添加时不显示 JButton

Question

Jbutton will not display on screen, and if I try to add from main frame I get a null pointer exception.

I have followed steps from other examples and problems but none seem to display the JButton, I have tried to add it directly to the main JFrame (which is where I am trying to add it) with Display.getFrame().add(buttons.get(id)); but that just gives me a nullpointer exception for some reason

this is my Display class

https://pastebin.com/Bq0VVL9v the method which I think might be why I cant add the buttons properly v

private void createDisplay() {

    frame = new JFrame(title);

    frame.setSize(width, height);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.setResizable(false);

    frame.setLocationRelativeTo(null);

    frame.setVisible(true);

    canvas = new Canvas();

    canvas.setPreferredSize(new Dimension(width, height));

    canvas.setMaximumSize(new Dimension(width, height));

    canvas.setMinimumSize(new Dimension(width, height));

    canvas.setFocusable(false);
    
    frame.add(canvas);
    frame.pack();
}

and this is the class I am trying to add buttons with, the button manager which is being called by one of my menu classes

https://pastebin.com/bUMvnmFf

public ButtonManager(Icon image,Icon image2, int positionX, int positionY,int width, int height, int id, String name) {

    buttons.add(id, new JButton(name, image));

    buttons.get(id).setRolloverIcon(image2);

    buttons.get(id).setLocation(positionX, positionY);

    buttons.get(id).setSize(width, height);

    System.out.println(buttons.get(0));

    add(buttons.get(id));
}

I would have expected the Jbutton to be displayed on screen but that does not happen, the most annoying thing is the nullpointer if I add() to the the main JFrame in Display class.

EDIT: another major problem was the canvas was being rendered over the buttons, so none would show up

Answer 1

ButtonManager never initializes Display , so: display.getFrame().add(buttons.get(id)); will NPE as display is null.

It seems ButtonManager does not need the reference to Display.

On another level, adding the button to the frame conflicts with the Canvas object.

I wrote this main method and it displays both the canvas and button:

  public static void main(String[] args) {
    Display display = new Display("title", 200, 200);

    ButtonManager buttonManager = new ButtonManager(... , 0, 0, 100, 100, 0, "name");

    display.getFrame().getContentPane().add(buttonManager, BorderLayout.SOUTH);
    display.getFrame().pack();
  }

And the Canvas adding code was modified to (note the last two lines):

    canvas = new Canvas();
    canvas.setPreferredSize(new Dimension(width, height));
    canvas.setMaximumSize(new Dimension(width, height));
    canvas.setMinimumSize(new Dimension(width, height));
    canvas.setFocusable(false);

    frame.getContentPane().setLayout(new BorderLayout());
    frame.getContentPane().add(canvas, BorderLayout.NORTH);

Answer 2

In the class Display which is your main frame, it appears you did not add an instance of your panel, that is ButtonManager class. Or was that done in another separate class that was not included here?

Answer 3

Here is a solution for you. I codified only the basics for the example, you can continue with the program as you need.

/* Button Manager Class */
package mainpackage.display;    

import java.util.ArrayList;    

import javax.swing.JButton;
import javax.swing.JPanel;    

public class ButtonManager extends JPanel {
    private static final long serialVersionUID = 1L;    

    public ArrayList<JButton> buttons = new ArrayList<JButton>();
    Display display;    

    public static JButton getJButton(String name) {
        //Put all your stuff of images and ids here
        JButton aButton = new JButton(name);
        return aButton;
    }    

    public static void putButton(JPanel panel, JButton button, int posX, int posY) {
        button.setLocation(posX, posY);
        panel.add(button);
    }    

    public void remove(int id) {
        buttons.remove(id);
        display.getFrame().remove(buttons.get(id));
    }    

    public void tick() {    

    }    

    public void render() {    

    }    

}

Display class:

package mainpackage.display;
import java.awt.Canvas;    

import java.awt.Component;
import java.awt.Dimension;    

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;    

import static mainpackage.display.ButtonManager.*;

public class Display {

    private JFrame frame;
    private Canvas canvas;

    private String title;
    private int width, height;

    public Display(String title, int width, int height) {
        this.title = title;
        this.width = width;
        this.height = height;
        createDisplay();
    }

    private void createDisplay() {
        frame = new JFrame(title);
        frame.setSize(width, height);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        canvas = new Canvas();

        canvas.setPreferredSize(new Dimension(width, height));
        canvas.setMaximumSize(new Dimension(width, height));
        canvas.setMinimumSize(new Dimension(width, height));
        canvas.setFocusable(false);
        frame.add(canvas);

        JPanel panel = new JPanel();
        //You can add many buttons as you need
        putButton(panel, getJButton("Button 1"), 100, 200);
        putButton(panel, getJButton("Button 2"), 800, 800);

        frame.setContentPane(panel);

        frame.pack();
    }

    public void postCreate(Component a) {
        frame.add(a);
    }
    public void postCreate(JButton j) {
        frame.add(j);
    }
    public Canvas getCanvas() {
        return canvas;
    }
    public JFrame getFrame() {
        return frame;
    }

    public static void main(String[] args) {
        Display display = new Display("Hello", 1000, 1000);
    }
}

Output: