title: Should input listeners be synchronized?
body: <p>My sample code posted below shows two classes. One implements KeyListener and the other implements Runnable and is running in an infinite loop sleeping every 20 ms. When a key is pressed the keyChar, which is in the form of an int, is used as an index setting the index of a boolean array true or false, representing with the key was pressed or not. At the same time the process loop is searching the key array for its true or false values and setting the true ones false then printing out the char.  My question is whether or not I need to use synchronization using a lock for accessing the charArray because it is used in two threads: the process thread and the key listener thread.</p>
  
  <p>Sample Code:</p>
  
  <pre><code>import java.awt.Component;
  import java.awt.event.KeyEvent;
  import java.awt.event.KeyListener;
  
  public class Input implements KeyListener {
  
  public boolean[] charArray;
  
  public Input(Component component) {
      charArray = new boolean[127];
      component.addKeyListener(this);
  }
  
  @Override
  public void keyPressed(KeyEvent e) {
              (possible synchronization with a lock?)
      int keyChar = e.getKeyChar();
      if (keyChar == 27 || keyChar == 9 || keyChar == 10 || keyChar == 127) //useless keys like del, tab, esc, etc..
          keyChar = 65535;
      if (keyChar &lt; 65535) //65535 represents no true char value
          charArray[keyChar] = true;
  }
  
  @Override
  public void keyReleased(KeyEvent e) {
  }
  
  @Override
  public void keyTyped(KeyEvent e) {
  }
  }
  
  
  
  
  import java.awt.Dimension;
  import javax.swing.JFrame;
  
  @SuppressWarnings("serial")
  public class Process extends JFrame implements Runnable {
  
  private boolean running;
  private Input input;
  
  public Process() {
      running = false;
      input = new Input(this);
      setTitle("Keyboard Test");
      setSize(new Dimension(200, 200));
      toFront();
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setVisible(true);
  }
  
  /**
   * @param args
   */
  public static void main(String[] args) {
      new Process().startThread();
  }
  
  public synchronized void startThread() {
      running = true;
      new Thread(this).start();
  }
  
  @Override
  public void run() {
      while (running) {
                      (possible synchronization with a lock?)
          for (int i = 0; i &lt; input.charArray.length; i++) {
              if (input.charArray[i] == true) {
                  input.charArray[i] = false;
                  System.out.println((char) i);
              }
          }
          try {
              Thread.sleep(20);
          } catch (InterruptedException e) {
          }
      }
  }
  }
  </code></pre>