How to properly connect classes (Stack overflow in constructors)

Question

I'm relatively new to Java and I'm coding extensions/plugins for online game Minecraft.

I've recently started "bigger" projects which require more classes to preserve neatness. I am still trying to fully understand how constructors work, but most recent try resulted into Stack Overflow (one constructor calling another, in loop).

In my case, for example, I have two classes, Main and MySQL.

Main class initializes plugin, MySQL class contacts database and returns data to any class which calls it.

Example problematic code:

Main.java

class Main extends JavaPlugin {

    private final MySQL mysql = new MySQL();

    //Method which is called first 
    void onEnable(){

        //Just an example of initializing
        mysql.init();
        mysql.open();
    }
}

MySQL.java

class MySQL {

    private final Main main = new Main();

    void init(){}
    void open(){}
}

I know what the problem is, I just don't know proper solution, which would work with even more classes.

Also if I would have another class, main would connect to MySQL, another class would communicate with it, I should do something like main.mysql.method ?

Simplified, easy to understand explanation on how constructors and instances ("new" keyword) work would be much appreciated. Thanks in advance !

In main class, there's a method which alerts specific users about error, so I have to call _main.mysqlErrors_ — ShiiNko, Aug 09 '12 at 09:48

score 1 · Accepted Answer · edited Aug 09 '12 at 09:57

1

First of all, just as a pointer for future applications, you might want to take a look at Design Patterns, they tend to make design cleaner and easier to maintain.

In your case you have created a Circular Reference. The easiest way to get around it would be to make something like so:

class Main extends JavaPlugin {

    private final MySQL mysql;

    public Main(MySQL mysql)
    {
        this.mysql = mysql
    }

    //Class which is called first 
    void onEnable() {

        //Just an example of initializing
        mysql.init();
        mysql.open();
    }
}

class MySQL {

    private final Main main = new Main(this);

    void init(){}
    void open(){}
}

This should solve the problem.

That being said, I am assuming that the MySQL class will have database related mechanisms. Such mechanisms are usually exposed through a Singleton design pattern. This design pattern should allow you to access the methods within the MySQL class only when you need them, thus also avoiding the circular reference issue.

edited Aug 09 '12 at 09:57

maba

47,113
10
108
118

answered Aug 09 '12 at 09:53

npinti

51,780
5
72
96

Thanks for an answer, however does _private final MySQL mysql;_ call MySQL class which than initializes _mysql_ variable ? – ShiiNko Aug 09 '12 at 09:57
That won't work if `Main` is the literal top-level entry point, which very often must have a constructor without arguments. Fixing `MySQL` seems easier to me. – Volker Stolz Aug 09 '12 at 09:57
Thanks to both of you, I've been doing this before, I just thought there's easier way. Anyway, thanks. – ShiiNko Aug 09 '12 at 10:02
@user1587051: The variable is initialized within the constructor. Final variables can only be assigned once, but should allow you to declare it and assign it at a later stage. – npinti Aug 09 '12 at 10:02
@ShiDoiSi: Yes I do agree. I just did it that way to get the point to the OP. – npinti Aug 09 '12 at 10:03

score 0 · Answer 2 · answered Aug 09 '12 at 09:54

The field mysql must get instantiated every time an object of type Main is created. When a new MySQL instance is created, since it has the field Main main, a new Main instance is created. 20 goto 10 :-)

In the MySQL class, you can avoid creating another Main instance in two ways: either make sure that you refer to static members of Main, or make MySQL a nested class within Main, in which case you don't need the extra field.

How to properly connect classes (Stack overflow in constructors)

2 Answers2