Java GUI Button Issue

Oh, I can see the screenshot now! TSR is weird sometimes!

Anyway, once you've killed off the default LayoutManager that comes with a JPanel, you can use setBounds on any component to specify a position relative to that JPanel's top-left corner.

TSR has a habit of acting up! I've tried absolute positioning once as it seems to be what my project leads to. However as soon as I set layout to null I get a NullPointerError come up on the paint method I'm using and It's absolutely beyond me why that's happening. Then of course because that happens nothing paints and I have a tiny empty panel display to screen.

That NullPointerError will most likely be caused by something in your program trying to use the LayoutManager, but if all you're doing is drawing lines in paintComponent that would seem odd. Does the NullPointerError give you a useful stack trace which identifies the place where this is happening? (That may of course be a wild goose-chase, because the exception might end up being triggered from somewhere in Swing itself and not directly from your code. but it's worth checking if you haven't already.)

There might be a better approach, because absolute positioning is a bit hacky anyway! Could you add the buttons to your "outermost" JFrame layout instead, using NORTH and SOUTH?

I'll use the same thing that I just knocked together above.. there's really just one really minor "tweak" in the class which creates the JFrame.

MyPanel.java (Now sans buttons!)


import javax.swing.*;
import java.awt.*;

public class MyPanel extends JPanel {

@Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}

@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
int width = getWidth() - 100;
int height = getHeight() - 100;
int x = (getWidth() - width) / 2;
int y = (getHeight() - height) / 2;
g2d.setColor(Color.RED);
g2d.drawRect(x, y, width, height);
g2d.dispose();
}
}




MyButtonPanel.java (All new panel with buttons)


import javax.swing.*;
import java.awt.*;

public class MyButtonPanel extends JPanel {
// Buttons
JButton btnHello = new JButton("Hello";
JButton btnWorld = new JButton("World";

public MyButtonPanel() {
this.setLayout(new FlowLayout());
this.add(btnHello);
this.add(btnWorld);
}
}



Test.java (updated to use the NORTH/SOUTH panes in its default layout manager)


import javax.swing.*;
import java.awt.*;

public class Test {
private static void createAndShowGUI() {
JFrame frame = new JFrame("HelloWorldSwing";
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

MyPanel myPanel = new MyPanel();
frame.add(myPanel, BorderLayout.NORTH);

MyButtonPanel myButtonPanel = new MyButtonPanel();
frame.add(myButtonPanel);

frame.pack();
frame.setVisible(true);
}

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



I'd still be really curious as to why you're getting the Null Pointer problem, but sometimes it's best not to waste the time and try something different.

@winterscoming Scratch that I still get the null pointer error as soon as I add a button
It highlights the line of code 'super.paintComponent(g)'
and it simply says:
'null (in javax.swing.jComponent)'

I very much want to tear my hair out about now.

Argh! Are you calling super.paintComponent(g); anywhere else aside from the first line within your MyPanel.paintComponent method body?

I'm guessing you've got places in your code which redraw those lines when your Dijkstra graph updates. But if you're calling super.paintComponent(g) to redraw your lines, then that could be the reason. Instead, could you try using this.repaint()?

I'm actually not, that's the only place that line is written, repaint however wasn't working when I was programming so I resorted to cheating and calling the paint methods again from within the algorithm and using getGraphics to make them work. This could be causing the issue I guess but I don't know how..

Right, that makes a lot of sense - I expect the problem you're having is that the algorithm is using an invalid Graphics instance - I seem to end up getting a lot of problems whenever I try to call 'paintComponent' directly. It's strange that 'repaint' wasn't working though. I always find the whole paint/repaint thing in Swing is a bit weird to be honest.

Do you think it'd be easy to add the buttons to a separate JPanel like the second example, and put those into the SOUTH part of the JFrame layout? I can't think of any reason why that would need to be a big change anyway

Of course, doing that you'd probably need the Button JPanel to have a reference to the main JPanel, but you could connect them together with a 'set' method:

// Test.Java - createAndShowGUI method:





MyPanel myPanel = new MyPanel();
frame.add(myPanel, BorderLayout.NORTH);

MyButtonPanel myButtonPanel = new MyButtonPanel();
frame.add(myButtonPanel);

myButtonPanel.setMainPanel(myPanel);





It sounds like you could end up digging endlessly deeper down into a rabbit hole with the null pointer and repaint problem,

I've also tried that, I tried just adding a panel to the south purely for buttons and again that gave me a null pointer error. At this point I think I have to give up since because this is my school coursework project I only have like 2 weeks to get the whole thing done.

I considered using a key press as a replacement for a button, since all I want the button to do is make the program move on to the next step through the algorithm. But a key press would involve writing a key listener which is something I've never done before and I'm not sure I can get my head around it, and especially how I'd implement it in to my already really long and complex program.

I'm already definitely way in to that rabbit hole of errors.