gistfile1.java

/**
     * the average value of all XValue of Data in values
     *
     * @see #values()
     */
    private DoubleBinding averageBinding = new DoubleBinding() {
        {
            //call #computeValue every time #values() changes
            super.bind(values());
        }
        @Override
        protected double computeValue() {
            double result = 0;
            if (values().isEmpty()) {
                return result;
            }
            for (XYChart.Data<Number, Number> d : values()) {
                result += d.getXValue().doubleValue();
            }
            return result / values().size();
        }
    };
    //....
        public static void main(String[] args) {
        Foo foo = new Foo();
        System.out.println(foo.averageBinding().get()); //Should print 0.2
        foo.values().add(new XYChart.Data<Number, Number>(-3d, -3d));
        System.out.println(foo.averageBinding().get()); //Should print -0.6
        foo.values().get(0).setXValue(30d);
        System.out.println(foo.averageBinding().get()); //Should  print 6.825
    }