this.black)
        main.setCENTER(5);
    }
}

The Top class is being used in Main and everytime each of the buttons is pressed the CENTER-area’s background of the Main class changes. Therefore I have programmed a small function setCENTER, which changes the background. the code for it looks like this (The code is in the Main class):

    public void setCENTER(int var){
    switch(var){
        case 1: pane.setBackground(Color.red);
        break;
        case 2: pane.setBackground(Color.green);
        break;
        case 3: pane.setBackground(Color.blue);
        break;
        case 4: pane.setBackground(Color.white);
        break;
        case 5: pane.setBackground(Color.black);
        break;
    }
}

So far the content is displayed properly (the top panel in the Main window and the Main window itself), but everytime I try to change the background by pressing on one of the 5 buttons in the top panel, I get errors and the first error points me to this line of the actionPerformed-method in the Top class:

        Main main = (Main)e.getSource();

I tried replacing it with “Main main = new Main();”. It worked, the background changes but everytime I pressed it it opened a new window (which is clearly not how it should behave).

EDIT: I forgot that Top is already a JPanel, so using another JPanel inside the class is pretty senseless. (Edited Source)

Answer 1

In the general case, you’ll need to follow the chain up the hierarchy, calling getParent(), until you find the parent you want. If you are looking for a specific class, through some generics magic you can write a utility to do this:

   public static <T extends Container> T findParent(Component comp, Class<T> clazz)  {
      if (comp == null)
         return null;
      if (clazz.isInstance(comp))
         return (clazz.cast(comp));
      else
         return findParent(comp.getParent(), clazz);     
   }

So, if you want to find the FooPanel that is an ancester of the Button that fired the event, you’d say

FooPanel fooPanel = findParent((Component)event.getSource(), FooPanel.class);
fooPanel.doSomething...