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Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
Section: Inheritance
Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Rewriting Interfaces
Summary of Interfaces
Questions and Exercises
Inheritance
Overriding and Hiding Methods
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Home Page > Learning the Java Language > Interfaces and Inheritance
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Using the Keyword super

> ### Accessing Superclass Members

> If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword `super`. You can also use `super` to refer to a hidden field (although hiding fields is discouraged). Consider this class, `Superclass`:

> >     public class Superclass {

> >         public void printMethod() {
> >             System.out.println("Printed in Superclass.");
> >         }
> >     }

> Here is a subclass, called `Subclass`, that overrides `printMethod()`:

> >     public class Subclass extends Superclass {

> >         public void printMethod() { //overrides printMethod in Superclass
> >             super.printMethod();
> >             System.out.println("Printed in Subclass");
> >         }
> >         public static void main(String[] args) {

> >         Subclass s = new Subclass();
> >         s.printMethod();
> >         }

> >     }

> Within `Subclass`, the simple name `printMethod()` refers to the one declared in `Subclass`, which overrides the one in `Superclass`. So, to refer to `printMethod()` inherited from `Superclass`, `Subclass` must use a qualified name, using `super` as shown. Compiling and executing `Subclass` prints the following:

> >     Printed in Superclass.
> >     Printed in Subclass

> ### Subclass Constructors

> The following example illustrates how to use the `super` keyword to invoke a superclass's constructor. Recall from the `Bicycle` example that `MountainBike` is a subclass of `Bicycle`. Here is the `MountainBike` (subclass) constructor that calls the superclass constructor and then adds initialization code of its own:

> >         public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) {
> >             super(startCadence, startSpeed, startGear);
> >             seatHeight = startHeight;
> >         }

> Invocation of a superclass constructor must be the first line in the subclass constructor.

> The syntax for calling a superclass constructor is

> >     super();
> >     --or--
> >     super(parameter list);

> With `super()`, the superclass no-argument constructor is called. With `super(parameter list)`, the superclass constructor with a matching parameter list is called.

> > Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. `Object` does have such a constructor, so if `Object` is the only superclass, there is no problem.

> If a subclass constructor invokes a constructor of its superclass, either explicitly or implicitly, you might think that there will be a whole chain of constructors called, all the way back to the constructor of `Object`. In fact, this is the case. It is called constructor chaining, and you need to be aware of it when there is a long line of class descent.

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