Question

In most of my java applications, I have a Controller (Logic) class and a GUI class. I usually need the Logic class to call methods on the GUI class, and vice versa. Now, when the Logic constructs the GUI, I pass a reference to “this” in the constructor so the GUI can call methods on the Logic. Here is some example code.

public class Logic {
   private int num = 2;
   private final GUI gui;

   public Logic(){
       gui = new GUI(this);
   }

   public int getNum(){
      return num;
   }
}


public class GUI {
   private final Logic logic;

   public GUI(Logic logic){
      this.logic = logic;
   }

   public void calledLater(){
      int num = logic.getNum();
   }
}

My question is: Is this the best OO way to create an aggregation relationship, or am I messing up my design?

Answer 1

Passing this to a UI class is not in itself a problem, as long as the methods of the class are not called through this, as in your example. You call getNum right in the constructor of the GUI, which should be avoided: your Logic class may not be ready to return the correct result.

One thing to note is that you couple your GUI to the Logic class, which may be too much coupling. You should consider extracting an interface from Logic, and let GUI interact only with what’s intended for it:

interface DataSourceForTheGui {
    int getNum();
}

class Logic implements DataSourceForTheGui {
    int getNum() {
        return n;
    }
}

class GUI {
    private readonly DataSourceForTheGui data;
    public GUI(DataSourceForTheGui data) {
        this.data = data;
    }
    public void display() {
        int k = data.getNum();
    }
}