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();
    }

}

drawpanel

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，每次按下按钮时都会画一个随机的彩色圆圈。

但目前，绘图板始终保持灰色，如此http://i.imgur.com/Kz84rKR.png

有什么明显的东西阻止代码像预期的那样工作吗？

现在圆圈总是黑色，永远不会变成随机颜色。

Answer 1

你拼写paintComponent错了。此方法始终位于@Override

之前

所以改变这个：

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

到此：

@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());

修改
您需要在括号中进行双重数学运算，因此正确的值将转换为int，因为：

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

等同于此

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

等于：

red = 0 * 256;

相反，请执行此操作：

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

但更重要的是你需要学习调试。使用printlns打印出您的颜色值，或使用调试器尝试隔离您的错误。在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没有绘制在drawpanel上？

1 个答案: