按下按钮时背景消失

Question

I have set up a game in Java using Swing UI.

Expected

When firing a projectile across the screen and I want the background to be constantly there

Issue

When I run the code the background appears to be fine. But when the button to fire the projectile is pressed then the background disappears.

Code

public class ProjectileShooterTest {

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

    private static void createAndShowGUI() {
        JFrame f = new JFrame() {
        };

        ImageIcon background=new ImageIcon("Background.png");
        Image img=background.getImage();
        Image temp=img.getScaledInstance(800,440,Image.SCALE_SMOOTH);
        background=new ImageIcon(temp);

        JLabel back=new JLabel(background);
        back.setBounds(0,0,800,500);

        f.getContentPane().setLayout(new BorderLayout());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(800,500);

        final ProjectileShooter projectileShooter = new ProjectileShooter();
        ProjectileShooterPanel projectileShooterPanel = new ProjectileShooterPanel(projectileShooter);
        projectileShooter.setPaintingComponent(projectileShooterPanel);

        JPanel controlPanel = new JPanel(new GridLayout(1,0));
        controlPanel.add(new JLabel("     Post-Launch Angle"));

        final JSlider angleSlider = new JSlider(70, 89, 85);
        angleSlider.setLayout( new FlowLayout() );

        controlPanel.add(angleSlider);
        f.add(back);

        controlPanel.add(new JLabel("                         Thrust"));
        final JSlider powerSlider = new JSlider(50, 80, 60);
        powerSlider.setLayout( new FlowLayout() );
        controlPanel.add(powerSlider);


        JButton shootButton = new JButton("Launch");
        shootButton.setLayout( new FlowLayout() );
        shootButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                int angleDeg = angleSlider.getValue();
                int power = powerSlider.getValue();
                projectileShooter.setAngle(Math.toRadians(angleDeg));
                projectileShooter.setPower(power);
                projectileShooter.shoot();
            }
        });
        f.add(back);
        controlPanel.add(shootButton);

        f.getContentPane().add(controlPanel, BorderLayout.NORTH);
        f.getContentPane().add(projectileShooterPanel, BorderLayout.CENTER);
        f.setVisible(true);
        f.setLayout(new BorderLayout());
    }
}

Question

How can I solve this?

Answer 1

f.add(back);

is equivalent to:

f.getContentPane().add(back, BorderLayout.CENTER);

Later in your code you do:

f.getContentPane().add(projectileShooterPanel, BorderLayout.CENTER);

Which will cause problems because you can't add two components to the CENTER.

A Swing GUI is parent/child design so you need something like:

f.getContentPane().add(back, BorderLayout.CENTER);
back.setLayout(new BorderLayout());
back.add(projectileShooterPanel, BorderLayout.CENTER);

I'll let you figure out the logic order and placement of the above statements.

Also why do you have:

f.setLayout(new BorderLayout());

at the end of the constructor? This replaces all the constraint information of your original layout.

Finally, your projectile shooter panel needs to be transparent, otherwise it will paint over top of the background. So you also need:

projectileShooterPanel.setOpaque( false );

Note: instead of using the JLabel as the background it would be easier to paint the background in your projectile shooter panel. Then you won't have issues with trying to add multiple panels to one another and the shooter panel won't need to be transparent.

Answer 2

While I cannot tell what projectileShooter.shoot() does, my gut feeling is that you are hogging the event dispatching thread. As long as your button's ActionListener is busy your GUI is unresponsive. Start a new Thread to perform the shooting itself.

So your ActionListener could look like this:

    shootButton.addActionListener(new ActionListener()
    {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            new Thread(new Runnable()
            {
                public void run() {
                    int angleDeg = angleSlider.getValue();
                    int power = powerSlider.getValue();
                    projectileShooter.setAngle(Math.toRadians(angleDeg));
                    projectileShooter.setPower(power);
                    projectileShooter.shoot();
                }
            }).start();
        }
    });