Why won't my comparative if statement of two variables work?

Question

In the following piece of code, once executed, it will change the state diet of an object hek based on a random number generator. I also have an undo() function which works perfectly fine in run-time.

What I am trying to do is: When the state of the object is changed but, the previous state prevDiet is equal to the changed state currentDiet, I want the program to call undo().

import java.util.Random;
import javax.swing.JOptionPane;

public class changeDietCom implements Command {

    Hek hek;
    Random rand = new Random();
    int x;
    DietBehaviour prevDiet;
    DietBehaviour currentDiet;

    public changeDietCom(Hek hek) {
        this.hek = hek;
    }

    public void execute() {

        x = rand.nextInt(3) + 1;

        prevDiet = hek.getDietBehaviour();
        if (x == 1) {

            hek.setDietBehaviour(new Enemies());
            JOptionPane.showMessageDialog(null, "Hek's diet has been changed!", "Changing diet", JOptionPane.INFORMATION_MESSAGE);

            currentDiet = hek.getDietBehaviour();
            hek.performDiet();

            if (prevDiet == currentDiet) {
                JOptionPane.showMessageDialog(null, "Diet hasn't changed!", "Current Diet = Prev Diet", JOptionPane.INFORMATION_MESSAGE);
                undo();
            }
        }
        else if (x == 2) {

            hek.setDietBehaviour(new ArmouredKnights());
            JOptionPane.showMessageDialog(null, "Hek's diet has been changed!", "Changing diet", JOptionPane.INFORMATION_MESSAGE);

            currentDiet = hek.getDietBehaviour();
            hek.performDiet();

            if (prevDiet == currentDiet) {
                JOptionPane.showMessageDialog(null, "Diet hasn't changed!", "Current Diet = Prev Diet", JOptionPane.INFORMATION_MESSAGE);
                undo();
            }
        }
        else if (x == 3) {

            hek.setDietBehaviour(new BigMac());
            JOptionPane.showMessageDialog(null, "Hek's diet has been changed!", "Changing diet", JOptionPane.INFORMATION_MESSAGE);

            currentDiet = hek.getDietBehaviour();
            hek.performDiet();

            if (prevDiet == currentDiet) {
                JOptionPane.showMessageDialog(null, "Diet hasn't changed!", "Current Diet = Prev Diet", JOptionPane.INFORMATION_MESSAGE);
                undo();
            }
        }
    }

    public void undo() {
        JOptionPane.showMessageDialog(null, "Undoing diet change...", "Undoing Diet Change", JOptionPane.INFORMATION_MESSAGE);
        hek.setDietBehaviour(prevDiet);
        hek.performDiet();
    }
}

But whenever both the previous state prevDiet and the current state currentDiet are equal to each other during run-time the if statement doesn't return true, and I can't see why not? Could someone please maybe shed some light as to why and how I can fix this?

I should mention that I am using the command pattern for the design of this part of the program.

It looks like you're new here, you should know that If an answer works for you its courteous to accept and upvote, if it doesn't work you may be able to comment on it and it will be altered or explained for you. — Kevin Klute, Nov 19 '15 at 15:00
This answered was useful, but didn't solve my problem. `DietBehaviour` is an interface that I have implemented from and not a concrete class so I'm confused when people are pointing out that I should override the `equals()` method? — PureShpongled, Nov 19 '15 at 15:08

Kevin Klute · Accepted Answer · 2015-11-19T14:44:27.267

1

DietBehavior objects are object types, so you can't use == to compare them. What you want to use is .equals(). If DietBehavior is your own class then you will have to implement the method, but its pretty simple.

public boolean equals(Object other) {
// compare them here based on instance variables
}

edited Nov 19 '15 at 14:44

answered Nov 19 '15 at 14:35

Kevin Klute

450
1
4
22

1

The argument for `equals` must be an `Object`, not a `DietBehaviour`. – Manu Nov 19 '15 at 14:43
@Manu edited, thank you – Kevin Klute Nov 19 '15 at 14:44
DietBehaviour is an interface and not a class, so I'm not really sure how I would go about that? – PureShpongled Nov 19 '15 at 15:03
@PureShpongled `getDietBehavior()` must return an object though and you need a class to have an object, so there must be a class that implements `DietBehavior`. Can you alter that class? – Kevin Klute Nov 19 '15 at 15:05
1

Ah! I see, go through the the classes which implement the interface, i.e. `ArmouredKnights()`, `BigMacs()` and `Enemies()`, and alter each `equals()` method of said objects? – PureShpongled Nov 19 '15 at 15:10
Yes if you implement `.equals()` correctly and then use it instead of `==` that should work. – Kevin Klute Nov 19 '15 at 15:11
Thanks so much! Finally have it fixed! You guys are awesome! – PureShpongled Nov 19 '15 at 15:28
@PureShpongled no problem, please accept the answer – Kevin Klute Nov 19 '15 at 15:30

score 1 · Answer 2 · edited May 23 '17 at 10:27

1

With if (prevDiet == currentDiet) {, you compare object references instead of the actual values. Use if (prevDiet.equals(currentDiet)) { instead, and make sure you override the equals() method of DietBehaviour.

Note that when you override equals(), you should also override hashCode() and the implementations should be consistent. More information about this can be found here.

edited May 23 '17 at 10:27

Community

1
1

answered Nov 19 '15 at 14:35

Manu

1,474
1
12
18

Why won't my comparative if statement of two variables work?

2 Answers2