Continuously moving an image in Java

Question

I'm trying to recreate a DigDug game in Java for my computer science class.

As I'm just a beginner, I'm only going to have the monsters move up and then back down, instead of chasing the player. I'm having difficulties making the monsters continuously move though. It will go up, but it will not go back down, and it doesn't repeat.

Previously it would just immediately fly off the screen if the player reached the point, or it wouldn't move at all. I have tried a for-loop, a while-loop and a do-while loop. I also tried a delay but it stopped my entire program. The goal is to make all of the monsters move up and down when the player reaches a certain point (340, 270).

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.awt.Font;
import java.awt.event.*;
import javax.swing.Timer;
import java.util.ArrayList;
import java.awt.event.KeyAdapter;

public class DigDug extends Applet implements MouseMotionListener, MouseListener, KeyListener
{
   private final Color c_background = new Color(32, 73, 150);
   private final Color c_first = new Color(230, 185, 9);
   private final Color c_second = new Color(224, 128, 18);
   private final Color c_third = new Color(232, 61, 14);
   private final Color c_last = new Color(184, 0, 0);
  private final Color c_score = new Color(232, 42, 96);
   public boolean ifEntered = false;
   public boolean isMoving1 = false;
   private int count = 0;
   private int otherCount = 0;
   private Image flower;
   private Image diggy;
   private Image enemies1;
   private Image enemies2;
   private Image enemies3;
   private Image arrowUp;
   private Image arrowDown;
   private Image arrowLeft;
   private Image arrowRight;
   //private ArrayList<Point> enemies = new ArrayList<Point>();
   Font myFont = new Font("SansSerif",Font.BOLD, 16);
   private Timer timer;
   private Point Up;
   private int digx = 685;
   private int digy = 45;
   private int e1x = 76;
   private int e1y = 210;  //i created a variable that would hold the enemies's values
   private int e2x = 465;
   private int e2y = 342;
   private int e3x = 546;
   private int e3y = 127;
   private boolean ifW = false;
   private boolean ifA = false;
   private boolean ifS = false;
   private boolean ifD = false;
   private boolean yaas = false;
   private int arrowX = digx+5;
   private int arrowY = digy+5;

   private ArrayList<Integer> xpts = new ArrayList<Integer>();
   private ArrayList<Integer> ypts = new ArrayList<Integer>();


   public void init()
   {
      diggy = getImage(getDocumentBase(), "Dig_Dug.png");
      flower = getImage(getDocumentBase(), "flower.gif");
      enemies1 = getImage(getDocumentBase(), "Pooka.png");
      enemies2 = getImage(getDocumentBase(), "Fygar.gif");
      enemies3 = getImage(getDocumentBase(), "Pooka.png");
      arrowUp = getImage(getDocumentBase(), "ArrowUp.png");
      arrowDown = getImage(getDocumentBase(), "ArrowDown.png");
      arrowRight = getImage(getDocumentBase(), "Arrow-Point-Right.png");
      arrowLeft = getImage(getDocumentBase(), "ArrowLeft copy.png");
     setBackground(c_background);
      setSize(700,500);
      addMouseMotionListener(this);
      addMouseListener(this);
      setFont(myFont);
      addKeyListener(this);
      Up = new Point(digx, digy);


   }
   public void paint(Graphics g)
   {
      score(g);
      background(g);
      g.setColor(Color.white);
      //g.fillRect(player.x, player.y, 10, 10);
      if(ifEntered)
      {
         if(digx > 330)
            digx--;   
        else if(digy < 247)
            digy++;
         repaint();
      }
      if(digx == 330 && digy == 247)
      {
         yaas = true;               //checks if the player has reached spot 
                                     //(random variable i created
      }

      if(yaas == true)             //this if statement is where i am having the problem
      {
         //int count = 0;       
            //int count = 0;
            boolean isUp = false;
            for(int i = 0; i < count; i++)
            {           

               if(e1y == 210)                        
               {
                  while(e1y != 120)
                  {
                    e1y--;
                  }
                  isUp = true;

               }
               if(e1y == 120 && isUp == true)
               {
                   while(e1y != 210)
                   {
                     e1y++;                   
                   }
                   isUp = false;
               }
               count++;
            }           
       }          



      g.drawImage(diggy, digx, digy, 20, 20, null);
      g.drawImage(flower, 680, 41, 20, 20, null);
      g.drawImage(enemies1, e1x, e1y, 20, 20, null);
     g.drawImage(enemies2, e2x, e2y, 20, 20, null);
      g.drawImage(enemies3, e3x, e3y, 20, 20, null);

      if(digy > e1y && digy < (e1y+20) && digx > e1x && digx < (e1x+20) ||e1y == digy || e1x == digx )
      {
         background(g);
         g.setColor(Color.white);
         g.drawString("GAME OVER", 294, 260);
     //    repaint();        
      }
      if(digy > e2y && digy < (e2y+20) && digx > e2x && digx < (e2x+20)||e2y == digy || e2x == digx )
      {
         background(g);
         g.setColor(Color.white);
         g.drawString("GAME OVER", 294, 260);
       //  repaint(); 
      }
      if(digy > e3y && digy < (e3y+20) && digx > e3x && digx < (e3x+20) || e3y == digy && e3x == digx)
      {
         background(g);
         g.setColor(Color.white);
         g.drawString("GAME OVER", 294, 260);
        // repaint(); 
      }



