Add listener to ArrayList

Question

I have an ArrayList which I add some Objects to it dynamically, and I have a JButton. The ArrayList is empty when running my program and the JButton is set to setEnabled(fal

Answer 1

Following SimpleIntegerProperty syntax I made an ArrayList which fire when the size change. In this case you need to now the current size of the ArrayList since you want to react when the size is 2, so a simple solution to this is doing the following:

ArrayListProperty.java

public class ArrayListProperty<E> extends ArrayList<E> implements ObservableValue<Number> {
    private ExpressionHelper<Number> helper = null;
    private SimpleIntegerProperty sizeProperty;

    public ArrayListProperty(){
        super();
        sizeProperty = new SimpleIntegerProperty(0);
    }

    @Override
    public boolean add(E e) {
        boolean returnValue = super.add(e);
        sizeProperty.set(size());
        fireValueChangedEvent();
        return returnValue;
    }

    @Override
    public void add(int index, E element) {
        super.add(index, element);
        sizeProperty.set(size());
        fireValueChangedEvent();
    }

    @Override
    public E remove(int index) {
        E returnValue = super.remove(index);
        sizeProperty.set(size());
        fireValueChangedEvent();
        return returnValue;
    }

    @Override
    public boolean remove(Object o) {
        boolean returnValue = super.remove(o);
        if(returnValue){
            sizeProperty.set(size());
            fireValueChangedEvent();
        }
        return returnValue;
    }

    protected void fireValueChangedEvent(){
        ExpressionHelper.fireValueChangedEvent(helper);
    }

    @Override
    public void addListener(ChangeListener<? super Number> listener) {
        helper = ExpressionHelper.addListener(helper, sizeProperty, listener);
    }

    @Override
    public void removeListener(ChangeListener<? super Number> listener) {
        helper = ExpressionHelper.removeListener(helper, listener);
    }

    @Override
    public Number getValue() {
        return null;
    }

    @Override
    public void addListener(InvalidationListener listener) {
        helper = ExpressionHelper.addListener(helper, sizeProperty, listener);
    }

    @Override
    public void removeListener(InvalidationListener listener) {
        helper = ExpressionHelper.removeListener(helper, listener);
    }
}

In this case the observable is the size of the array, then, when declaring your arraylist, you can add a listener and act when the size is 2.

ArrayListProperty<Object> arrayList = new ArrayListProperty<>();
arrayList.addListener((ob, ov, nv) -> { 
    if(nv.intValue() == 2) doSomething();
});

This implementation also allows you to add a comparison in the ChangeListener so you can know when there was something added or something removed. Also you can store the object added or removed and have it in the listener.

Answer 2

ObservableList<DynamicObjects> ol = FXCollections.ObservableArrayList(new ArrayList<DynamicObjects>());
ListProperty lp = new SimplePropertyList(ol);

lp.addListener(new ChangeListener() {
    @Override public void changed(ObservableValue o, Object oldVal, Object newVal) {
        if (ol.size() > 1 && !JButton.isEnabled()) {
            JButton.setEnable(true);
        } else if (ol.size < 2 && JButton.isEnabled()) {
            JButton.setEnable(false);
        }
    }
});