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Java - super keyword
The super keyword is similar to this keyword. Following are the scenarios where the super keyword is used.
It is used to differentiate the members of superclass from the members of subclass, if they have same names.
It is used to invoke the superclass constructor from subclass.
Differentiating the Members
If a class is inheriting the properties of another class. And if the members of the superclass have the names same as the sub class, to differentiate these variables we use super keyword as shown below.
super.variable super.method();
Sample Code
This section provides you a program that demonstrates the usage of the super keyword.
In the given program, you have two classes namely Sub_class and Super_class, both have a method named display() with different implementations, and a variable named num with different values. We are invoking display() method of both classes and printing the value of the variable num of both classes. Here you can observe that we have used super keyword to differentiate the members of superclass from subclass.
Copy and paste the program in a file with name Sub_class.java.
Example
class Super_class { int num = 20; // display method of superclass public void display() { System.out.println("This is the display method of superclass"); } } public class Sub_class extends Super_class { int num = 10; // display method of sub class public void display() { System.out.println("This is the display method of subclass"); } public void my_method() { // Instantiating subclass Sub_class sub = new Sub_class(); // Invoking the display() method of sub class sub.display(); // Invoking the display() method of superclass super.display(); // printing the value of variable num of subclass System.out.println("value of the variable named num in sub class:"+ sub.num); // printing the value of variable num of superclass System.out.println("value of the variable named num in super class:"+ super.num); } public static void main(String args[]) { Sub_class obj = new Sub_class(); obj.my_method(); } }
Output
Compile and execute the above code using the following syntax.
javac Super_Demo java Super
On executing the program, you will get the following result −
This is the display method of subclass This is the display method of superclass value of the variable named num in sub class:10 value of the variable named num in super class:20
Invoking Superclass Constructor
If a class is inheriting the properties of another class, the subclass automatically acquires the default constructor of the superclass. But if you want to call a parameterized constructor of the superclass, you need to use the super keyword as shown below.
super(values);
Sample Code
The program given in this section demonstrates how to use the super keyword to invoke the parametrized constructor of the superclass. This program contains a superclass and a subclass, where the superclass contains a parameterized constructor which accepts a integer value, and we used the super keyword to invoke the parameterized constructor of the superclass.
Copy and paste the following program in a file with the name Subclass.java
Example
class Superclass { int age; Superclass(int age) { this.age = age; } public void getAge() { System.out.println("The value of the variable named age in super class is: " +age); } } public class Subclass extends Superclass { Subclass(int age) { super(age); } public static void main(String args[]) { Subclass s = new Subclass(24); s.getAge(); } }
Output
Compile and execute the above code using the following syntax.
javac Subclass java Subclass
On executing the program, you will get the following result −
The value of the variable named age in super class is: 24
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