Can anyone help me with this error? I have included the code after the
explaination.
This code uses the rectangle class (which I just included in the code,
although I was supposed to import it). The code should take in values from
the user, calculate them using the methods stored in the rectangle class, and
display them in the GUI. I was able to get my code to compile, but when it
runs, I get "Exception in thread "main" java.lang.NoSuchMethodError: main. I
have been over and over the code. I can't see what I'm doing wrong. If you
can help in any way, I would really appreciate it. I think I included
everything I was supposed to, but I am a beginner and can't always tell if I
need more. Any help will be appreciated. thank you.

import java.text.*;
import java.io.*;
import java.lang.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Program81 {

    JFrame outputFrame;
    JLabel enterWidth;
    JLabel enterLength;
    JLabel calculateArea;
    JLabel calculatePerim;
    JTextField dataWidth;
    JTextField dataLength;
    JButton calculate;
    JButton clear;
    ButtonHandler button;

    private class ButtonHandler implements ActionListener {

            public void actionPerformed(ActionEvent event)  {

                rectangle r = new rectangle();

                r.setWidth(Integer.parseInt(dataWidth.getText()));
                r.setLength(Integer.parseInt(enterLength.getText()));

                calculateArea.setText("The area of your rectangle is " + r.calcArea());
                calculatePerim.setText("The perimeter of your rectangle is " + r.
calcPerimeter());
            }
    }//end of action handler

   public void main ( String args[] ) throws IOException {
    BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));

    //Declare a class JFrame variable
    outputFrame = new JFrame();

    //Declare a container variable
    Container outputPane;
    outputPane = outputFrame.getContentPane();

    //default close action
    outputFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //default size
    outputFrame.setSize(300, 300);

    //set class flow layout argument
    outputPane.setLayout(new FlowLayout());

    //create components
    enterWidth = new JLabel("Enter Width");
    enterLength = new JLabel("Enter Length");
    calculateArea = new JLabel("");
    calculatePerim = new JLabel("");
    dataWidth = new JTextField(15);
    dataLength = new JTextField(15);
    calculate = new JButton("Calculate");
    clear = new JButton("Clear fields");

    button = new ButtonHandler();
    calculate.addActionListener(button);

    //add components to the frame
    outputPane.add(BorderLayout.WEST, enterWidth);
    outputPane.add(BorderLayout.EAST, dataWidth);
    outputPane.add(BorderLayout.WEST, enterLength);
    outputPane.add(BorderLayout.EAST, dataLength);
    outputPane.add(BorderLayout.SOUTH, calculate);
    outputPane.add(BorderLayout.SOUTH, clear);
    outputPane.add(BorderLayout.SOUTH, calculateArea);
    outputPane.add(BorderLayout.SOUTH, calculatePerim);

    // make visible
    outputFrame.setVisible(true);

}//end of main

public  class rectangle  {
        private int length;
        private int width;

        public rectangle() {
            length = 1;
            width = 1;
        }

        public rectangle(int l, int w){
            length = l;
            width = w;
        }

        public int calcArea() {
            int area = length * width;
            return area;
        }

        public int calcPerimeter() {
            int perimeter = 2 * (length + width);
            return perimeter;
        }

        public void setWidth(int w1) {
                if (w1 >= 1 && w1 <= 20)
                width = w1;
                else
                width = 1;
        }

        public void setLength(int l1) {
                if (l1 >= 1 && l1 <= 20)
                length = l1;
                else
                length = 1;
        }

        public int getWidth() {
            return width;
            }

        public int getLength() {
            return length;
            }

}//end of rectangle class

    }//end of program81