How do I implement ActionListener inside a JPanel class that's inside another class?

Question

I have a class called 'Panel' that extends JPanel and it is inside another class called 'Main'. The constructor instantiates JFrame, and all the GUI components, and sets it all up, such as size.

The class 'Panel' which extends JPanel has a method public void paintComponent(Graphics g){} and inside it I added a few JButtons and used g.drawString's.

Then in the 'Main' class, I added the 'Panel' to the JFrame.

My question is, I am trying to implement an actionListener to a button added inside the 'Panel' class. The actionListener function would add more buttons and use g.drawString's as well. Now where would I place the ActionListener in order to do so? How can I use g.drawString for a particular panel and the g.drawString line is inside another class, which is the ActionListener class? I would need to use Graphics g of paintComponent inside the actionPerformed.

Thank you!

EDIT - CODE EXAMPLE:

public class Main{

private JFrame jf;
private JTextField jtf1;
private JTextField jtf2;    
private Panel p;    
private JComboBox jcb1;
private JComboBox jcb2;     
private JButton button;     
private Object options[];

//ActionListener Variables
private int string1 = 150;
private int string2 = 150;      
private int yJtf1 = 150;
private int yJtf2 = 160;        
private int cb1 = 140;
private int cb2 = 165;      
private int count = 0;

    public Main(){

        jf= new JFrame();
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(700, 700);

        p = new Panel();

        jtf1 = new JTextField("", 20);
        jtf2= new JTextField("", 20);

        Object options[] = {""};

        jcb1 = new JComboBox(tools);
        jcb2 = new JComboBox(tools);

        button = new JButton("+");

        jf.add(p);
        jf.setVisible(true);`
    }

    public class Panel extends JPanel{

        public Panel(){
            this.setLayout(null);
        }

        public void paintComponent(Graphics g){

            super.paintComponent(g);

            /*button.addActionListener(new ActionListener(){  //Would this work or should the ActionListener be a class as shown below?
                public void actionPerformed(ActionEvent e){

                    if(count < 3){ //Won't be allowed to add anymore after 3 times
                        string1 += 50;
                        string2 += 50;

                        jtf1 += 50;
                        jtf2 += 50;

                        cb1 += 50;
                        cb2 += 45;

                        //Would like to add the following components to the 'Panel' (which is a JPanel) whenever the JButton 'button' already added to 'Panel' is clicked.
                        p.add(jtf1); //Would doing p.add really add to the panel when the ActionListener is called?
                        jtf1.setBounds(60, yJtf1, 50, 40);
                        p.add(jtf2);
                        jtf2.setBounds(60, yJtf2, 50, 40);

                        add(jcb1);
                        jcb1.setBounds(250, cb1, 50, 40);
                        add(left2);
                        jcb2.setBounds(250, cb2, 50, 40);

                        Font font = new Font("TimesRoman", Font.BOLD, 18);
                        g.setFont(font);  //Getting error on 'g' regardless

                        g.drawString("Hi", 15, string1); //This is the main problem, how would I be able to add this strings to the 'Panel' (which is a JPanel)
                        g.drawString("There", 330, string1);
                    }
                    count++;
                }
            });*/

            add(jtf1);
            jtf1.setBounds(100, 30, 120, 30);
            add(jtf2);
            ljtf2.setBounds(100, 60, 120, 30);

            add(button);
            plusButton.setBounds(200,150, 50, 50);
            //button.addActionListener(new ButtonClicked()); if doing ActionListener via class like below

            add(jcb1);
            jcb1.setBounds(300, 350, 100, 50);
            add(ljcb2);
            jcb2.setBounds(300, 350, 100, 25);

            Font font = new Font("Arial", Font.BOLD, 12);
            g.setFont(font);
            g.drawString("Item:", 40, 45);
            g.drawString("Cost:", 40, 75);
        }
    }


    public static void main(String [] args){
        new Main();
    }

    class ButtonClicked implements ActionListener{ //Action Listener: The follow is what I am trying to implement
        public void actionPerformed(ActionEvent ae){
            if(count < 3){ //Won't be allowed to add anymore after 3 times
                string1 += 50;
                string2 += 50;

                jtf1 += 50;
                jtf2 += 50;

                cb1 += 50;
                cb2 += 45;

                //Would like to add the following components to the 'Panel' (which is a JPanel) whenever the JButton 'button' already added to 'Panel' is clicked.
                p.add(jtf1); //Would doing p.add really add to the panel when the ActionListener is called?
                jtf1.setBounds(60, yJtf1, 50, 40);
                p.add(jtf2);
                jtf2.setBounds(60, yJtf2, 50, 40);

                mp.add(jcb1);
                jcb1.setBounds(250, cb1, 50, 40);
                mp.add(left2);
                jcb2.setBounds(250, cb2, 50, 40);

                Font font = new Font("TimesRoman", Font.BOLD, 18);
                g.setFont(font); 

                g.drawString("Hi", 15, string1); //This is the main problem, how would I be able to add this strings to the 'Panel' (which is a JPanel)
                g.drawString("There", 330, string1);
            }
            count++;
        }
    }
}

Answer 1

I would need to use Graphics g of paintComponent inside the actionPerformed."`

No, you wouldn't since Swing graphics is passive not active. The Graphics object would reside in the JPanel's paintComponent method and would be passed to it from the JVM when it and only it calls the paintComponent method. The actionPerformed method would then change the value of a String variable or perhaps an ArrayList<String>, call repaint(), and then the JPanel's paintComponent method would use the changed String to draw the appropriate text.

