Question

In the following Java code I am populating an ArrayList in one class then using the ArrayList in another class. However, the array transfers but is filled with null values. I have debugged the code and the ArrayList does get populated but its values do not transfer.

First Class:

public String[] log = new String[100];
public ArrayList llog= new ArrayList();
int arraypos= 0;
public void nativeKeyPressed(NativeKeyEvent e) {
  if (e.getKeyCode() == NativeKeyEvent.VK_ESCAPE) {
    GlobalScreen.unregisterNativeHook();
  }
  System.out.println(" " + NativeKeyEvent.getKeyText(e.getKeyCode()).toLowerCase() + " ");
  llog.add(NativeKeyEvent.getKeyText(e.getKeyCode()).toString().toLowerCase());
  System.out.println("list:" + llog.get(arraypos));
  arraypos = arraypos + 1;  
}

public ArrayList getStrokes (){
  return this.llog;
}

Second Class:

public void TeslaTimer()
{
  int delay = 10000; //1 sec = 1000 
  int period = 90000; // repeat every sec.

  Timer timer = new Timer();
  timer.scheduleAtFixedRate(new TimerTask()
  {
    @Override
    public void run()
    {
      GlobalKeyListener keyl = new GlobalKeyListener();
      ArrayList keylog=keyl.getStrokes();

      //TeslaLogger savetoFile = new TeslaLogger();
      //savetoFile.TeslaLogger(keylog);
      System.out.println("Ring Ring");
      for(int i =0; i < keylog.size(); i++){
        System.out.println(keylog.get(i));
      }
    }
  }, delay, period);
}

Answer 1

The instance of GlobalKeyListener in TeslaTimer is not the same instance as the one you are adding to – the are different objects each with their own list.

You must make the same instance of GlobalKeyListener available to your run() method, or perhaps use a static variable (which is like a global variable).

---

Here’s the Singleton pattern in action that you could use to make it work:

public class GlobalKeyListener {

    // rest of class omitted
    private static GlobalKeyListener INSTANCE = new GlobalKeyListener();

    public static GlobalKeyListener getInstance() { 
        return INSTANCE;
    }

}

then wherever you need to use it locally:

GlobalKeyListener globalKeyListener = GlobalKeyListener.getInstance();
globalKeyListener.getStrokes(); // etc

All code will then be using the same instance.