Q:

JavaFX - Bind ObjectProperty to member inside another ObjectProperty?

I have a class called Tree
public class Tree
{
    private Color c;

    public Color getColor()
    {
        return c;   
    }   
}

I have an ObjectProperty...
ObjectProperty<Tree> importantTree = new SimpleObjectProperty();

I want to make another ObjectProperty of type Color that is always equal to importantTree.get().getColor(). Whenever the Tree changes, I want the other ObjectProperty to change to that Tree's color.
For example.
ObjectProperty<Tree> importantTree = new SimpleObjectProperty();
ObjectProperty<Color> importantTreesColor = ... 

Tree a = new Tree(Color.RED);
Tree b = new Tree(Color.GREEN);

importantTree.set(a);
System.out.println(importantTreesColor.get()); // This should print RED.

importantTree.set(b);
System.out.println(importantTreesColor.get()); // This should print GREEN.

A:

Just use a binding:
ObjectProperty<Tree> importantTree = new SimpleObjectProperty();
Binding<Color> importantTreesColor = Bindings.createObjectBinding(() -> 
    importantTree.get() == null ? null : importantTree.get().getColor(),
    importantTree);

Tree a = new Tree(Color.RED);
Tree b = new Tree(Color.GREEN);

importantTree.set(a);
System.out.println(importantTreesColor.getValue()); // Prints RED.

importantTree.set(b);
System.out.println(importantTreesColor.getValue()); // Prints GREEN.

You can also do
Binding<Color> importantTreesColor = new ObjectBinding<Color>() {
    { bind(importantTree); }
    @Override
    protected Color computeValue() {
        return importantTree.get()==null ? null : importantTree.get().getColor();
    }
};

if you prefer.

