Java inheritance and string interpolation

Question

I'm learning OOP in Java and I'm trying to decide if I can/should use inheritance in this example. Do not pay attention to access modifiers and things like that.

There are two kind of pets:

dogs (name, breed, age, owner)
cats (name, breed, color, owner)

Dog.toString() should return "Name: name, Breed: breed, Age: age, Owner: owner"

Cat.toString() should return "Name: name, Breed: breed, Color: color, Owner: owner"

My question is: should I implement toString() in an abstract class and override in derived classes? How? I mean, there is only one field. I can't see the gain.

So, I did:

abstract class Pet {

    String name;
    String breed;
    String owner;

    // getters setters

    public String toString() { 

         return ????
    } 

}

class Dog extends Pet {

    int age; 

    public String toString() {

       return String.format("Name: %s, Breed:  %s, Age: %d, Owner: %s", name, breed, age, owner)

    }

}

class Cat extends Pet {

    String color; 

    public String toString() {

        return String.format("Name: %s, Breed:  %s, Color: %s, Owner: %s", name, breed, color, owner)

    }

}

Why does a Dog have an age and not a Cat? Or a Turtle? Or a Rock? Age should be a property of Pet rather than just Dog. — NomadMaker
– NomadMaker, Commented Dec 29, 2020 at 21:23

score 1 · Accepted Answer · 2020-12-29 21:50:36Z

1

This is a situation where you could consider applying the Template Method design pattern. In this pattern, you define the reusable part of your method in the abstract class, and implement only the variable parts in the subclasses. For example:

abstract class Pet {

// ...

    protected abstract String extra(); // Or, return the empty String

    public final String toString() { 
        return String.format("Name: %s, Breed:  %s, %s, Owner: %s", 
            name, breed, extra(), owner);
    } 
}

In your subclasses:

class Dog extends Pet {

    int age; 

    public String extra() {
        return String.format("Age: %d");
    }
}

edited Dec 29, 2020 at 21:50

answered Dec 29, 2020 at 21:00

user11595728

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1 Comment

Turing85 Over a year ago

A slight variation would be to make extra() not abstract and let it return an empty String. This avoids overriding extra() if it needs not to be overridden at the expense that overriding is not enforced, i.e. the class's author could forget to override extra().

mjlitz · Accepted Answer · 2020-12-29 20:56:52Z

1

I wouldn't write the toString() in the abstract class. It adds very little in terms of reusing code and makes the statement quite a bit harder to read. Having super.toString() in the middle of the statement would be confusing.

answered Dec 29, 2020 at 20:56

mjlitz

19212 bronze badges

1 Comment

InglouriousBastard Over a year ago

yes, that was my initial thought. but i was looking something like the accepted answer.

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