Java Swing未绘制paintComponent

Question

Im trying to draw a simple rectangle on a frame with some scrollbars and textfields(just testing) but the paintComponent its not showing, ive seen some similar cases here but i cant manage to make it work, any help please?

package appletdeslizadores;

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

public class frame extends JPanel {

    JFrame f1;
    JPanel p1, p2;
    JLabel lbl1, lbl2, lbl3;
    JTextField txtfld1, txtfld2, txtfld3;
    JScrollBar sbar1, sbar2, sbar3;       


    public frame() {

        f1 = new JFrame("Applet ScrollBars");
        f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f1.setSize(380, 350);
        f1.setLayout(new FlowLayout());  
        p1 = new JPanel(new GridLayout(3,3,10,10));
        lbl1 = new JLabel("Scroll Bar 1");
        lbl2 = new JLabel("Scroll Bar 2");
        lbl3 = new JLabel("Scroll Bar 3");
        sbar1 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        sbar2 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        sbar3 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        txtfld1 = new JTextField(3);
        txtfld1.setText(String.valueOf(sbar1.getValue()));
        txtfld1.setEditable(false);
        txtfld2 = new JTextField(3);
        txtfld2.setText(String.valueOf(sbar2.getValue()));
        txtfld2.setEditable(false);
        txtfld3 = new JTextField(3);
        txtfld3.setText(String.valueOf(sbar3.getValue()));
        txtfld3.setEditable(false);
        p1.add(lbl1);
        p1.add(lbl2);
        p1.add(lbl3);
        p1.add(sbar1);
        p1.add(sbar2);
        p1.add(sbar3);
        p1.add(txtfld1);
        p1.add(txtfld2);
        p1.add(txtfld3);
        f1.add(p1);
        f1.setVisible(true);

    }


    public void paintComponent(Graphics2D g) {

        g.drawRect(50,50,70,100);
        g.setColor(Color.red);        

    }

    public static void main(String[] args) {

        new frame();


    }

}

Answer 1

Problem

You're not sticking to conventions. This is causing problems small mistakes. Your frame class is actually a JPanel , not a JFrame .

There are two main problems: You never added the panel to the frame, and the paintComponent() method has a Graphics object, not a Graphics2D object as a parameter.

The changes to your code are at the bottom of this answer.

Solution

1. Stick to conventions. (You should also rename your class to a more appropriate name, but that is your choice.) Add the @Override annotation to your paintComponent() method since you wish to override this method from the original JPanel . If it crashes because of the annotation, it means you are not overriding correctly.
2. Change the paintComponent() parameter from Graphics2D to Graphics .
3. Add the JPanel to the JFrame .
4. Make sure you call setPreferredSize() on your JPanel and specify a size.
5. Call pack() on the JFrame right before making it visible so the layout manager can place everything accordingly.

Now I'm sure by the end of all this you are still not going to be happy with what you see because the code still needs some work, but at least this should give you a boost into the right direction. Also, you may want to call setColor() before drawing the rectangle. ;)

Hope this helped.

Code

public class frame extends JPanel {

    JFrame f1;
    JPanel p1, p2;
    JLabel lbl1, lbl2, lbl3;
    JTextField txtfld1, txtfld2, txtfld3;
    JScrollBar sbar1, sbar2, sbar3;       


    public frame() {

        f1 = new JFrame("Applet ScrollBars");
        f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f1.setSize(380, 350);
        f1.setLayout(new FlowLayout());  
        p1 = new JPanel(new GridLayout(3,3,10,10));
        lbl1 = new JLabel("Scroll Bar 1");
        lbl2 = new JLabel("Scroll Bar 2");
        lbl3 = new JLabel("Scroll Bar 3");
        sbar1 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        sbar2 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        sbar3 = new JScrollBar(JScrollBar.HORIZONTAL, 0, 10, 0, 255);
        txtfld1 = new JTextField(3);
        txtfld1.setText(String.valueOf(sbar1.getValue()));
        txtfld1.setEditable(false);
        txtfld2 = new JTextField(3);
        txtfld2.setText(String.valueOf(sbar2.getValue()));
        txtfld2.setEditable(false);
        txtfld3 = new JTextField(3);
        txtfld3.setText(String.valueOf(sbar3.getValue()));
        txtfld3.setEditable(false);
        p1.add(lbl1);
        p1.add(lbl2);
        p1.add(lbl3);
        p1.add(sbar1);
        p1.add(sbar2);
        p1.add(sbar3);
        p1.add(txtfld1);
        p1.add(txtfld2);
        p1.add(txtfld3);
        f1.add(p1);
        setPreferredSize(new Dimension(512, 512));
        f1.add(this);
        f1.pack();
        f1.setVisible(true);


    }

    @Override
    public void paintComponent(Graphics g) {

        g.drawRect(50,50,70,100);
        g.setColor(Color.red);        

    }

    public static void main(String[] args) {

        new frame();


    }

}