我们如何正确扩展javafx属性类？

Question

我正在尝试扩展SimpleDoubleProperty并覆盖set方法，以便在新值和旧值相等时使侦听器代码运行。但是当我运行代码时，它没有触发监听器代码。这里有什么问题？我很难搞清楚这一点：\

SimpleDouble.java

import javafx.beans.property.SimpleDoubleProperty;

public class SimpleDouble extends SimpleDoubleProperty {

    public SimpleDouble() {

    }

    public SimpleDouble(double arg0) {
        super(arg0);

    }

    public SimpleDouble(Object arg0, String arg1) {
        super(arg0, arg1);

    }

    public SimpleDouble(Object arg0, String arg1, double arg2) {
        super(arg0, arg1, arg2);

    }

    @Override
    public void set(double arg0) {
           if (isBound()) {
                throw new java.lang.RuntimeException("A bound value cannot be set.");
           }
           fireValueChangedEvent();
    }

}

Bill.java

class Bill {


    private DoubleProperty amountDue = new SimpleDouble();


    public final double getAmountDue(){return amountDue.get();}


    public final void setAmountDue(double value){amountDue.set(value);}


    public DoubleProperty amountDueProperty() {return amountDue;}

}

主要

public class Main {

    public static void main(String[] args) {

      Bill electricBill = new Bill();

       electricBill.amountDueProperty().addListener(new ChangeListener(){
        @Override public void changed(ObservableValue o,Object oldVal, 
                 Object newVal){
             System.out.println("Electric bill has changed!");
        }
      });

      electricBill.setAmountDue(100.00);
      electricBill.setAmountDue(100.00);



    }
}