错误：在类mainGUI中找不到主要方法，请将该主要方法定义为：public static void main（String [] args）

Question

I am creating a GUI in java and am getting the following error:

Error: Main method not found in class mainGUI, please define the main method as:
public static void main(String[] args)

I cant work out why this is happening, considering that i do have a main method in my code and it does contain some code. My GUI code is as follows:

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.SwingConstants;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;


public class mainGUI extends JFrame {

/**
 * 
 */
private static final long serialVersionUID = 1L;

private static final int WIDTH = 400;
private static final int HEIGHT = 300;

private JLabel lengthL, widthL, areaL, perimeterL;
private JTextField lengthTF, widthTF, areaTF, perimeterTF;
private JButton calculateB, exitB;

private CalculateButtonHandler cbHandler;
private ExitButtonHandler ebHandler;

public mainGUI() {

    lengthL = new JLabel("Enter the length: ", SwingConstants.RIGHT);
    widthL = new JLabel("Enter the width: ", SwingConstants.RIGHT);
    areaL = new JLabel("Area: ", SwingConstants.RIGHT);
    perimeterL = new JLabel("Perimeter: ", SwingConstants.RIGHT);

    lengthTF = new JTextField(10);
    widthTF = new JTextField(10);
    areaTF = new JTextField(10);
    calculateB = new JButton("Calculate");
    cbHandler = new CalculateButtonHandler();
    calculateB.addActionListener(cbHandler);
    exitB = new JButton("Exit");
    ebHandler = new ExitButtonHandler();
    exitB.addActionListener(ebHandler);

    lengthTF = new JTextField(10);
    widthTF = new JTextField(10);
    areaTF = new JTextField(10);
    perimeterTF = new JTextField(10);

    calculateB = new JButton("Calculate");
    exitB = new JButton("Exit");
    Container pane = getContentPane();
    pane.setLayout(new GridLayout(5, 2));

    pane.add(lengthL);
    pane.add(lengthTF);
    pane.add(widthL);
    pane.add(widthTF);
    pane.add(areaL);
    pane.add(areaTF);
    pane.add(calculateB);
    pane.add(exitB);

    setTitle("Main Menu");
    setSize(WIDTH, HEIGHT);
    setVisible(true);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
}

private class CalculateButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            double width, length, area;
            length = Double.parseDouble(lengthTF.getText()); //We use the     getText & setText methods to manipulate the data entered into those fields.
            width = Double.parseDouble(widthTF.getText());
            area = length * width;           
            areaTF.setText("" + area);
        }
    }

public class ExitButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            System.exit(0);
        }
}

public static void main(String[] args) {
        mainGUI rectObj = new mainGUI();
}     
}

Could anyone shed some light on as to why this is happening?

Thanks very much :)

Answer 1

Your code works fine, try running it from command line, or try reinstalling eclipse, because its not a code error, its an environmental error.

If that doesn't work, try recreating it, starting from main, then walk your way up, it should pinpoint where eclipse is acting up.

good luck!

Answer 2

You know, I came here looking for an answer to a similar problem. Turns out an older incorrect version of a java file with the same name was in my Java home directory. The correct file was saved in some other dir. Perhaps you are experiencing a similar problem.

Edit: I see this post is several years old. OP is probably a CIO by now. Leaving comment for those of you who go saving files to odd places.