创建子类时出现StackOverflowError

Question

My superclass:

public class Node {
    public MyPoint myPoint = new MyPoint();
}

My empty subclass:

public class MyPoint extends Node {

}

The question below was the closest to the problem I could find, however my subclass, in this case, is empty and has no constructors.

Calling a subclass constructor from a superclass

I get the following error:

Exception in thread "JavaFX Application Thread" java.lang.StackOverflowError
...
at engine.MyPoint.<init>(MyPoint.java:3)
at engine.Node.<init>(Node.java:28)
at engine.MyPoint.<init>(MyPoint.java:3)
at engine.Node.<init>(Node.java:28)
at engine.MyPoint.<init>(MyPoint.java:3)
at engine.Node.<init>(Node.java:28)
...

I don't understand why and how there is no recursion as I see it. How is this

public MyPoint myPoint = new MyPoint();

Different than this (for example in JavaFX), while creating a new Group doesn't give me the error?

public Group group = new Group();

Answer 1

MyPoint extends Node , so when you create an instance of MyPoint , you'll first have to initialize its parent Node . This means initializing the myPiont member and so on until you get a StackOverflowError .

There isn't enough context here to suggest changes, but you should probably lose this member.

Answer 2

If your Node class absolutely needs a MyPoint field, you have a few options:

• Are you absolutely sure that MyPoint needs to extend from Node? Is it in fact a Node creature? If not, get rid of this, problem solved. If so, then there are other fixes....
• Is MyPoint not unique for each Node instance? If not, make it a static field, problem solved. I doubt this is so, leading us to...
• If MyPoint absolutely needs to extend from Node, and needs to be unique for each Node instance, then you must pass the reference in, meaning change this:

public class Node {
    public MyPoint myPoint = new MyPoint();
}

to this:

public class Node {
    public MyPoint myPoint;  // note that MyPoint NOT created here

    public Node(MyPoint myPoint) {
        this.myPoint = myPoint; // but rather passed in HERE
    }
}   

This way you don't recursively create Node objects each time a Node is created, but rather separate the creation of MyPoint instances from Node instances. Alternatively, this can be also be solved using a setter method.