Question

java中drawLine无法画线，求解。。。

以下是书上的例子，结果如图，而书本上的结果应该是窗口左上到Banner有条线的，求解释啊，谢谢
import javax.swing.*;
import java.awt.Graphics;
public class TestGetGraphics extends JFrame{
private JLabel jlblBanner = new JLabel("Banner");
TestGetGraphics(){
add(jlblBanner);
System.out.println(jlblBanner.getGraphics());
}

public static void main(String[] args){
TestGetGraphics frame = new TestGetGraphics();
frame.setTitle("TestGetGraphics");
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200,100);
frame.setVisible(true);
//JOptionPane.showMessageDialog(null,"Delay on purpose\nClick OK to dismiss the dialog");
Graphics graphics = frame.jlblBanner.getGraphics();
graphics.drawLine(0,0,50,50);
}
}

Answer 1

1. 把JLabel替换为Label对象吧、如果不设置文字、就会划出直线了
2. 重写JLabel的paint方法，在paint方法里面划线就可以了
import javax.swing.*;

import java.awt.Color;
import java.awt.Graphics;

public class TestGetGraphics extends JFrame {

private JLabel jlblBanner = new JLabel() {

@Override
public void paint(Graphics g) {
super.paint(g);

g.setColor(Color.red);
g.drawLine(0, 0, 50, 50);
}

};

TestGetGraphics() {

jlblBanner.setBackground(null);
jlblBanner.setForeground(null);
jlblBanner.setOpaque(true);

add(jlblBanner);

}

public static void main(String[] args) {
TestGetGraphics frame = new TestGetGraphics();
frame.setTitle("TestGetGraphics");
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200, 100);
frame.setVisible(true);
}

}

Answer 2

是你的画布问题吧