JavaFX Experiments – Order of Declaration Matters

Given the following piece of JavaFX Script

import javafx.scene.CustomNode;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.shape.Rectangle;
import javafx.scene.paint.Color;
import java.lang.Math.sqrt;

class C extends CustomNode
{
    // Test #1: Commented out
    //var node: Group = Group {};

    protected override function create():Node
    {
        return node;
    }

    public var data:Number[] on replace olddata
    {
        for (item in data)
        {
            var side:Number = sqrt(item);
            insert Rectangle{
                width:side height:side
                stroke:Color.RED strokeWidth:2 fill:Color.TEAL
            }
            into node.content;
        }
    }

    // Test 2: Comment out
    var node: Group = Group {};
}

C { data: [21000, 10000]; }

One of these declares node at the end of the class, and the other declares it at the beginning of the class. Take a look at it for a while and see if you can guess the difference in behaviour?

Answer

The “on replace” trigger for data attribute does not throw an exception in Test #2 despite the fact that node is null.

Moral of the story

Order of initialization matters. Make sure you initialize variables before triggers are defined.