在Java中使用Keylistener用箭头键在GUI中绘制线条

Question

I am working on a keylistener exercise for my java class, but have been stuck for the past week. I appreciate any helpful suggestions. The exercise is:

"Write a program that draws line segments using the arrow keys. The line starts from the center of the frame and draws toward east, north, west, or south when the right-arrow key, up-arrow key, left-arrow key, or down-arrow key is clicked."

Through debugging I figured out that the KeyListener works to the point of getting to drawComponent(Graphics g), but it only draws when I press down or right and that only works the first couple times. Here is my code:

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

@SuppressWarnings("serial")
public class EventProgrammingExercise8 extends JFrame {

    JPanel contentPane;
    LinePanel lines;
    public static final int SIZE_OF_FRAME = 500;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    EventProgrammingExercise8 frame = new EventProgrammingExercise8();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public EventProgrammingExercise8() {
        setTitle("EventExercise8");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(SIZE_OF_FRAME, SIZE_OF_FRAME);
        contentPane = new JPanel();
        lines = new LinePanel();
        contentPane.add(lines);
        setContentPane(contentPane);
        contentPane.setOpaque(true);
        lines.setOpaque(true);

        lines.setFocusable(true);
        lines.addKeyListener(new ArrowListener());
    }

    private class LinePanel extends JPanel {

        private int x;
        private int y;
        private int x2;
        private int y2;

        public LinePanel() {
            x = getWidth() / 2;
            y = getHeight() / 2;
            x2 = x;
            y2 = y;
        }

        protected void paintComponent(Graphics g) {
            g.drawLine(x, y, x2, y2);
            x = x2;
            y = y2;
        }

        public void drawEast() {
            x2 += 5;
            repaint();
        }

        public void drawWest() {
            x2 -= 5;
            repaint();
        }

        public void drawNorth() {
            y2 -= 5;
            repaint();
        }

        public void drawSouth() {
            y2 += 5;
            repaint();
        }

    }

    private class ArrowListener extends KeyAdapter {

        public void keyPressed(KeyEvent e) {
            int key = e.getKeyCode();
            if (key == KeyEvent.VK_RIGHT) {
                lines.drawEast();
            } else if (key == KeyEvent.VK_LEFT) {
                lines.drawWest();
            } else if (key == KeyEvent.VK_UP) {
                lines.drawNorth();
            } else {
                lines.drawSouth();
            }
        }

    }

}

Thanks.

Answer 1

A few things jump out at me...

public LinePanel() {
    x = getWidth() / 2;
    y = getHeight() / 2;

This will be an issue, because at the time you construct the class, it's size is 0x0

Apart from the fact that you haven't called super.paintComponent which breaks the paint chain, you seem to think that painting is accumaltive...

protected void paintComponent(Graphics g) {
    g.drawLine(x, y, x2, y2);
    x = x2;
    y = y2;
}

Painting in Swing is destructive. That is, you are expected to erase to the Graphics context and rebuild the output from scratch. The job paintComponent is to clear the Graphics context ready for painting, but you've not called super.paintComponent , breaking the paint chain and opening yourself up to a number of very ugly paint artifacts

Calling setSize(SIZE_OF_FRAME, SIZE_OF_FRAME); on a frame is dangerous, as it makes no guarantee about the frames border insets, which will reduce the viewable area available to you.

This....

contentPane = new JPanel(); 
lines = new LinePanel();
contentPane.add(lines);
setContentPane(contentPane);

Is not required, it just adds clutter to your code. It's also a good hint as to what is going wrong with your code.

JPanel uses a FlowLayout by default. A FlowLayout uses the component's preferred size to determine how best to layout the components. The default preferred size of a component is 0x0

You could use...

lines = new LinePanel();
add(lines);

instead or set the contentPane to use a BorderLayout which will help...

Try adding lines.setBorder(new LineBorder(Color.RED)); add see what you get...

Oddly, during my testing, your KeyListener worked fine...

Basically...

• Override the getPreferredSize method of the LinePanel and return the size of the panel you would like to use.
• Use a java.util.List to maintain a list of Point s that need to be painted.
• In you paintComponent method, use the Point List to actually render you lines. This will be a bit tricky, as you need two points and the List may contain an odd number of points, but's doable.
• Calculate the start Point either by using the preferred size or some other means (like using a ComponentListener and monitoring the componentResized method. This becomes tricky as your component may be resized a number of times when it is first created and released to the screen and you will want to ignore future events once you have your first point)