How to access an object within another class (Java)

Question

Here's an abrevated version of what my code looks like:

public class ColorFactory extends JFrame {
            public ColorFactory(){
                buildTopPanel();
            }

            public void buildTopPanel(){
            JPanel topPanel = new JPanel();
            this.add(topPanel, BorderLayout.NORTH);
            }

}

As you can see I have a method that makes a new JPanel object when called. How can I access that particular JPanel object from another class? I have a button listener class that I want to change the color of the JPanel from outside the ColorFactory class. This code is right after the ColorFactory class.

public class ButtonListener implements ActionListener{
     public void actionPerformed(ActionEvent e) {
           //Change JPanel color here. 
     }
}

Would it be better just to instantiate JPanel in the ColorFactory constructor and then just access it through there?

Answer 1

For starters, you need to make the JPanel a field in ColorFactory, so references to it don't disappear when you exit buildTopPanel(). Once you've saved a reference to it, then you have a couple of choices. From the design standpoint, the bad choice is to expose it, eg:

JPanel getTopPanel(){
    return topPanel;
}

The better choice is to have your action listener send a message to ColorFactory that says "respondToButton(Color newColor)", and have ColorFactory decide to change topPanel's color... eg:

public void respondToButton(Color newColor){     
    topPanel.setBackground(newColor);
}

Answer 2

You are facing a design issue; in general, this type of situations require more investigation to understand how to end up with a clean and maintainable design. However, For the specific problem you are reporting, I would:

1. Create a constructor of ButtonListener that receives a parameter (ie the ColorFactory ) which could access the information you need, so that you can initialize a field in ButtonListener itself
2. Create a method changeColor in the ColorFactory . This method actually applies the color change
3. In the ButtonListener , invoke changeColor on the field, ie the reference to the ColorFactory

Answer 3

You should make the JPanel a field of the class like this:

public class ColorFactory extends JFrame {

        JPanel topPanel;


        public ColorFactory(){
            buildTopPanel();
        }

        public void buildTopPanel(){
            topPanel = new JPanel();
            this.add(topPanel, BorderLayout.NORTH);
        }

        public void changeColor(Color color) {
            //color changing code here
        }


}

now You can get the JPanel from another class. All you have to do now, is get the ColorFactory into your Button listener:

public class ButtonListener implements ActionListener{

    ColorFactory colorFactory;
    public ButtonListener(ColorFactory colorFactory) {
        this.colorFactory = colorFactory;
    }

    public void actionPerformed(ActionEvent e) {
           colorFactory.changeColor(/* color here */);
    }
}