GUI 不在绘图面板上绘图？

Question

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class SimpleGui3C implements ActionListener{
    JFrame frame;

    public static void main (String[] args){
        SimpleGui3C gui = new SimpleGui3C();
        gui.go();
    }

    public void go() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton button = new JButton("Change colours");
        button.addActionListener(this);

        MyDrawPanel drawPanel = new MyDrawPanel();

        frame.getContentPane().add(BorderLayout.SOUTH, button);
        frame.getContentPane().add(BorderLayout.CENTER, drawPanel);
        frame.setSize(300,300);
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent event ){
        frame.repaint();
    }

}

画板

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

public class MyDrawPanel extends JPanel{
    protected void paintComponenet(Graphics g){
        g.fillRect(0,0,this.getWidth(),this.getHeight())


        Graphics2D g2d = (Graphics2D) g;

        int red = (int) Math.random() * 256;
        int green = (int) Math.random() * 256;
        int blue = (int) Math.random() * 256;
        Color startColour = new Color(red,green,blue);

        red = (int) Math.random() * 256;
        green = (int) Math.random() * 256;
        blue = (int) Math.random() * 256;
        Color endColour = new Color(red,green,blue);


        GradientPaint gradient = new GradientPaint(70,70,startColour,150,150,endColour);
        g2d.setPaint(gradient);
        g2d.fillOval(70,70,100,100);
    }


}

我正在尝试了解 GUI，我有一些代码可以在每次按下按钮时绘制一个随机颜色的圆圈。

但目前拉板总是像这样保持灰色http://i.imgur.com/Kz84rKR.png

是否有任何明显的因素阻止代码按预期运行？

现在圆圈总是黑色的，永远不会变成随机颜色。

Answer 1

你拼错了paintComponent。始终在此方法之前加上 @Override

所以改变这个：

public void paintComponenet(Graphics g) {
    g.fillRect(0, 0, this.getWidth(),this.getHeight());

对此：

@Override    // add so the compiler will warn you if you spell it wrong
protected void paintComponent(Graphics g) {  // should be protected, not public
    super.paintComponent(g); // so that you erase old images
    g.fillRect(0, 0, this.getWidth(),this.getHeight());

编辑
您需要在括号中进行双重数学运算，以便将正确的值转换为 int，因为：

red = (int) Math.random() * 256;

相当于这个

red = ((int) (Math.random())) * 256;

这等于：

red = 0 * 256;

所以，改为这样做：

red = (int) (Math.random() * 256);

但更重要的是你需要学会调试。使用 printlns 打印出您的颜色值或使用调试器来尝试隔离您的错误。来这里之前先做这件事。例如：

    red = (int) Math.random() * 256;
    green = (int) Math.random() * 256;
    blue = (int) Math.random() * 256;

    // this will show you what your values are
    // and key you in to where you should look:
    System.out.printf("Colors: [%d, %d, %d]%n", red, green, blue);

    Color endColour = new Color(red,green,blue);

GUI 不在绘图面板上绘图？

GUI not drawing on drawpanel?

java

swing

jpanel

paintcomponent