为什么会出现NoClassDefFoundError？

Question

I'm creating a basic program that is to make a "life bar" that fills when you press the space bar, and when you get to the maximum, you win. Every second it decreases, so just spam the space bar to win. It's in attempt to test for a basic game throug java. My program is caught at this error, giving it a runtime error that I'm unfamiliar with. Here is my code:

import javax.swing.JOptionPane;
import javax.swing.*;               
import java.util.*;                             
import static java.lang.System.*;
import static java.lang.Math.*;

import java.awt.*;                              
import java.awt.event.*;                                
import java.io.*;   //for files                     
public class clicker
{
    public static void main(String args[])
    {
        new Clicker();  //Make a window
    }

}
class Clicker extends Frame   implements KeyListener, MouseListener
{
    // global variables
    private final static int SCHEIGHT=768,SCWIDTH=1024;
    // direction constants

    final static int N = 0,NE = 1,E = 2,SE = 3, S = 4, SW = 5, W = 6, NW =7,STILL = 8;
    // movement change constants
    final  int X = 0,Y = 1,Z = 2;
    final int TITLE = 30, STATUS = 40;
    final static int size = 2;
    Image myPic;

    private boolean gameNotOver = true;
    private boolean keyPressed = false;
    private Image myScreen;
    private int whichScreen;
    private int numScreen;
    private int timer;
    private int amt;

    public Clicker()
    {       
        setSize(SCWIDTH,SCHEIGHT);
        addWindowListener(new WindowAdapter()
              {
                  public void windowClosing(WindowEvent e)
                    {
                      System.exit(0);
                    }
               });

       this.setVisible(true);


       gameLoop();
    }
public void gameLoop()
{   
    do   
     {
        if (keyPressed)
           {
             amt++;
             out.println(keyPressed + " " +gameNotOver+" "+ "Time "+timer);
             this.repaint();
             keyPressed = false;
           }
        if(amt >= 450)
          gameNotOver = false;
        this.setVisible(true);
        //this.repaint();
        pause(30);
     }
   while (gameNotOver);     
   if(!gameNotOver)
    {
        pause(5000);
        System.exit(0);
    }
}
public int getAmt()
{
    return amt;
}
public void paint(Graphics pen)
 {    
        if(whichScreen==0)
            {
                pen.drawImage(myPic,100,200,506,279,this);   //Draws the image
                pen.setFont(new Font("Timesroman", Font.ITALIC, 50));
                pen.drawString("Welcome to the Clicker Test", 200, 75);
                pen.setFont(new Font("Timesroman", Font.ITALIC, 40));
                pen.drawString("Created by Cody Coulter",150,150);
                pen.setFont(new Font("Timesroman", Font.ITALIC, 25));
                pause(2000); // 2000 final          
                whichScreen++;
            }
        else
            {
                myScreen =createImage(getSize().width,getSize().height);
                Graphics o = myScreen.getGraphics();
                doubleBuffer(o);
                pen.drawImage(myScreen,0,0,this);
            }
 }   
public void doubleBuffer(Graphics pen)  // Draws the window
 {     
          pause (500);
          numScreen++;
          if (numScreen > 0)          
               {      
                    setBounds(0,0,SCWIDTH,SCHEIGHT);
                    Color HPRed = new Color(213, 0, 0);
                    pen.setColor(Color.BLACK);
                    setTitle("Clicker Test  -- Cody Coulter."+
                             " To click, press the space bar");
                    pen.setFont(new Font("Timesroman",Font.PLAIN,48));
                    pen.drawString("Clicker Test ",350,75);
                    pen.setFont(new Font("TimesRoman",Font.BOLD,33));
                    pen.fillRect((SCWIDTH/4), (SCHEIGHT/3), 50, 500);

                    pen.setColor(HPRed);
                    pen.fillRect((SCWIDTH/4)-5, (SCHEIGHT/3)+5, 40, amt);


                    if(!gameNotOver)
                     {
                        if(amt > 100)
                        {
                            pen.setColor(HPRed);
                            pen.fillRect(SCWIDTH/4-75, SCHEIGHT/3, SCWIDTH/2+150, SCHEIGHT/3-60);
                            pen.setColor(Color.GRAY);
                            pen.fillRect(SCWIDTH/4-65, SCHEIGHT/3+10, SCWIDTH/2+130, SCHEIGHT/3-80);
                            pen.setColor(Color.YELLOW);
                            pen.setFont(new Font("Verdana", Font.BOLD, 40));
                            pen.drawString("You got it!", SCWIDTH/2-290, SCHEIGHT/2-20);
                        }
                     }
                    this.repaint();
               }
}    
public void keyPressed(KeyEvent e)
{
    keyPressed = true;
    setTitle(""+ KeyEvent.getKeyText(e.getKeyCode()));  
    System.out.println("hit + "+ KeyEvent.getKeyText(e.getKeyCode())+ " " + keyPressed);
    switch(e.getKeyCode())
       {
          case KeyEvent.VK_SPACE: amt= amt + 4;
                                  break; 
       }
}
public void mouseClicked(MouseEvent m)
{
}   
public void mouseEntered(MouseEvent m)
{
}   
public void mouseExited(MouseEvent m)
{
}   
public void mousePressed(MouseEvent m)
{
}   
public void mouseReleased(MouseEvent m)
{
}
public void keyReleased(KeyEvent e)
{
}
public void keyTyped(KeyEvent e)
{
}

public void update(Graphics G)
{
      paint(G);
}       
public static void pause (long r)
{
    try
       {
           Thread.sleep(r);
       }
    catch (Exception e) 
       {
           out.println(" sleep error " + e);
        }          
}
}

I'm new to this forum board, and I am most likely using arbitrary information to solve this particular problem, and am new to programming in general. Any insight, or how to narrow the question would be greatly appreciated.

Again, the error stated is "Exception in thread "main" java.lang.NoClassDefFoundError: clicker (wrong name: Clicker)

Thank You

Answer 1

public class clicker
{
    public static void main(String args[])
    {
        new Clicker();  //Make a window
    }

}

You're essentially declaring your class twice. Take the main function and put it in your other clicker class, and delete the lower case one.

class Clicker extends Frame   implements KeyListener, MouseListener
{
    public static void main(String args[])
    {
        new Clicker();  //Make a window
    }
    //.....rest of class.....
}

Answer 2

You main() method should be placed in the Clicker class.

So the solution is :

class Clicker extends Frame implements KeyListener, MouseListener
{
    public static void main(String args[])
    {
        new Clicker();  // Make a window
    }
...
}

Also, make sure that your file is named Clicker.java , to match your class's name.

Answer 3

public Clicker(){       

setSize(SCWIDTH,SCHEIGHT);

    addWindowListener(new WindowAdapter(){
        public void windowClosing(WindowEvent e){
            System.exit(0);
        }
    });

    this.setVisible(true);

    //ADD THIS
    this.addKeyListener(this);
    this.addMouseListener(this);

    gameLoop();
}