Java Event Listener to detect a variable change

Question

I cannot seem to find an answer anywhere to my question. Is there any event listener which can detect the changing of a boolean or other variable and then act on it. Or is it possible to create a custom event listener to detect this?

Please I cannot seem to find a solution to this anywhere and I found this website explaining how to create custom events

Answer 1

Just like you need to create an event listener, you will also need to create the event firer -- since there is nothing automatic that will do this for you. I've provided sample code that shows you how to implement such a firer.

This test implementation isn't perfect. It only includes a way to add listeners. You may wish to include a way to remove listeners who are no longer interested in receiving events. Also note that this class is not thread-safe.

import java.util.List;
import java.util.ArrayList;
import java.util.EventListener;
import java.util.EventObject;
import java.awt.EventQueue; 

/**
 * This class uses the EventQueue to process its events, but you should only 
 * really do this if the changes you make have an impact on part of a GUI 
 * eg. adding a button to a JFrame.
 *
 * Otherwise, you should create your own event dispatch thread that can handle
 * change events
 */
public class BooleanChangeTest implements BooleanChangeDispatcher {

    public static void main(String[] args) {

        BooleanChangeListener listener = new BooleanChangeListener() {
            @Override
            public void stateChanged(BooleanChangeEvent event) {
                System.out.println("Detected change to: "
                    + event.getDispatcher().getFlag()
                    + " -- event: " + event);
            }
        };

        BooleanChangeTest test = new BooleanChangeTest(false);
        test.addBooleanChangeListener(listener);

        test.setFlag(false); // no change, no event dispatch
        test.setFlag(true); // changed to true -- event dispatched

    }

    private boolean flag;
    private List<BooleanChangeListener> listeners;

    public BooleanChangeTest(boolean initialFlagState) {
        flag = initialFlagState;
        listeners = new ArrayList<BooleanChangeListener>();
    }

    @Override   
    public void addBooleanChangeListener(BooleanChangeListener listener) {
        listeners.add(listener);
    }

    @Override
    public void setFlag(boolean flag) {
        if (this.flag != flag) {
            this.flag = flag;
            dispatchEvent();
        }
    }

    @Override
    public boolean getFlag() {
        return flag;
    }

    private void dispatchEvent() {
        final BooleanChangeEvent event = new BooleanChangeEvent(this);
        for (BooleanChangeListener l : listeners) {
            dispatchRunnableOnEventQueue(l, event);
        }
    }

    private void dispatchRunnableOnEventQueue(
                final BooleanChangeListener listener, 
                final BooleanChangeEvent event) {

        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                listener.stateChanged(event);
            }
        });
    }

}

interface BooleanChangeDispatcher {

    public void addBooleanChangeListener(BooleanChangeListener listener);
    public boolean getFlag();
    public void setFlag(boolean flag);

}

/**
 * Listener interface for classes interested in knowing about a boolean
 * flag change.
 */
interface BooleanChangeListener extends EventListener {

    public void stateChanged(BooleanChangeEvent event);

}

/** 
 * This class lets the listener know when the change occured and what 
 * object was changed.
 */
class BooleanChangeEvent extends EventObject {

    private final BooleanChangeDispatcher dispatcher;

    public BooleanChangeEvent(BooleanChangeDispatcher dispatcher) {
        super(dispatcher);
        this.dispatcher = dispatcher;
    }

    // type safe way to get source (as opposed to getSource of EventObject
    public BooleanChangeDispatcher getDispatcher() {
        return dispatcher;
    }
}

Answer 2

Use PropertyChangeSupport. You wont have to implement as much and it is thread safe.

public class MyClassWithText {
    protected PropertyChangeSupport propertyChangeSupport;
    private String text;

    public MyClassWithText () {
        propertyChangeSupport = new PropertyChangeSupport(this);
    }

    public void setText(String text) {
        String oldText = this.text;
        this.text = text;
        propertyChangeSupport.firePropertyChange("MyTextProperty",oldText, text);
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        propertyChangeSupport.addPropertyChangeListener(listener);
    }
}

public class MyTextListener implements PropertyChangeListener {
    @Override
    public void propertyChange(PropertyChangeEvent event) {
        if (event.getPropertyName().equals("MyTextProperty")) {
            System.out.println(event.getNewValue().toString());
        }
    }
}

public class MyTextTest {
    public static void main(String[] args) {
        MyClassWithText interestingText = new MyClassWithText();
        MyTextListener listener = new MyTextListener();
        interestingText.addPropertyChangeListener(listener);
        interestingText.setText("FRIST!");
        interestingText.setText("it's more like when you take a car, and you...");
    }
}

Answer 3

you can also try to implement an Observer.

First create the observable object:

import java.util.Observable;

public class StringObservable extends Observable {
 private String name;


 public StringObservable(String name) {
  this.name = name;
 }

 public String getName() {
  return name;
 }


 public void setName(String name) {
  this.name = name;
  setChanged();
  notifyObservers(name);
 }

}

Then the observer:

import java.util.Observable;
import java.util.Observer;

public class NameObserver implements Observer {
 private String name;

 public NameObserver() {
  name = null;
 }

 public void update(Observable obj, Object arg) {
  if (arg instanceof String) {
   name = (String) arg;
   System.out.println("NameObserver: Name changed to " + name);
  } else {
   System.out.println("NameObserver: Some other change to subject!");
  }
 }
}

And in your main (or wherever else):

public class TestObservers {
 public static void main(String args[]) {

  // Create the Subject and Observers.
  StringObservable s = new StringObservable("Test");
  NameObserver nameObs = new NameObserver();

  // Add the Observer
  s.addObserver(nameObs);


  // Make changes to the Subject.
  s.setName("Test1");
  s.setName("Test2");
 }
}

Mostly found here

Answer 4

Very late to answer, but this is a problem that can be solved with Observer/Observable. Example

Answer 5

The boolean you are setting should be allowed to do only through a setter method like:

public void setFlag(boolean flag){
    //Method code goes here
}

Now in now set method, you can decide based on what value comes in, what event needs to be fired. I am explaining in simple terms without introducing complex terms so you can understand better, so code snippet would look like:

public void setFlag(boolean flag){

    //if flag is TRUE do something
    //If flag is FALSE then do something 

    //And finally do what you needed to do with flag
}

Ask questions if you need more info