Use of 'super' keyword when accessing non-overridden superclass methods

Question

I'm trying to get the hang of inheritance in Java and have learnt that when overriding methods (and hiding fields) in sub classes, they can still be accessed from the super class by using the 'super' keyword.

What I want to know is, should the 'super' keyword be used for non-overridden methods?

Is there any difference (for non-overridden methods / non-hidden fields)?

I've put together an example below.

public class Vehicle {
    private int tyreCost;

    public Vehicle(int tyreCost) {
         this.tyreCost = tyreCost;
    }

    public int getTyreCost() {
        return tyreCost;
    }        
}

and

public class Car extends Vehicle {
    private int wheelCount;

    public Vehicle(int tyreCost, int wheelCount) {
        super(tyreCost);
        this.wheelCount = wheelCount;
    }   

    public int getTotalTyreReplacementCost() {
        return getTyreCost() * wheelCount;
    }   
}

Specifically, given that getTyreCost() hasn't been overridden, should getTotalTyreReplacementCost() use getTyreCost(), or super.getTyreCost() ?

I'm wondering whether super should be used in all instances where fields or methods of the superclass are accessed (to show in the code that you are accessing the superclass), or only in the overridden/hidden ones (so they stand out).

Answer 1

Don't use the super keyword to refer to other methods which aren't overridden. It makes it confusing for other developers trying to extend your classes.

Let's look at some code which does use the super keyword in this way. Here we have 2 classes: Dog and CleverDog:

/* file Dog.java */
public static class Dog extends Animal {

    private String name;

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

    public String getName() {
        return name;
    }

}

/* file CleverDog.java */
public class CleverDog extends Dog {

    public CleverDog(String name) {
         super(name);
    }

    public void rollover() {
        System.out.println(super.getName()+" rolls over!");
    }

    public void speak() {
        System.out.println(super.getName() + " speaks!");
    }

}

Now, imagine you are a new developer on the project, and you need some specific behavior for a clever dog who is on TV: that dog has to do all its tricks, but should go by its fictitious TV name. To accomplish this, you override the getName(...) method...

/* file DogOnTv.java */
public class DogOnTv extends CleverDog {

    String fictionalName;

    public DogOnTv(String realName, String fictionalName) {
        super(realName);
        fictionalName = fictionalName;
    }

    public String getName() {
        return fictionalName;
    }

}

... and fall into a trap set by the original developer and their unusual use of the super keyword!

The code above isn't going to work - because in the original CleverDog implementation, getName() is invoked using the super keyword. That means it always invokes Dog.getName() - irrelevant of any overriding. Consequently, when you use your new DogOnTv type...

    System.out.println("Showcasing the Clever Dog!");
    CleverDog showDog = new CleverDog("TugBoat");
    showDog.rollover();
    showDog.speak();

    System.out.println("And now the Dog on TV!");
    DogOnTv dogOnTv = new DogOnTv("Pal", "Lassie");
    dogOnTv.rollover();

... you get the wrong output:

Showcasing the Clever Dog!
Tugboat rolls over!
Tugboat speaks!

And now the Dog on TV!
Pal rolls over!
Pal speaks!

This is not the usual expected behavior when you override a method, so you should avoid creating this kind of confusion using the super keyword where it doesn't belong.

If, however, this is actually the behavior you want, use the final keyword instead - to clearly indicate that the method can't be overridden:

/* file CleverDog.java */
public class CleverDog extends Dog {

    public CleverDog(String name) {
         super(name);
    }

    public final String getName() { // final so it can't be overridden
        return super.getName();
    }

    public void rollover() {
        System.out.println(this.getName()+" rolls over!"); // no `super` keyword
    }

    public void speak() {
        System.out.println(this.getName() + " speaks!"); // no `super` keyword
    }

}

Answer 2

You are doing the right way by not using the super keyword for accessing getTyreCost.

But you should set your members private and only use the getter method.

Using super keyword should be reserved for constructors and overridden methods which need to explicitly call the parent method.