Having trouble with an error Error: Main method not found in class GUITest

Question

我的 GuiTest class 有问题。这里是报错，“错误：在class GUITest中找不到主要方法，请将主要方法定义为： public static void main(String[] args) 或 JavaFX 应用程序 class 必须扩展 javafx.application.Application”。我该如何解决这个问题？

    import java.awt.BorderLayout;
    import java.awt.Color;
    import java.awt.FlowLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;

    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JLabel;
    import javax.swing.JPanel;
    //Here i modified this class to add button and implement its action        listener.
   public class GUITest {

static class buttonlistener implements ActionListener
{
JFrame l1 ;
buttonlistener(JFrame l1)
{
this.l1 = l1;


public static void main(String[] args){
    final JFrame f=new JFrame();
    f.setLayout(new BorderLayout());
    f.add(new PatternPanel());
    f.setSize(500,500);
    f.setVisible(true);
    f.setBackground(Color.CYAN);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JPanel buttonPanel = new JPanel();
    buttonPanel.setLayout(new FlowLayout(FlowLayout.RIGHT));
    JButton b1 = new JButton("change color");
    b1.addActionListener(new ActionListener(){

        public void actionPerformed(ActionEvent ae)
        {
            f.setLayout(new BorderLayout());
            f.add(new CircleTile());
            f.setSize(500,500);
            f.setVisible(true);
            f.setBackground(Color.CYAN);
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            repaint();
        }
        });
          ;
          buttonPanel.add(b1);
          f.add(buttonPanel,BorderLayout.SOUTH);


    }

}

protected void repaint() {
    // TODO Auto-generated method stub

}

@Override
public void actionPerformed(ActionEvent e) {
    // TODO Auto-generated method stub

        }   
     }
    }




    import java.awt.Graphics;
``  import java.awt.Color;
    import javax.swing.*;
    import java.awt.Graphics2D;

   public class PatternPanel extends JPanel {
    int i,j,x,y;
    @Override
    public void paint(Graphics g){
    Graphics2D g1=(Graphics2D)g;        
    g1.setColor(Color.GREEN);       
    g1.fillRect(0,0,100,100);
    g1.drawRect(0,0,100,100);
    g1.setColor(Color.BLACK);
    g1.drawOval(0,0,100,100);
    g1.setColor(Color.YELLOW);
    g1.fillOval(0,0,100,100);
    g1.setColor(Color.BLACK);
    g1.drawOval(10,10,80,80);   
    g1.drawOval(20,20,60,60);       
    g1.drawOval(30,30,40,40);       
    g1.drawOval(40,40,20,20);
    g1.drawLine(50,0,50,100);
    g1.drawLine(0,50,100,50);
    for(i=0;i<3;i++){           
        x=0 + 50*i;
        g1.drawLine(x,0,x,0+100);           
    }
    for(i=0;i<3;i++){           
        y=0 + 50*i;
        g1.drawLine(0,y,0+100,y);           
    }

    //2nd Block
    g1.setColor(Color.GREEN);       
    g1.fillRect(10,120,120,120);
    g1.drawRect(10,120,120,120);                
    for(i=0;i<5;i++){
        y=120 + 24*i;
        for(j=0;j<5;j++){
            x=10 + 24*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,24,24);
            g1.setColor(Color.YELLOW);
            g1.fillOval(x,y,24,24);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 3,y + 3,18,18);
            g1.drawOval(x + 6,y + 6,12,12);
            g1.drawOval(x + 9,y + 9,6,6);               
        }                           
    }
    for(i=0;i<11;i++){          
        x=10 + 12*i;
        g1.drawLine(x,120,x,120+120);           
    }
    for(i=0;i<11;i++){          
        y=120 + 12*i;
        g1.drawLine(10,y,10+120,y);         
    }       

