Java: super keyword

May 19, 2016 SJ OOPS 0

In this article, we will discuss super keyword in Java

super keyword in java is used to refer immediate parent-class

 

Usage of super keyword in Java

  • Instance variable: super keyword is used to refer instance-variable of immediate parent-class (super-class)
  • Super constructor: super() constructor call; is used to invoke constructor of immediate parent-class (super-class)
  • Overridden Method: <methodName> is used to invoke instance method of immediate parent-class (super-class)

Note: super cannot be used refer anything in static context (Cannot use super in a static context)

 

Let us understand each one in detail with examples

Usage 1: super keyword to access variables of immediate parent-class (super-class)

  • Syntax: super.<variable_name>

Case 1.1. Variable Hiding

  • Variable Hiding: If instance-variable name of both child-class and parent-class are same, then child-class instance-variable hides the parent-class’ instance-variable

Let us take a close look at the need of super in inheritance concept

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge = 20;
	String siteName = "oracle.com";

	// other things
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	// display() method to print website details
	void displaySiteDetail() {
		System.out.println("Website age in years : " + siteAge);
		System.out.println("Website name : " + siteName);
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		child.displaySiteDetail();
	}
}

Output:

Website age in years : 2
Website name : BenchResources.Net

Explanation:

  • There are 2 instance-variables viz.; siteAge and siteName in both parent/child classes
  • When we access these instance-variables from display() method, then it prints current class’ values of same variable-name (instead of immediate super-class)
  • Reason: current class’ instance-variables hides parent-class’s instance-variable which are having same variable-name

 

1.1.Q. What if we need to access parent class’ instance-variables ?

  • By use of super keyword, we can invoke/call/refer immediate parent-class’ instance-variable

 

Case 1.2. Accessing parent-class variables using super keyword

Let us look at the modified code to access parent-class’ instance-variable

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge = 20;
	String siteName = "oracle.com";

	// other things
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	// display() method to print website details
	void displaySiteDetail() {
		System.out.println("Website age in years : " 
				+ super.siteAge);
		System.out.println("Website name : " 
				+ super.siteName);
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		child.displaySiteDetail();
	}
}

Output:

Website age in years : 20
Website name : oracle.com

Explanation:

  • Using super keyword, we can invoke immediate parent-class’ variable
  • siteAge accesses parent-class’ site-age instance-variable
  • similarly, siteName accesses parent-class’ site-name instance-variable
  • Note: If the instance-variable names of child-class and parent-class is different, then there is no need of super keyword; instead we can directly access it

 

Usage 2: Invoke super() constructor of immediate parent class

  • Syntax: super() constructor call; for invoking default constructor of immediate parent-class
  • Syntax: super(args) constructor call; for invoking parameterized constructor of immediate parent-class

Case 2.1. Implicit super() constructor call inserted by compiler during compilation

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge = 20;
	String siteName = "oracle.com";

	//	default no-arg constructor 
	ParentClass(){
		System.out.println("default constructor of Parent class");
	}

	// other codes here
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	//	default no-arg constructor 
	ChildClass() {
		System.out.println("default constructor of Child class");
	}

	// display() method to print website details
	void displaySiteDetail() {
		System.out.println("Website age in years : " 
				+ super.siteAge);
		System.out.println("Website name : " 
				+ super.siteName);
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		//		child.displaySiteDetail();
	}
}

Output:

default constructor of Parent class
default constructor of Child class

Explanation:

  • there is no explicit super() constructor call
  • but still parent-class’ constructor is invoked
  • Because, compiler during compilation inserts super() constructor call, as the 1st statement in all constructor
  • Due to which parent-class’ constructor is invoked as you can see from the above output

 

Case 2.2. Explicit super() constructor call inserted by programmer

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge = 20;
	String siteName = "oracle.com";

	//	default no-arg constructor 
	ParentClass(){
		// here super() will be inserted; 
		// which will invoke Object class' default-constructor
		System.out.println("default constructor of Parent class");
	}

	// other codes here
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	//	default no-arg constructor 
	ChildClass() {
		// explicitly inserted
		super(); // it should be 1st statement of constructor
		System.out.println("default constructor of Child class");
	}

	// display() method to print web-site details
	void displaySiteDetail() {
		System.out.println("Website age in years : " 
				+ super.siteAge);
		System.out.println("Website name : " 
				+ super.siteName);
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		// child.displaySiteDetail();
	}
}

Output:

default constructor of Parent class
default constructor of Child class

Explanation:

  • there is an explicit super() constructor call inserted by programmer
  • So, it invokes the parent-class’ default no-arg constructor and output remain same as that of previous example
  • Note: Make sure while inserting super() constructor call explicitly; it must be 1st statement of the constructor;
  • Otherwise compilation error will be thrown stating reason “Constructor call must be the first statement in a constructor”
  • Note :- There is no point in explicitly invoking super() constructor call, when we know compiler inserts super call implicitly during compilation

 

2.2.Q. So, when it is required ?

