The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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) { 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();
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.
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.