JavaFX - 如何使用When创建条件绑定

import javafx.beans.binding.DoubleBinding;
import javafx.beans.binding.When;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.SimpleDoubleProperty;

public class Main {
  public static void main(String[] args) {
    DoubleProperty a = new SimpleDoubleProperty(0);
    DoubleProperty b = new SimpleDoubleProperty(0);

    DoubleBinding s = a.add(b).divide(2.0D);

    final DoubleBinding aBinding = 
        new When(a.add(b).greaterThan(b).and(a.add(a).greaterThan(b)))
                .then(s.multiply(s.subtract(a)).multiply(s.subtract(b))).otherwise(0.0D);

    a.set(3);
    b.set(4);
    System.out.println(a.get());
    System.out.println(b.get());
    System.out.println(aBinding.get());

    a.set(2);
    b.set(2);
    System.out.println(a.get());
    System.out.println(b.get());
    System.out.println(aBinding.get());

  }
}