Java KeyAdapter

Question

I am somewhat unfamiliar with how the Java KeyAdapter works, and am getting unexpected results with the following code using KeyAdapter . The problem occurs when a key is pressed while another key is already held down, regardless of whether isKeyPressed() is called.

Note: I know this is a lot of code, and I apologize. I tried the best I could to isolate it, and I think it resides primarily around the comments in the keyHandler method below (how keyHandler() puts the keys currently pressed into keysHeld ). Hopefully the thorough comments are helpful.

keyHandler:

ArrayList keysHeld = new ArrayList<KeyEvent>();

private void keyHandler()
{
    KeyAdapter keyListnr = new KeyAdapter()
    {
        public void keyPressed(KeyEvent e)
        { 
            int keyCode = e.getKeyCode();

            int index = 0;
            boolean found = false;
            while(!found && index<keysHeld.size()) //While not already found, and end of ArrayList not reached
            {
                System.out.print("errorCheck: keysHeld: "+keysHeld+", "+(Object)keyCode+" "); //PRINT
                if(keysHeld.get(index) == (Object)keyCode)
                {
                    System.out.println("found"); //PRINT
                    found = true; //This key is already recognized as held
                }
                else
                {
                    System.out.println("not found"); //PRINT
                    //This key is not recognized as held
                }
            }
            if(!found) //If key must be added to keysHeld
            {
                keysHeld.add(keyCode); //Add to list of held keys
            }
        System.out.println(keysHeld.toString()); //PRINT ArrayList of all held keys
    } //end of keyPressed


        public void keyReleased(KeyEvent e) //similar in concept to keyPressed
        {
         int keyCode = e.getKeyCode();

         int index = 0;
         boolean found = false;
         while(!found && index < keysHeld.size())
         {
          if(keysHeld.get(index) == (Object)keyCode)
          {
           keysHeld.remove(index); //remove key from keysHeld
           found = true;
          }
          else
          {
           index++;
          }
         }
         System.out.println(keysHeld.toString()); //PRINT ArrayList of all held keys
        } //end of keyReleased
    };
    addKeyListener( keyListnr );
}

isKeyHeld:

public boolean isKeyHeld(int e)
{
 int keyCode = e;
 Object key = (Object)keyCode;

 if(!keysHeld.isEmpty())
 {
  int index = 0;
  while(index<keysHeld.size())
  {
   if(keysHeld.get(index) == key)
   {
    return true;
   }
   index++;
  }
 }
 return false;
}

Console output: (held leftArrow[37], and then pressed rightArrow[39])

[37]
errorCheck: keysHeld: [37], 39 not found
errorCheck: keysHeld: [37], 39 not found
errorCheck: keysHeld: [37], 39 not found
errorCheck: keysHeld: [37], 39 not found
...

Answer 1

A couple points:

• You aren't populating your keysHeld array with instances of KeyEvent , but with autoBoxed Integer objects derived from the int keyCodes.
• You need to increment your index variable if you want to get out of the while loop in keyPressed
• You shouldnt use == to compare the two Objects in your while loop

You can test with something like the following:

    if(keysHeld.get(index++).equals(new Integer(keyCode))

Answer 2

When handling multiple keys, it's best to use the keyReleased(KeyEvent) method: it makes it easier to handle multiple key combinations during a key release.

What I've noticed was that when inside the keyPressed() , I would only get to capture one key character. On a keyReleased , I was able to capture multiple characters (such as CTRL - V ).