  • To invoke immediate parent-class’ parameterized-constructor using super(arguments);

 

Case 2.3. Explicit super() constructor call inserted by programmer

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance-variable
	int siteAge;
	String siteName;

	//	2-arg parameterized-constructor 
	ParentClass(int siteAge, String siteName) {
		// here super() will be inserted; 
		// which will invoke Object class' default-constructor
		System.out.println("2-arg parameterized-constructor"
				+ " of Parent class");

		// assigning instance variables of this class
		this.siteAge = siteAge;
		this.siteName = siteName;
	}

	// other codes here
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	//	default no-arg constructor 
	ChildClass() {
		// explicitly inserted to invoke 2-arg constructor
		// it should be 1st statement of constructor
		super(20, "oracle.com"); 
		System.out.println("default constructor of Child class");
	}

	// display() method to print website details
	void displaySiteDetail() {
		System.out.println("\nWebsite age in years : " 
				+ super.siteAge);
		System.out.println("Website name : " 
				+ super.siteName);
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		child.displaySiteDetail();
	}
}

Output:

2-arg parameterized constructor of Parent class
default constructor of Child class

Website age in years : 20
Website name : oracle.com

Explanation:

  • there is an explicit super(int, String) constructor call inserted by programmer
  • So, it invokes the parent-class’ 2-arg parameterized-constructor; while creating object of child-class through inheritance relationship
  • Note: Make sure while inserting super(arguments) constructor call explicitly; it must be 1st statement of the constructor;
  • Otherwise compilation error will be thrown stating reason “Constructor call must be the first statement in a constructor”

 

Usage 3: super keyword to invoke instance-method of immediate parent-class

  • Syntax: super.<overridden_instance_method>
  • Both parent-class and child-class have method with same method-name i.e.; child-class overriding parent-class’ instance-method

Case 3.1. Without super keyword

  • When child-class overrides parent-class’ instance-method
  • and if we invoke method directly without any keyword, then it calls current class-method i.e.; overriding method

Let us see an example

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge;
	String siteName;

	// overridden display() method
	void displayMessage() {
		System.out.println("Parent-class "
				+ "instance-method displayMessage()");
	}
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	// overriding display() method
	void displayMessage() {
		System.out.println("Child-class "
				+ "instance-method displayMessage()");
	}

	// test method for super keyword
	void callDsiplayTest() {
		// invokes overriding method
		displayMessage(); // without super keyword
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		child.callDsiplayTest();
	}
}

Output:

Child-class instance-method displayMessage()

Explanation:

  • Directly invoking instance-method from child-class –> calls overriding-method
  • Because priority is given to local method
  • To invoke overridden-method of parent-class from child-class –> we need to explicitly invoke using super keyword

Let us see an example on the use of super keyword for overridden method invocation

 

Case 3.2. Invoking parent-class’ method using super keyword

  • Invoking parent-class’ overridden-method using super keyword

ParentClass.java

package in.bench.resources.superkeyword.example;

public class ParentClass {

	// instance variables
	int siteAge;
	String siteName;

	// overridden display() method
	void displayMessage() {
		System.out.println("Parent class' "
				+ "instance-method displayMessage()");
	}
}

ChildClass.java

package in.bench.resources.superkeyword.example;

public class ChildClass extends ParentClass {

	// instance variables
	int siteAge = 2;
	String siteName = "BenchResources.Net";

	// overriding display() method
	void displayMessage() {
		System.out.println("Child-class' "
				+ "instance-method displayMessage()");
	}

	// test method for super keyword
	void callDsiplayTest() {
		// invokes overriding method
		super.displayMessage(); // invoking parent-class method
	}

	// main() method - entry point to JVM to begin execution
	public static void main(String[] args) {

		ChildClass child = new ChildClass();
		child.callDsiplayTest();
	}
}

Output:

Parent class instance method displayMessage()

Explanation:

  • We are explicitly invoking parent-class method using super keyword
  • Reason: Local instance-method hides parent-class overridden-method while invoking without super keyword, as seen in the earlier case 3.1
  • Note: If instance-method names are different in parent-class and child-class, then there is no need of super keyword to invoke parent-class methods; instead directly parent-class’ instance-method can be invoked through inheritance

 

Point to remember about super keyword:

  • super keyword is used to refer immediate parent-class
  • super.<variable_name> is used to refer immediate parent-class’ instance-variable
  • super() constructor call; is used to invoke immediate parent-class’ default no-arg constructor
  • super(arguments) constructor call; is used to invoke immediate parent-class’ parameterized-constructor
  • super.<overridden_method> is used to invoke immediate parent-class’ instance-method

That’s all about super keyword in Java

 

References:

 

Read Also:

 

Happy Coding !!
Happy Learning !!

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