   }
   public void score(Graphics g)
   {
      g.setColor(c_score);
      g.drawString("1UP", 101, 21);
      g.drawString("HIGH SCORE", 271, 21);
      g.setColor(Color.white);
      g.drawString(" " + count, 118, 35);
      g.drawString(" " + count, 317, 35);
   }
   public void background(Graphics g)
   {
      g.setColor(c_first);
      g.drawRect(0,65, 700, 120);
      g.fillRect(0, 65, 700, 120);
      g.setColor(c_second);
      g.drawRect(0,166, 700, 120);
      g.fillRect(0,166, 700, 120);
      g.setColor(c_third);
      g.drawRect(0, 267, 700, 120);
      g.fillRect(0, 267, 700, 120);
      g.setColor(c_last);
      g.drawRect(0, 388, 700, 120);
      g.fillRect(0, 388, 700, 120);
   }

Answer 1

First, you will need an understanding of Painting in AWT and Swing and Concurrency in Java. I would also discourage the use of Applet, applets have there own bunch of issues which you probably don't need to deal with right now.

The paint method is not the place to update the state of your UI, the paint method should just paint the current state. You should also not change the state of the UI from within the paint method, this could cause no end of issues

To start with, you need some kind of "game loop", this loop is responsible for updating the state of the game and scheduling the updates to the UI

In the main loop, we maintain a movement "delta", this describes the amount of change that a particular object has and in what direction, in this example, I'm only dealing with enemies1, but you can imagine anything that moves will need it's own as well.

On each cycle of the game loop, we apply the delta to the objects current position, do a bounds check and invert the delta as required...

e1y += moveDelta;
if (e1y >= 210) {
    e1y = 209;
    moveDelta *= -1;
} else if (e1y <= 120) {
    e1y = 121;
    moveDelta *= -1;
}

The mainLoop below should probably go in the init or start method, depending on if you can work out how to "pause" the Thread

Thread mainLoop = new Thread(new Runnable() {

    @Override
    public void run() {

        int moveDelta = -1;
        gameOver = false;
        while (!gameOver) {
            if (ifEntered) {
                if (digx > 330) {
                    digx--;
                } else if (digy < 247) {
                    digy++;
                }
                repaint();
            }
            // Removed some code here for testing
            e1y += moveDelta;
            if (e1y >= 210) {
                e1y = 209;
                moveDelta *= -1;
            } else if (e1y <= 120) {
                e1y = 121;
                moveDelta *= -1;
            }

            if (digy > e1y && digy < (e1y + 20) && digx > e1x && digx < (e1x + 20) || e1y == digy || e1x == digx) {
                gameOver = true;
            } else if (digy > e2y && digy < (e2y + 20) && digx > e2x && digx < (e2x + 20) || e2y == digy || e2x == digx) {
                gameOver = true;
            } else if (digy > e3y && digy < (e3y + 20) && digx > e3x && digx < (e3x + 20) || e3y == digy && e3x == digx) {
                gameOver = true;
            }

            try {
                Thread.sleep(40);
            } catch (InterruptedException ex) {
            }
            repaint();
        }
    }
});
mainLoop.start();

You then need to replace the paint method with code that only paints the current state of the game

@Override
public void paint(Graphics g) {
    super.paint(g);
    score(g);
    background(g);
    g.setColor(Color.white);
    //g.fillRect(player.x, player.y, 10, 10);

    if (gameOver) {
        g.setColor(Color.white);
        g.drawString("GAME OVER", 294, 260);
    } else {
        g.drawImage(diggy, digx, digy, 20, 20, null);
        g.drawImage(flower, 680, 41, 20, 20, null);
        g.drawImage(enemies1, e1x, e1y, 20, 20, null);
        g.drawImage(enemies2, e2x, e2y, 20, 20, null);
        g.drawImage(enemies3, e3x, e3y, 20, 20, null);      
    }
}

Now, I'd recommend that you dump Applet in favour of a JPanel (for the core logic) and a JFrame (for displaying) and a Swing Timer instead of the Thread. You get double buffering for free to start with