The super keyword in Java is a reference variable which is used to refer immediate parent class object.
Whenever you create the instance of subclass, an instance of parent class is created implicitly which is referred by super reference variable.
Usage of Java super Keyword
- super can be used to refer immediate parent class instance variable.
- super can be used to invoke immediate parent class method.
- super[] can be used to invoke immediate parent class constructor.
1] super is used to refer immediate parent class instance variable.
We can use super keyword to access the data member or field of parent class. It is used if parent class and child class have same fields.
Test it NowOutput:
In the above example, Animal and Dog both classes have a common property color. If we print color property, it will print the color of current class by default. To access the parent property, we need to use super keyword.
2] super can be used to invoke parent class method
The super keyword can also be used to invoke parent class method. It should be used if subclass contains the same method as parent class. In other words, it is used if method is overridden.
Test it NowOutput:
In the above example Animal and Dog both classes have eat[] method if we call eat[] method from Dog class, it will call the eat[] method of Dog class by default because priority is given to local.
To call the parent class method, we need to use super keyword.
3] super is used to invoke parent class constructor.
The super keyword can also be used to invoke the parent class constructor. Let's see a simple example:
Test it NowOutput:
animal is created dog is created
Note: super[] is added in each class constructor automatically by compiler if there is no super[] or this[].
As we know well that default constructor is provided by compiler automatically if there is no constructor. But, it also adds super[] as the first statement.
Another example of super keyword where super[] is provided by the compiler implicitly.
Test it NowOutput:
animal is created dog is created
super example: real use
Let's see the real use of super keyword. Here, Emp class inherits Person class so all the properties of Person will be inherited to Emp by default. To initialize all the property, we are using parent class constructor from child class. In such way, we are reusing the parent class constructor.
Test it NowOutput:
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 { // 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
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
or:
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.