java.lang.StackOverflowError but i'm not looping anything

Question

I have two methods one in the main class and one in a class called 'DeleteWall' .I'm creating instances of each class so I can access their methods .I've read all the posts on stackOverflows but I don't see what i'm doing wrong ...like i'm not looping it to make a new instance as far as I can see anyway.I'll highlight the lines which are causing the errors as intellij says.I'm only calling demo once aswell .Oh and the method 'delNode' is in the main class hence the instance of it 'werk'.

    //Main class
    DeleteWall ok=new DeleteWall(); //error line
    public void demo(){
    System.out.println("running");
    ok.delWalls(30,0,30,30);
    System.out.println("didnt stop");
    }
     //DeleteWall class
     public class DeleteWall  {

     Main werk = new Main(); //error line

     public  void delWalls(int Sx, int Sy, int Ex, int Ey) {


     werk.delNode(Sx, Sy, Ex, Ey);
     }

     }

the error

In your main class, a `DeleteWall` instance is created, which in turn creates a `Main` instance, which in turn creates a `DeleteWall` instance, which in turn ... well, you get the idea. — MC Emperor, Jul 13 '19 at 21:57
Related: https://stackoverflow.com/questions/57023090/java-lang-stackoverflowerror-but-im-not-looping-anything — MC Emperor, Jul 13 '19 at 21:58
@khelwood Indeed, perhaps this will give a rough idea of recursion. — MC Emperor, Jul 13 '19 at 22:33

score 4 · Accepted Answer · answered Jul 13 '19 at 22:47

The main problem is reduced to

public class Main {
    DeleteWall ok = new DeleteWall();
}

and

public class DeleteWall  {
    Main werk = new Main();
}

if any of these class gets instantiated, an instance of the other must be created which in turn will again create a new instance of first, and so on. The whole design is strange, like declaring that each House always contains a Car and each Car always contains a House.

Solving the problem: have the DeleteWall receive the instance of Main instead of creating a new one. Some possibilities:

in the constructor

public class Main {
    DeleteWall ok = new DeleteWall(this);
}

////

public class DeleteWall  {
    Main werk;
    public DeleteWall(Main main) {
        werk = main;
    }
}

when calling

public class Main {
    DeleteWall ok = new DeleteWall();
    public void demo() {
        ok.delWalls(this, 30, 0, 30, 30);
    }
}

////

public class DeleteWall  {
    public void delWalls(Main werk, ...) {
        werk.delNodes(...);
    }
}

(there are many other possibilities, it depends heavily on what the program should do, how the problem is modeled, preferences, ...)

ayy Carlos coming throughhhhhh, thanks alot man. It works now ,Doing my dance moves to celebrate — Fred, Jul 13 '19 at 23:09

score 3 · Answer 2 · answered Jul 13 '19 at 21:56

3

You have an instance of DeleteWall inside Main and and instance of Main inside DeleteWall. Now when it tries to create the Main class it will need to create an DeleteWall instance for the field DeleteWall ok= new DeleteWall(); when it tries to do that, it will need to create an instance of Main for the field Main werk = new Main(); inside DeleteWall. Then it goes back to create Main and there you have your infinite loop.

answered Jul 13 '19 at 21:56

Willem

992
6
13

Ok yeah I sorta get what your saying . What's the convention for calling methods from other classes then? cheerzzz – Fred Jul 13 '19 at 22:00
You can just create an instance of the other class in a non-static method and call the method on that instance – Willem Jul 13 '19 at 22:02
so like , public happy void(){ Main werk = new Main();} ? – Fred Jul 13 '19 at 22:07
yes that's right – Willem Jul 13 '19 at 22:09
okkkkk but then where do I call that method? – Fred Jul 13 '19 at 22:11
Like you already have: `werk.delNode(Sx, Sy, Ex, Ey);` Just move `Main werk = new Main(); //error line` Inside the method below (where you use it) that will fix it – Willem Jul 13 '19 at 22:13

score 1 · Answer 3 · edited Sep 19 '19 at 19:51

Your Main class has a field ok which is initialized with a DeleteWall object. Your DeleteWall class has a field werk which is initialized with a Main object.

So, when the program starts running, it instantiates the Main class. Since all fields are assigned their default values when the class is instantiated, this needs to instantiate the DeleteWall class, so that werk can be initialized. Instantiating the DeleteWall class needs to instantiate the Main class, so that ok can be initialized. Instantiating the Main class needs to instantiate the DeleteWall class, so that werk can be initialized. Instantiating the DeleteWall class needs to instantiate the Main class, so that ok can be initialized. Instantiating the Main class needs to instantiate the DeleteWall class, so that werk can be initialized. ...

java.lang.StackOverflowError but i'm not looping anything

3 Answers3