Java - Interview question and answer on super keyword

In this article, we will cover some of the interview questions with their justification on Super keyword in Java

These are most frequently asked interview question from Super keyword in Java

Usage of super keyword in Java:

Instance variable: super keyword is used to refer instance variables of immediate parent class
Super constructor: super() constructor call; is used to invoke constructor of immediate parent class
Overridden Method: <methodName>is used to invoke instance method of immediate parent class (when it is overridden in the sub-class)
Note:super keyword cannot be used to refer in static context

Rules for Constructor chaining:

Whenever an Object is created, then its respective class constructor is invoked
But before the execution of its own class constructor, its immediate parent class constructor is invoked
This is repeated till root of the hierarchy is reached i.e.; until Object class

Reason:

In every constructor, compiler by default inserts call to super class constructor using super(); statement
Therefore, every-time Object is created, its parent class constructor is invoked
Programmer can also write call to super(); constructor statement explicitly, but impacts is very same much to the default one
If this super() constructor call is present, then it must be 1^st statement of the constructor
Otherwise, compile-time error will be thrown

There won’t be any error by inserting explicit super() constructor call
Rather, inserting explicit super(); constructor call inside child class constructor results in invocation of parent class constructor
Let us insert super(); constructor explicitly in the child class and see result
Example, as shown in the below screen-capture

As programmer can write explicit call to super class constructor using super keyword from child class constructor
Likewise, using parameterized super(…) constructor call is also possible
This is required, whenever programmer required to invoke parameterized constructor of the immediate parent class
Let us see one example with its results (as shown in the below screen-capture)

To invoke parent class variable from child class, use super keyword (when names of the variable in both parent & child class are same)
Syntax: super.<variable-name>
Example, as shown in the below screen-capture

Although, it is very possible to access parent class’s static variable using super keyword
But its usage is discouraged with compiler warning with message “The static field <parent-class-name>.<static-variable-name> should be accessed in a static way”
As it can be accessed using class-name
Syntax: <class-name>.<static-variable-name>
Let us see one example where we are accessing static variable of parent class using super keyword

Suppose method is overridden in the sub-class from parent-class
Then to access overridden method of parent-class from child-class, use super keyword
Syntax: super.<overridden-method-name>
In the below example, display() is overridden from parent class
So to access the parent class display() method from child class display() method, we have used super keyword, as shown in the below screen-capture

In the next article, we will see couple of question on this keyword

References:

Happy Coding !!
Happy Learning !!