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 < // overrides printMethod in Superclass public void printMethod() < 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
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)Invocation of a superclass constructor must be the first line in the subclass constructor.
The syntax for calling a superclass constructor is
super();
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.