    //3rd Block
    g1.setColor(Color.GREEN);       
    g1.fillRect(150,20,240,240);
    g1.drawRect(150,20,240,240);                
    for(i=0;i<6;i++){
        y=20 + 40*i;
        for(j=0;j<6;j++){
            x=150 + 40*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,40,40);
            g1.setColor(Color.YELLOW);
            g1.fillOval(x,y,40,40);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 4,y + 4,32,32);
            g1.drawOval(x + 8,y + 8,24,24);
            g1.drawOval(x + 12,y + 12,16,16);
            g1.drawOval(x + 16,y + 16,8,8);             
        }               
    }
    for(i=0;i<13;i++){          
        x=150 + 20*i;
        g1.drawLine(x,20,x,20+240);         
    }
    for(i=0;i<13;i++){          
        y=20 + 20*i;
        g1.drawLine(150,y,150+240,y);           
    }

    //4th Block
    g1.setColor(Color.GREEN);       
    g1.fillRect(130,275,108,108);
    g1.drawRect(130,275,108,108);               
    for(i=0;i<3;i++){
        y=275 + 36*i;
        for(j=0;j<3;j++){
            x=130 + 36*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,36,36);
            g1.setColor(Color.YELLOW);
            g1.fillOval(x,y,36,36);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 6,y + 6,24,24);
            g1.drawOval(x + 12,y + 12,12,12);                               
        }               
    }
    for(i=0;i<7;i++){           
        x=130 + 18*i;
        g1.drawLine(x,275,x,275+108);           
    }
    for(i=0;i<7;i++){           
        y=275 + 18*i;
        g1.drawLine(130,y,130+108,y);           
    }       
}

}

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

import javax.swing.*;

import java.awt.Graphics2D;
public class CircleTile extends JPanel {
int i,j,x,y;
@Override
public void paint(Graphics g){
    Graphics2D g1=(Graphics2D)g;        
    g1.setColor(Color.GREEN);       
    g1.fillRect(0,0,100,100);
    g1.drawRect(0,0,100,100);
    g1.setColor(Color.BLACK);
    g1.drawOval(0,0,100,100);
    g1.setColor(Color.YELLOW);
    g1.fillOval(0,0,100,100);
    g1.setColor(Color.BLACK);
    g1.drawOval(10,10,80,80);   
    g1.drawOval(20,20,60,60);       
    g1.drawOval(30,30,40,40);       
    g1.drawOval(40,40,20,20);
    g1.drawLine(50,0,50,100);
    g1.drawLine(0,50,100,50);
    for(i=0;i<3;i++){           
        x=0 + 50*i;
        g1.drawLine(x,0,x,0+100);           
    }
    for(i=0;i<3;i++){           
        y=0 + 50*i;
        g1.drawLine(0,y,0+100,y);           
    }

    //2nd Block
    g1.setColor(Color.GRAY);        
    g1.fillRect(10,120,120,120);
    g1.drawRect(10,120,120,120);                
    for(i=0;i<5;i++){
        y=120 + 24*i;
        for(j=0;j<5;j++){
            x=10 + 24*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,24,24);
            g1.setColor(Color.YELLOW);
            g1.fillOval(x,y,24,24);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 3,y + 3,18,18);
            g1.drawOval(x + 6,y + 6,12,12);
            g1.drawOval(x + 9,y + 9,6,6);               
        }                           
    }
    for(i=0;i<11;i++){          
        x=10 + 12*i;
        g1.drawLine(x,120,x,120+120);           
    }
    for(i=0;i<11;i++){          
        y=120 + 12*i;
        g1.drawLine(10,y,10+120,y);         
    }       

    //3rd Block
    g1.setColor(Color.BLUE);        
    g1.fillRect(150,20,240,240);
    g1.drawRect(150,20,240,240);                
    for(i=0;i<6;i++){
        y=20 + 40*i;
        for(j=0;j<6;j++){
            x=150 + 40*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,40,40);
            g1.setColor(Color.RED);
            g1.fillOval(x,y,40,40);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 4,y + 4,32,32);
            g1.drawOval(x + 8,y + 8,24,24);
            g1.drawOval(x + 12,y + 12,16,16);
            g1.drawOval(x + 16,y + 16,8,8);             
        }               
    }
    for(i=0;i<13;i++){          
        x=150 + 20*i;
        g1.drawLine(x,20,x,20+240);         
    }
    for(i=0;i<13;i++){          
        y=20 + 20*i;
        g1.drawLine(150,y,150+240,y);           
    }

