java - 面板为什么不绘制(paint)？

Question

代码

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

class tester {
   public static void main(String args[]) {
     JFrame fr = new JFrame();
     JPanel p = new JPanel();
     p.setBackground(Color.RED);
     p.paintImmediately(20,20,500,500);  
     fr.add(p);
     fr.setVisible(true);
     fr.setSize(2000,2000);
  }
}

我得到一个完全涂成红色的面板。为什么我不接电话？我怎样才能得到它？

Answer 1

I get a panel painted completely of red color.

那是因为你设置了背景，没有做任何进一步的绘画......

Why dont i get the line ? How can i get it?

这不是做这件事的方法。为什么调用 paintImmediately？以下是文档中的内容:

Paints the specified region in this component and all of its descendants that overlap the region, immediately.

It's rarely necessary to call this method. In most cases it's more efficient to call repaint, which defers the actual painting and can collapse redundant requests into a single paint call. This method is useful if one needs to update the display while the current event is being dispatched.

我建议您继续阅读 AWT/Swing 中的绘画。

• Lesson: Performing Custom Painting

---

得到这样的东西

您可以这样更改您的代码:

JFrame fr = new JFrame();
JPanel p = new JPanel() {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(20, 20, 500, 500);
    }
};
p.setBackground(Color.RED);
fr.add(p);
fr.setVisible(true);
fr.setSize(200, 200);