Edit C:\Program Files\Java\jdk1.8.0_121\com\sun\javafx\collections\ElementObserver.java

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 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package com.sun.javafx.collections;

import javafx.collections.ObservableListBase;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.RandomAccess;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.util.Callback;

final class ElementObserver<E> {

private static class ElementsMapElement {
        InvalidationListener listener;
        int counter;

public ElementsMapElement(InvalidationListener listener) {
            this.listener = listener;
            this.counter = 1;
        }

public void increment() {
            counter++;
        }

public int decrement() {
            return --counter;
        }

private InvalidationListener getListener() {
            return listener;
        }
    }

private Callback<E, Observable[]> extractor;
    private final Callback<E, InvalidationListener> listenerGenerator;
    private final ObservableListBase<E> list;
    private IdentityHashMap<E, ElementObserver.ElementsMapElement> elementsMap =
            new IdentityHashMap<E, ElementObserver.ElementsMapElement>();

ElementObserver(Callback<E, Observable[]> extractor, Callback<E, InvalidationListener> listenerGenerator, ObservableListBase<E> list) {
        this.extractor = extractor;
        this.listenerGenerator = listenerGenerator;
        this.list = list;
    }

void attachListener(final E e) {
        if (elementsMap != null && e != null) {
            if (elementsMap.containsKey(e)) {
                elementsMap.get(e).increment();
            } else {
                InvalidationListener listener = listenerGenerator.call(e);
                for (Observable o : extractor.call(e)) {
                    o.addListener(listener);
                }
                elementsMap.put(e, new ElementObserver.ElementsMapElement(listener));
            }
        }
    }

void detachListener(E e) {
        if (elementsMap != null && e != null) {
            ElementObserver.ElementsMapElement el = elementsMap.get(e);
            for (Observable o : extractor.call(e)) {
                o.removeListener(el.getListener());
            }
            if (el.decrement() == 0) {
                elementsMap.remove(e);
            }
        }
    }

}

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