Override compareTo() method in custom Java class?

Question

I have both a Dog and Cat class which look something like this:

public class Dog implements Speakable
{
private String name;

public Dog(String name)
{
    this.name = name;
}

public void speak()
{
    System.out.println("Woof! Woof!");
}

public String toString()
{
    return "Dog: " + name;
}

}

These classes both implement an interface I have created called Speakable which looks like this:

public interface Speakable
{
  public void speak();
}

This Speakable interface exists because I need a reference variable that allows me to add Dogs and Cats to the same ArrayList and still invoke the speak() method on them.

I also need to override the compareTo() method of the Comparable interface so I can compare the names of dogs. When calling this method, I think my code will look like this: a.compareTo(b). I want this new method to return -1 if a is greater than b, 1 if a is less than b, and 0 if they are equal.

I think I need the Dog class to specifically implement Comparable in addition to Speakable so the Comparable interface I have written is this:

public interface Comparable<Dog>
{
    public int compareTo(Object obj);
}

How do I override the compareTo() method to meet the needs I listed above? I know I will need several if statements, but I can't think of a way to write this without calling the compareTo() method within the new compareTo() method. Is that legal? Shouldn't the compareTo() method from the comparable interface already contain these decisions?

"How do I override the compareTo() method to meet the needs I listed above" - what are your needs. I can see that you want to compare only name — Rahman, Oct 08 '15 at 14:27
What do you mean by `a is greater than b, a is less than b`? How do you compare `dog`s? — TDG, Oct 08 '15 at 14:27
You are not supposed to create your own `Comparable` interface. That's why it's generic - each class can implement it for itself. — RealSkeptic, Oct 08 '15 at 14:28

score 3 · Answer 1 · answered Oct 08 '15 at 14:28

3

Remove your custom Comparable, and use the Java Comparable<T>. And you could modify Speakable to extend Comparable. Something like,

public interface Speakable extends Comparable<Speakable>

Then every Speakable is also a Comparable<Speakable>

answered Oct 08 '15 at 14:28

Elliott Frisch

198,278
20
158
249

When I do this, I get an error that says Dog is not abstract and does not override abstract method compareTo(speakable) – JoeSchmuck Oct 08 '15 at 15:23
You'd have to override `compareTo(Speakable)`. If you want to be able to compare `Cat`s and `Dog`s. – Elliott Frisch Oct 08 '15 at 15:34

score 2 · Answer 2 · answered Oct 08 '15 at 14:37

2

Use Java Comparable interface in Dog class. Somenthing like this:

public class Dog implements Speakable, Comparable<Dog>{
    @Override
    public int compareTo(Dog o) {
        return this.name.compareTo(o.name);
    }

}

answered Oct 08 '15 at 14:37

ocastillo

21
3

Error: Cannot find variable name when I write it like this. – JoeSchmuck Oct 08 '15 at 15:20

Override compareTo() method in custom Java class?

2 Answers2