If you need more specific help, consider telling us a few more details and posting a minimal example program.

---

Edit
On review of your code I have several suggestions:

• Please try to fix your code a bit so that it is either compilable or as close to compilable as possible.
• Never add components or do anything but painting inside of your paintComopnent(...) method. You don't have full control over when or even if that method will be called, and placing something in inside of this method will result in unwanted side effects, like combo boxes that simply won't work.
• If you ever wanted text shown in your program, you can always add a JLabel.
• Your program uses null layout and setBounds(...) something that results in a rigid GUI that may look good on one system but will usually look poor on any other system or screen resolution. Also programs created this way are very hard to debug, maintain and upgrade. Instead use the layout managers as this is what they excel at: at creating complex flexible GUI's that can be enhanced and changed easily.

---

Edit
I'm guessing what you really want to do, and that possibly is to add more components to allow the user to enter more data into the GUI. You also want to add text perhaps to guide the user in what the purpose of the components may be. If this is so, then the best solution is not to add Strings to the GUI in paintComponent but rather to add Strings/Components in an organized fashion with the Strings being displayed in JLabels, and the components and the labels all held in a JPanel or JPanels, and added to the GUI using layout managers.

For example, if you want the user to add data in two JTextfields and have two JComboBoxes, and then allow the user to add 3 more of these guys if need be, a GUI could look something like this:

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.Insets;
import java.awt.Window;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.util.ArrayList;
import java.util.List;

import javax.swing.*;

@SuppressWarnings("serial")
public class Main2 extends JPanel {
   private List<DataPanel> dataPanelList = new ArrayList<>();
   private JPanel dataPanelHolder = new JPanel();

   public Main2() {
      DataPanel dataPanel = new DataPanel();
      dataPanelList.add(dataPanel);
      setLayout(new BorderLayout());      
      dataPanelHolder.setLayout(new BoxLayout(dataPanelHolder, BoxLayout.PAGE_AXIS));
      dataPanelHolder.add(dataPanel);
      JPanel innerBorderLayoutPanel = new JPanel(new BorderLayout());
      innerBorderLayoutPanel.add(dataPanelHolder, BorderLayout.PAGE_START);

      JScrollPane scrollPane = new JScrollPane(innerBorderLayoutPanel);
      scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
      int w = dataPanel.getPreferredSize().width;
      int h = dataPanel.getPreferredSize().height * 4;
      Dimension viewPortSize = new Dimension(w, h);
      scrollPane.getViewport().setPreferredSize(viewPortSize);

      JPanel buttonPanel = new JPanel(new GridLayout(1, 0, 5, 0));
      buttonPanel.add(new JButton(new AddDatatAction("Add")));
      buttonPanel.add(new JButton(new ExitAction("Exit", KeyEvent.VK_X)));

      add(scrollPane, BorderLayout.CENTER);
      add(buttonPanel, BorderLayout.PAGE_END);
   }

   private class AddDatatAction extends AbstractAction {
      private int maxCount = 4;
      public AddDatatAction(String name) {
         super(name);
         int mnemonic = (int)name.charAt(0);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         if (dataPanelList.size() < maxCount) {
            DataPanel dataPanel = new DataPanel();
            dataPanelList.add(dataPanel);
            dataPanelHolder.add(dataPanel);
            dataPanelHolder.revalidate();
            dataPanelHolder.repaint();
         }
      }
   }

   private class ExitAction extends AbstractAction {
      public ExitAction(String name, int mnemonic) {
         super(name);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         Window win = SwingUtilities.getWindowAncestor(Main2.this);
         win.dispose();
      }
   }

   private static void createAndShowGui() {
      JFrame frame = new JFrame("Main2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(new Main2());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

@SuppressWarnings("serial")
class DataPanel extends JPanel {
   private static final String[] TOOLS = {"Tool 1", "Tool 2", "Tool 3", "Tool 4"};
   private static final String[] FIELD_LABELS = {"Item", "Cost"};
   private static final String[] COMBO_LABELS = {"Foo", "Bar"};
   private JTextField[] fields = new JTextField[FIELD_LABELS.length];
   private List<JComboBox<String>> comboList = new ArrayList<>();

   public DataPanel() {
      setBorder(BorderFactory.createTitledBorder("Data"));
      setLayout(new GridBagLayout());
      for (int i = 0; i < FIELD_LABELS.length; i++) {
         add(new JLabel(FIELD_LABELS[i]), createGbc(0, i));
         fields[i] = new JTextField(10);
         add(fields[i], createGbc(1, i));

         JComboBox<String> combo = new JComboBox<>(TOOLS);
         comboList.add(combo);
         add(combo, createGbc(2, i));
         add(new JLabel(COMBO_LABELS[i]), createGbc(3, i));         
      }
   }

   public static GridBagConstraints createGbc(int x, int y) {
      GridBagConstraints gbc = new GridBagConstraints();
      gbc.gridx = x;
      gbc.gridy = y;
      gbc.gridwidth = 1;
      gbc.gridheight = 1;
      gbc.weightx = 1.0;
      gbc.weighty = 1.0;
      int ins = 4;
      gbc.insets = new Insets(ins, ins, ins, ins);
      return gbc;
   }
}

And could look like:

one datapanel added

four added: