Java GUI PaintComponent（新手）

Question

Thanks for the help. The objects are all showing now.

But I ran into a new problem. Im trying to use a For loop to draw 10 copys of the same box with a little space in between so they don't just stack in the same position.

But for some reason they keep getting painted on top of eachother and in the center instead of starting at x = 20...

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

public class CarWashPanel extends JPanel {

    public int i;
    public int x = 20;

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.black);
        for (i=0; i < 10; i++){
        g.fillRoundRect(x, 10, 50, 100, 55, 25);
        x = x + 10;
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(500, 150);
    }

}

//

I'm trying to add graphics in my CarWashPanel class to I want to add to my GUI. I've read some tutorials and other questions but I can't figure out what i'm doing wrong.

The buttons and label that i've added to the GUI show up just fine but when I add something to my CarWashPanel it doesn't show up in the GUI.

I feel like I need to tell my GUI to add all elements from the CarWashPanel but I'm not sure how.

        public class Main {

            public static void main(String[] args) {
                GUI g = new GUI();        

            }

        }

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

        public class GUI extends JFrame {

            private JTextField t1 = new JTextField(2);
            private JLabel l1 = new JLabel("enter position");
            private JButton b1 = new JButton("new customer");
            private JButton b2 = new JButton("wash car");

            public GUI() {
                setDefaultCloseOperation (
                JFrame.EXIT_ON_CLOSE );
                add(l1);
                add(t1);
                add(b1);
                add(b2);        
                setTitle("Carwash");
                setSize(500, 200);
                setVisible(true);
                setLayout(new FlowLayout());
                add(new CarWashPanel());
            }

        }


public class Carwash {

    private boolean[] positions = new boolean[10];
    private int washing = 10;

    public void addCar(int p) {        
        positions[p] = true;
    }

    public void removeCar(int p) {        
        positions[p] = false;
    }

    public boolean[] getPositions() {
        return positions;
    }

    public int getWashing() {
        return washing;
    }

}


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

        public class CarWashPanel extends JPanel {

            public CarWashPanel(){

            }

        public void paintComponent(Graphics g) {
                super.paintComponent(g);    
               g.setColor(Color.black);       
               g.fillRoundRect(150, 50, 100, 100, 50, 25);
           }

            }

Answer 1

If you don't need to add things dynamically the best thing to do is call setVisible(true) after you've added all your components.

However, if you want to add things after the frame is visible you can do so and then call the frame's revalidate() method to cause it to redraw.

Secondly I'd recommend you set the layout before you add any components.

Answer 2

It's very often issue. You are calling setVisible before you are adding your components. Add your components on CarWashPanel , add CarWashPanel on JFrame and then call setVisible . Also, remove this line: setLayout(new FlowLayout()); - FlowLayout is default layout for JPanel (CarWashPanel in your case) and this makes it sufficient.

Your code should look something like this:

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

public class Main {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(GUI::new);

    }

    public static class GUI extends JFrame {
        private JTextField t1 = new JTextField(2);
        private JLabel l1 = new JLabel("enter position");
        private JButton b1 = new JButton("new customer");
        private JButton b2 = new JButton("wash car");

        CarWashPanel carWashPanel = new CarWashPanel();

        public GUI() {
            setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            carWashPanel.add(l1);
            carWashPanel.add(t1);
            carWashPanel.add(b1);
            carWashPanel.add(b2);
            add(carWashPanel,BorderLayout.CENTER);
            setTitle("Carwash");
            pack();
            setVisible(true);

        }

    }


    public class Carwash {

        private boolean[] positions = new boolean[10];
        private int washing = 10;

        public void addCar(int p) {
            positions[p] = true;
        }

        public void removeCar(int p) {
            positions[p] = false;
        }

        public boolean[] getPositions() {
            return positions;
        }

        public int getWashing() {
            return washing;
        }

    }

    public static class CarWashPanel extends JPanel {


        public CarWashPanel() {

        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.black);
            g.fillRoundRect(150, 50, 100, 100, 50, 25);
        }


        @Override
        public Dimension getPreferredSize() {
            return new Dimension(500, 200);
        }
    }
}

Other sidenotes:

Don't call setSize for JFrame , call pack . Rather override getPreferredSize for JPanel and return some dimensions.

Avoid extending your class with JFrame unless you want to define new methods or override existing ones.