arrays - java.util.ConcurrentModificationException with iterator (character movement) -

i'm coding game when hold d character moves right , when hold s character moves left. program works should, 1 exception. i'm moving character (and character moves fine), java keeps throwing error "java.util.concurrentmodificationexception". did research , learned cannot adding arraylist "keys" , iterating @ same time. said, how edit code still have character move seamlessly now? way know of make sure character moves @ same steady pace second hold key down, not take quick movement, pause, , continue motion.

package leveleditor;  import java.awt.canvas; import java.awt.dimension; import java.awt.graphics; import java.awt.toolkit; import java.awt.event.keyevent; import java.awt.event.keylistener; import java.awt.event.mouseevent; import java.awt.event.mouselistener; import java.awt.event.mousemotionlistener; import java.awt.image.bufferstrategy; import java.util.arraylist; import java.util.iterator;   public class leveleditor extends canvas implements keylistener, runnable, mouselistener, mousemotionlistener{  private object[][] grid = new object[50][50];  private graphics buffergraphics = null; //the graphics buffer private bufferstrategy bufferstrategy = null;  private thread thread;  private boolean running;  private int selectedblock = 0;  private int mousex; private int mousey;  private int playerx; private int playery;  private arraylist<integer> keys = new arraylist<integer>(); iterator itr;  player player1;  public leveleditor(dimension size){     //constructor     this.setpreferredsize(size);     this.addkeylistener(this);     this.thread = new thread(this);     this.addmouselistener(this);     this.addmousemotionlistener(this);     running = true;     mousex = 0;     mousey = 0;     playerx = 0;     playery = 0;     itr = keys.iterator();   }    public void paint(graphics g){     if (bufferstrategy == null){         this.createbufferstrategy(2);         bufferstrategy = this.getbufferstrategy();         buffergraphics = bufferstrategy.getdrawgraphics();         player1 = new player(playerx, playery);         this.thread.start();     } }  @override public void run() {     //this runs when level editor running     while (running){         //program's logic         dologic();         draw();         drawbackbuffertoscreen();          thread.currentthread();         try{             thread.sleep(10);         }         catch(exception e){             e.printstacktrace();         }     } }   public void dologic(){  }  public void draw(){     //clear secondary screen     buffergraphics = bufferstrategy.getdrawgraphics();     try{         buffergraphics.clearrect(0, 0, this.getsize().width, this.getsize().height);          //this drawn buffer         (int x = 0; x < grid.length; x++){             for(int y = 0; y < grid[x].length; y++){                 object o = grid[x][y];                 if (o instanceof block){                     block blocktodraw = (block)o;                     blocktodraw.draw(buffergraphics);                 }             }         }          block.getblock(selectedblock, mousex, mousey).draw(buffergraphics);         player1.draw(buffergraphics);          (integer x  : keys){             if(x == 68 && itr.hasnext()){                 playerx += 5;                 player1.updateplayer(playerx, playery);                 player1.draw(buffergraphics);                 itr.next();                 itr.remove();               }             if (x == 65 && itr.hasnext()){                 playerx -= 5;                 player1.updateplayer(playerx, playery);                 player1.draw(buffergraphics);                 itr.next();                 itr.remove();             }         }         }     catch(exception e){         e.printstacktrace();     }     finally{         buffergraphics.dispose();     } }   public void drawbackbuffertoscreen(){     bufferstrategy.show();     toolkit.getdefaulttoolkit().sync(); }   @override public void mousereleased(mouseevent e) {     int mousex = e.getx();     int mousey = e.gety();      mousex = (mousex / 25);     mousey = (mousey / 25);      if (e.getmodifiers() == 16){         grid[mousex][mousey] = block.getblock(selectedblock, mousex, mousey);     }     else if (e.getmodifiers() == 4){         grid[mousex][mousey] = block.getblock(100, mousex, mousey);     }   }  @override public void keytyped(keyevent e) {     // todo auto-generated method stub  }  @override public void keyreleased(keyevent e) {     keys.clear();  }  @override public void keypressed(keyevent e) {      keys.add(e.getkeycode());  }   public void mousemoved(mouseevent e) {     this.mousex = e.getx() / 25;     this.mousey = e.gety() / 25;  }    @override public void mouseclicked(mouseevent e) {     // todo auto-generated method stub  }    @override public void mousepressed(mouseevent e) {     // todo auto-generated method stub  }    @override public void mouseentered(mouseevent e) {     // todo auto-generated method stub  }    @override public void mouseexited(mouseevent e) {     // todo auto-generated method stub  }    @override public void mousedragged(mouseevent e) {     int mousex = e.getx();     int mousey = e.gety();      mousex = (mousex / 25);     mousey = (mousey / 25);       if(e.mouse_dragged == 506 && e.getmodifiers() == 16){         grid[mousex][mousey] = block.getblock(selectedblock, mousex, mousey);     }     else if (e.mouse_dragged == 506 && e.getmodifiers() == 4){         grid[mousex][mousey] = block.getblock(100, mousex, mousey);     } } 

}

using iterator wrong model here: aren't iterating through list, looking see if have element, , processing if have something.

a better fit use queue - particularly, sort of concurrent queue, e.g. java.util.concurrent.blockingqueue, since modifying , reading in different threads.

you can add key codes using queue.add(e.getkeycode()), , remove them using queue.poll() (or queue.peek()), returns null value if there nothing in queue (akin itr.hasnext() being false).