How do I use an object reference as an argument if it is instantiated after the class taking the argument?

Question

So I have this code:

package com.erikbalen.game.rpg;
import com.erikbalen.platform.*;
import javax.swing.JFrame;

public class World extends Engine {

    public static void main(String[] args) {        

    Gui display = new Gui(/*takes a Player argument so i can get certain variables*/);
    display.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    display.setSize(300,220);
    display.setVisible(true);

    Player player1 = new Dps("ebalen", display);
    Player player2 = new Healer("frankypanky", display);

    }

}

package com.erikbalen.game.rpg;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class Gui extends JFrame implements ActionListener {

    /**
     * 
     */
    private static final long serialVersionUID = -384241835772507459L;
    private JLabel playerInfo;
    private JTextField textField;
    private final static String newline = "\n";
    private JTextArea feed;
    private JScrollPane scrollPane;
    private Player player;

    public Gui(Player currentPlayer) {
        super("Erik's RPG");
        this.player = currentPlayer;
        setLayout(new FlowLayout());

        playerInfo = new JLabel("<html>Health = " + currentPlayer.getHealth() + " | " + "Mana = " + currentPlayer.getMana() + "</html>");

        playerInfo.setBorder(BorderFactory.createTitledBorder(currentPlayer.getName()));

        textField = new JTextField(20);

        textField.addActionListener(this);

        feed = new JTextArea(5, 20);
        scrollPane = new JScrollPane(feed);
        feed.setEditable(false);

        add(playerInfo);
        add(feed);
        add(textField);
        add(scrollPane);

    }

    public void actionPerformed(ActionEvent textBox) {
         String text = textField.getText();
         this.player.chat(text);

    }

    public void printText(String text) {
        feed.append(text + "\n");
        feed.setCaretPosition(feed.getDocument().getLength());
    }

}

My problem is that class Gui takes a Player as an argument and Player takes Gui as an argument. How do I let both objects take each other as arguments? Feel free to tell me if my code is inefficient.

Why would `Gui` take a `Player` as a constructor argument? I think this is probably bad design. Please show your `Gui` code. — Paul Bellora, Sep 13 '11 at 05:22
Because Gui displays some of the player's information and I didn't know how else for it to access the players variables. — Erik Balen, Sep 13 '11 at 05:24
You can make `setPlayer` method instead of using constructor parameters. — , Sep 13 '11 at 05:30

score 5 · Accepted Answer · answered Sep 13 '11 at 05:29

Well, ideally you should try to break the circular dependency, but otherwise, you could:

Create the GUI, create the players by passing in the GUI, then add the players to the GUI
Create the players, create the GUI by passing in the players, then set the GUI for the players

Create the GUI within the player constructor:

Player(String name)
{
    GUI gui = new GUi(this);
    ...
}

All of these are non-ideal:

You may well want your classes to be immutable, which makes the first two options nasty
Publishing the this reference before your constructor has finished executing has various issues, both in terms of thread safety and the memory model, as well as potentially allowing the GUI constructor to call back to the Player object before it's fully initialized.

This goes back to "try to break the dependency" - but if that's really impossible, I would probably favour the first option, without knowing anything else. It makes sense to be able to add players to a game - it makes less sense to set the GUI for a player after the fact, IMO.

+1, I would emphasise more on *try to break dependency*. I wouldn't want a `StackOverflowError` to occur (sarcasm). Besides, debugging a circular depency is not a simple eye-opener without breaking few teeth and bones. — Buhake Sindi, Sep 13 '11 at 05:33
@ebalen: It's hard to say without knowing why there's a circular dependency. It seems very odd for the GUI constructor to take a *single* Player as the "current" one. Have you considered a setCurrentPlayer method which is called each time the current player changes? — Jon Skeet, Sep 13 '11 at 05:58

Paul Bellora · Answer 2 · 2011-09-13T05:36:11.533

Try giving the Gui class methods for updating what is displayed to the user/users. Make the directing code (main() for example) responsible for updating the Gui with the right information as events happen.

Neither Gui nor Player should have to take each other as constructor arguments - Gui should only be responsible for displaying information it is told to, and Player should only be a logical representation of a game piece. Event-driven functionality should be reserved for the directing code.

score 1 · Answer 3 · answered Sep 13 '11 at 05:30

Your stuck in what we call a circular dependency. This is almost everytime the result of a bad design.

There are still solutions, but there are not really pretty. For an elegant way, you should rethink your design. Does the GUI really need the player ? Maybe you could create a method to set the player later on. If it's not possible, you could also create a setter in the player. You won't be able to set both at construction time.

How do I use an object reference as an argument if it is instantiated after the class taking the argument?

3 Answers3