图形在Java中不起作用

Question

I am trying to mess around with some graphics in java, however i can't get it to work. The JFrame comes up with the button i created, but the JFrame is just gray with no red line that i want it to draw.

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;


public class Shapes extends JFrame implements ActionListener{

JButton button = new JButton("click");
public Shapes() {
    setVisible(true);
    setSize(500, 500);
    button.addActionListener(this);
    button.setSize(20, 20);
    setLayout(new FlowLayout());
    add(button);
    repaint();
}
public static void main(String[] args){
    Shapes s = new Shapes();   
}


public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    g2.setColor(Color.red);
    g2.drawLine(5, 10, 10, 20);
}

@Override
public void actionPerformed(ActionEvent e) {
    if (e.getSource() == button){
        repaint();
    }
 }
}

Answer 1

Two things:

1). You don't really want to do custom painting on a top-level container such as a JFrame . Instead you want to use a JPanel

class Panel extends JPanel
{
    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setColor(Color.red);
        g2.drawLine(5, 10, 10, 20);
    }
}

And add it to your JFrame : add(new Panel()); (Or create an object if you want).

2). setVisible(true); should be the very last thing that you do while setting up your window. So change your constructor:

public Shapes() {
setSize(500, 500);
button.addActionListener(this);
button.setSize(20, 20);
setLayout(new FlowLayout());
add(button);
add(new Panel()) // added from part 1
repaint();
setVisible(true);
}

For more information go through the "performing custom painting tutorials."