    //4th Block
    g1.setColor(Color.GRAY);        
    g1.fillRect(130,275,108,108);
    g1.drawRect(130,275,108,108);               
    for(i=0;i<3;i++){
        y=275 + 36*i;
        for(j=0;j<3;j++){
            x=130 + 36*j;
            g1.setColor(Color.BLACK);
            g1.drawOval(x,y,36,36);
            g1.setColor(Color.PINK);
            g1.fillOval(x,y,36,36);
            g1.setColor(Color.BLACK);
            g1.drawOval(x + 6,y + 6,24,24);
            g1.drawOval(x + 12,y + 12,12,12);                               
        }               
    }
    for(i=0;i<7;i++){           
        x=130 + 18*i;
        g1.drawLine(x,275,x,275+108);           
    }
    for(i=0;i<7;i++){           
        y=275 + 18*i;
        g1.drawLine(130,y,130+108,y);           
    }   
}
}

Answer 1

您的主要方法隐藏在内部 class 的构造函数中 — 为什么？您所要做的就是修复错误告诉您要修复的内容：将其放在主外部 class、GUITest class.

永远不要忽略编译器错误消息，事实上，解决这个错误和大多数编译器错误的关键是尝试批判性地阅读错误消息，然后修复它告诉您要修复的内容。这样做，您将修复 90% 的此类错误。

您的代码的另一个问题：您的缩进到处都是，变化很大，正因为如此，您看不到自己做错了什么。如果你正确且有规律地缩进，你很快就会发现你的 main 方法是如何深深嵌入到错误中的 class。了解缩进和格式化规则并不是为了让代码看起来更漂亮（尽管它有助于做到这一点），而是为了帮助您更好地理解和调试代码，一目了然。

例如，如果您的代码格式正确，您会发现 main 方法显然深埋在缩进代码中：

public class GUITest {

    static class buttonlistener implements ActionListener {
        JFrame l1 ;
        buttonlistener(JFrame l1) {
            this.l1 = l1;

            // ***** here is your main method, where it does not belong
            public static void main(String[] args){
                final JFrame f=new JFrame();
                f.setLayout(new BorderLayout());
                f.add(new PatternPanel());
                f.setSize(500,500);
                f.setVisible(true);
                f.setBackground(Color.CYAN);
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                JPanel buttonPanel = new JPanel();
                buttonPanel.setLayout(new FlowLayout(FlowLayout.RIGHT));
                JButton b1 = new JButton("change color");
                b1.addActionListener(new ActionListener() {
                    public void actionPerformed(ActionEvent ae) {
                        f.setLayout(new BorderLayout());
                        f.add(new CircleTile());
                        f.setSize(500,500);
                        f.setVisible(true);
                        f.setBackground(Color.CYAN);
                        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                        repaint();
                    }
                });
                buttonPanel.add(b1);
                f.add(buttonPanel,BorderLayout.SOUTH);
            }
        }

        protected void repaint() {
            // TODO Auto-generated method stub
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
        }   
    }
}

所以修复它：

public class GUITest {

    // ***** So move it to where it now does  belong
    public static void main(String[] args){
        final JFrame f=new JFrame();
        f.setLayout(new BorderLayout());
        f.add(new PatternPanel());
        f.setSize(500,500);
        f.setVisible(true);
        f.setBackground(Color.CYAN);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.RIGHT));
        JButton b1 = new JButton("change color");
        b1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                f.setLayout(new BorderLayout());
                f.add(new CircleTile());
                f.setSize(500,500);
                f.setVisible(true);
                f.setBackground(Color.CYAN);
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                repaint();
            }
        });
        ;
        buttonPanel.add(b1);
        f.add(buttonPanel,BorderLayout.SOUTH);
    }    

    static class buttonlistener implements ActionListener {
        JFrame l1 ;
        buttonlistener(JFrame l1) {
            this.l1 = l1;
        }

        protected void repaint() {
            // TODO Auto-generated method stub
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
        }   
    }
}

Having trouble with an error Error: Main method not found in class GUITest

Having trouble with an error Error: Main method not found in class GUITest

java

swing

main

main-method