Why is this coming up with missing main method error?

Question

So this source code comes free from this website. http://math.hws.edu/javamath/basic_applets/SliderGraph.html It seems to be somewhat old but I don't know if that is the issue. I would really like to write my own applet but this is exactly what I need and I am very short on time. The code compiles fine with no errors but comes up Exception in thread main java.lang.NoSuchMethodError: main Why is this happening? Where should I add the main method?

import java.awt.*;
import edu.hws.jcm.data.*;
import edu.hws.jcm.draw.*;
import edu.hws.jcm.awt.*;

public class SliderGraph extends java.applet.Applet {



   private DisplayCanvas canvas;

   public void stop() {
      canvas.releaseResources();
   }

   JCMPanel makeSliderPanel(VariableSlider v) {
         // A small utility routing that makes a JCMPanel that contains
         // a VariableSlider and a DisplayLabel that shows the value
         // of the variable associated with that slider.
      JCMPanel p = new JCMPanel();
      p.add(v, BorderLayout.CENTER);
      p.add( new DisplayLabel(v.getName() + " = #", new Value[] { v } ), BorderLayout.EAST);
      return p;
   }


   public void init() {

      Parser parser = new Parser();
      Variable x = new Variable("x");
      parser.add(x);

        // Create the three VariableSliders.  In this case, the sliders have
        //   names.  There is also a Variable associated with each slider,
        //   which has the same name.  This variable is added to the parser
        //   which is passed as the fourth parameter to the constructor, making
        //   it possible to use "a", "b", and "c" in expressions parsed by the
        //   parser.  Adjusting the value on a slider changes the value of the
        //   associated variable, and therefore changes the value of any
        //   expression that refers to that variable.  The second and third
        //   parameters to the constructor give the minimum and maximum Values
        //   on the slider.  Passing "null,null" uses the defaults, namely
        //   new Constant(-5) and new Constant(5).
      VariableSlider a = new VariableSlider("a",null,null,parser);
      VariableSlider b = new VariableSlider("b",null,null,parser);
      VariableSlider c = new VariableSlider("c",null,null,parser);

      canvas = new DisplayCanvas();
      canvas.setHandleMouseZooms(true);
      canvas.add(new Panner());

      LimitControlPanel limits =
           new LimitControlPanel( LimitControlPanel.SET_LIMITS | LimitControlPanel.RESTORE
                                    | LimitControlPanel.EQUALIZE,  false);
      limits.addCoords(canvas);

      ExpressionInput input = new ExpressionInput("a*x^2 + b*x + c", parser);
      Graph1D graph = new Graph1D(input.getFunction(x));

      ComputeButton button = new ComputeButton("Graph it!");

      canvas.add(new Axes());
      canvas.add(graph);
      canvas.add(new DrawBorder(Color.darkGray, 2));

      JCMPanel main = new JCMPanel();  // Build interface out of JCMPanels!
      main.setInsetGap(3);
      main.add(canvas, BorderLayout.CENTER);
      main.add(limits, BorderLayout.EAST);
      JCMPanel bot = new JCMPanel(5,1);
      main.add(bot, BorderLayout.SOUTH);

      bot.add(new Label("Enter a function f(x), which can use the constants a, b, and c:"));
      JCMPanel inputPanel = new JCMPanel();
      bot.add(inputPanel);
      inputPanel.add(input, BorderLayout.CENTER);
      inputPanel.add(button, BorderLayout.EAST);

      bot.add( makeSliderPanel(a) );  // Create and add the sliders.
      bot.add( makeSliderPanel(b) );
      bot.add( makeSliderPanel(c) );

      Controller controller = main.getController();  // Set up error reporting.
      controller.setErrorReporter(canvas);
      limits.setErrorReporter(canvas);

      main.gatherInputs();  // Set up main panel to respond to changes in input objects.
                            // This works since the interface is built of JCMPanels.  For
                            // the same reason, I don't have to add the objects the the
                            // controller.

      button.setOnUserAction(controller);  // Set controller to respond to button.

      setBackground(Color.lightGray);
      setLayout(new BorderLayout());
      add(main,BorderLayout.CENTER);

   } // end init()

} // end class SliderGraph

Answer 1

In addition to the other answers saying run as an applet. The main method in your sample code is closed too late, all other methods are enclosed by the main method in your sample, to be honest I can't see how it could compile in it's current state. Remove the main method and the closeing curly brace after the end init() comment.

Answer 2

You are likely encountering this issue because you are trying to run the Applet as an Application. Applets do not contain main methods, Applications do. If you want to run the Applet as an Application, just add a main method that does the following:

1. Create a window (JFrame) to hold the applet.
2. Make the window's close box stop the applet.
3. Create a new applet object, and add it to the window.
4. Start the applet by calling init(), then start().
5. Finalize the layout.
6. Make the window (with the applet in it) visible.

Answer 3

It's an applet, it shouldn't need a main method because it has init(). Compile it as an applet, not a program (if you're using eclipse, that is).

EDIT: on the command line, you'd use the "appletviewer" command on the .class file.