multithreading - Oval collision method never called - Java Swing -

i need calculate 2 ovals collision in java swing mini game.

i have jpanel draws arraylist of balls thread's , player ball. in run() method of non player balls check collision between player ball , balls thread arraylist.

the problem collision method never executed. it's not getting collision if statement. never calls method.

ball.java:

public class ball extends thread {     int x;     int y;     int velocity = 1;     public int radius = 20;     public boolean directionx = true; // true - right, false - left     public boolean directiony = false; // true - up, false - down      public static final int y_starting_position = 0;     public static final int x_starting_position = 0;      public ball()     {            switch (this.getstartingside())         {             case 0: // left             {                 this.directionx = true;                 this.directiony = true;                  this.x = this.getrandomheight();                 this.y = this.getrandomwidth();                  break;             }              case 1: // right             {                 this.directionx = false;                 this.directiony = false;                  this.x = this.getrandomheight();                 this.y = this.getrandomwidth();                  break;             }              case 2: // top             {                 this.directionx = true;                 this.directiony = false;                  this.x = this.getrandomwidth();                 this.y = this.getrandomheight();                  break;             }              case 3: // bottom             {                 this.directionx = false;                 this.directiony = true;                  this.x = this.getrandomwidth();                 this.y = this.getrandomheight();                  break;             }          }     }       public int getx()      {         return this.x;     }     public void setx(int x)      {         this.x = x;     }     public int gety()     {         return this.y;     }     public void sety(int y)      {         this.y = y;     }      public void move()     {         if (this.directionx) // right         {             this.x += this.velocity;         }         else // left         {             this.x -= this.velocity;         }         if (this.directiony) //         {             this.y -= this.velocity;         }         else // down         {             this.y += this.velocity;         }     }      @override     public void run()     {         try         {             this.iscollision(); // never called             thread.sleep(20);         }         catch (interruptedexception e)         {             e.printstacktrace();         }     }      /**      * random number varies 0 screen width.      *       * @return random number.      *       */     public int getrandomwidth()     {         random random = new random();         return random.nextint((program.getpanelwidth() - 0) + 1) + 0; // minimum = 0,maximum = program.panelwidth     }     /**      * random number varies 0 screen height.      *       * @return random number.      *       */     public int getrandomheight()     {         random random = new random();         return random.nextint((program.getpanelheight() - 0) + 1) + 0; // minimum = 0,maximum = program.panelheight     }     /**      * starting side of ball.      *       * left - 0.      * right - 1.      * top - 2.      * bottom - 3.      *       * @return      *       */     public int getstartingside()     {         random random = new random();         return random.nextint((4 - 0) + 1) + 0; // minimum = 0,maximum = 3     }       public void iscollision()     {         system.out.println("ssssssssssssssss");         if (math.sqrt(math.pow(mypanel.playerx + mypanel.playerradius - this.getx() + this.radius,2)                 + math.pow(mypanel.playery + mypanel.playerradius - this.gety() + this.radius,2))                 <= mypanel.playerradius + this.radius) // collision         {             system.exit(0);         }     }   } // end of ball class 

mypanel.java:

public class mypanel extends jpanel implements keylistener {     private static final long serialversionuid = 1l;      private static final color background_color = color.white;      private static final color npc_balls_color = color.red;      // player oval     public static int playerradius = 35;      public static int playerx;     public static int playery;      // true - first player position, false - otherwise     private boolean playerposition = true;      // array of balls threads     public static arraylist<ball> balls = new arraylist<ball>();       public mypanel()     {         this.setbackground(mypanel.background_color);         this.setfocusable(true);         this.addkeylistener(this);          new timer(10,new updateui());     }       // drawing      @override     protected void paintcomponent(graphics g)     {         super.paintcomponent(g);          final double panel_width = this.getwidth();         final double panel_height = this.getheight();          if (this.playerposition)         {             mypanel.playerx = (int)(panel_width / 2 - mypanel.playerradius);             mypanel.playery = (int)(panel_height / 2 - mypanel.playerradius);             this.playerposition = false;         }          // drawing player         g.setcolor(color.black);         g.filloval(playerx,playery, mypanel.playerradius * 2, mypanel.playerradius * 2);          // drawing npc's balls         g.setcolor(mypanel.npc_balls_color);         (ball ball: mypanel.balls) // concurrentmodificationexception         {             if (ball.isalive())             {                 ball.start();             }              ball.move();             repaint();             g.filloval(ball.getx(), ball.gety(),                      ball.radius * 2, ball.radius * 2);         }      }      // keyboard listeners      @override     public void keypressed(keyevent e)      {         switch (e.getkeycode())         {             case keyevent.vk_w: //             {                 mypanel.playery -= 10;                 repaint();                 break;             }             case keyevent.vk_s: // down             {                 mypanel.playery += 10;                 repaint();                 break;             }             case keyevent.vk_d: // right             {                 mypanel.playerx += 10;                 repaint();                 break;             }             case keyevent.vk_a: // left             {                 mypanel.playerx -= 10;                 repaint();                 break;             }         }     }      @override     public void keyreleased(keyevent e)      {      }      @override     public void keytyped(keyevent e)      {      }      public class updateui implements actionlistener     {          @override         public void actionperformed(actionevent e)         {             repaint();         }      }       } // end of mypanel class 

program.java:

public class program {     private static int panelwidth;     private static int panelheight;      public static void main(string[] args)      {         myframe frame = new myframe();          program.panelwidth = frame.getwidth();         program.panelheight = frame.getheight();          // generate ball each 2 seconds         while (true)         {             try             {                 mypanel.balls.add(new ball());                   thread.sleep(500);             }             catch (interruptedexception e)             {                 e.printstacktrace();             }         }      } // end of main method      public static int getpanelwidth()      {         return program.panelwidth;     }       public static int getpanelheight()      {         return program.panelheight;     }     } 

the jframe nothing special , adds jpanel , so.

so iscollision() method never called if it's on run() method of thread. how come?

you never started thread implemented ball class.

the concurrentmodificationexception due fact adding balls arraylist nature thread-unsafe datastructure if don't synchronize externally. here adding in thread while iterating list in edt (event dispatch thread). either synchronize on list, or use thread-safe data structure (for iteration still may need lock full list).

have @ collections.synchronizedlist or copyonwritearraylist (the latter doesn't need synchronizing iteration).

however, synchronization on each insertion , each iteration seems inefficient me, because aiming game requires fast rendering , background processing. may sufficient basic game though.

as sidenote: better technique game use double buffering: render game graphics in background thread e.g. on bufferedimage, , when done, switch 2 buffers 1 drew displayed on screen, , other 1 can used again drawing next frame. might need synchronization checkpoint between frames. more advanced, if starting in java, wouldn't spend time on , keep things simple.