When one of the properties is bound with bind(), and you then apply a bindBidirectional between that property and another, the direction in which you do this (a.bindBidirectional(b) or b.bindBidirectional(a)) matters. One will throw an exception -- the other will not.

package hs.mediasystem.screens.collection.detail;

import static org.junit.Assert.assertEquals;
import hs.mediasystem.test.JavaFXRunningRule;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;

import org.junit.Rule;
import org.junit.Test;

public class BoundBidirectionalPropertyTest {

  @Rule
  public final JavaFXRunningRule jfxRunningRule = new JavaFXRunningRule();

  @Test(expected = RuntimeException.class)
  public void shouldThrowException() {
    StringProperty a = new SimpleStringProperty("a");
    StringProperty b = new SimpleStringProperty("b");
    StringProperty c = new SimpleStringProperty("c");

    b.bind(a);
    b.bindBidirectional(c); // Throws exception "A bound value cannot be set"

    assertEquals("a", a.get());
    assertEquals("a", b.get());
    assertEquals("a", c.get());
  }

  @Test(expected = RuntimeException.class)
  public void shouldAlsoThrowException() {
    StringProperty a = new SimpleStringProperty("a");
    StringProperty b = new SimpleStringProperty("b");
    StringProperty c = new SimpleStringProperty("c");

    b.bind(a);
    c.bindBidirectional(b); // Does not throw exception...

    assertEquals("a", a.get());
    assertEquals("a", b.get());
    assertEquals("a", c.get());
  }
}