This and Super Keyword in Java

Introduction

Java provides many implicit keywords among those this and super are two reference variables. The this and super keywords resemble the reference variable pointing to an instance of a class or a parent class (superclass), respectively.

this: this is the reserved keyword in Java that can be used to invoke the constructors, methods, static members, etc. of the current instance of a class.

super: super is the reserved keyword in Java that is used to invoke constructors and methods of the parent class. This is possible only when one class inherits another class (child class extends parent class).

What is this keyword in Java?

Let us understand the use of this keyword using the above code snippet. As we can see that we have an instance variable value and a parameter var of the assign method. If we want to assign var to the instance variable value inside the class then we have to use the this keyword. In technical terms we are using this keyword to refer to the current object of the Code class.

this is a special keyword in Java that is used to refer to an instance of the current class. Using the this keyword, we can refer to the current class's methods, constructors, instance variables, and many more. We cannot use the this keyword as an identifier in Java.

Uses of “this” keyword in Java

• It can be used to refer to the current class instance variables.
• It can be passed as an argument in the method call representing the current object of the class.
• It can be used to return the current class instance from any of its instance methods.
• It can be used to access instance and static variables of the current instance.
• It can be passed as an argument in the constructor call.
• It is used to initiate a constructor of the current class.
• It can be used to invoke the current class methods.

Example of this keyword in Java

Example 1

The this keyword can be used to access and modify the instance and static variables. Thus, we can change the values of the variables that are declared inside the class through the this keyword. Also, this keyword can be used to differentiate between method-specific and class variables. Let us see an example.

Code:

Output:

Explanation:

• In the Illustration class, instanceVar and staticVar are instance and static variables respectively. Both the variables are referred in the Scaler method by using the "this" keyword.
• instanceVar is initialized with value 5, and staticVar is initialized with value 10. The Scaler method assigns 50 and 100 to the instanceVar and staticVar variables respectively.
• There are other two method-specific variables having the same names. We are able to differentiate between variables inside the class and methods by using this keyword.

Example 2

The “this” keyword is used to invoke the methods of the current object or class. Let us see an example.

Code:

Output:

Explanation:

In the above example, the Illustration class has two methods scaler and name. The scaler method is invoked inside the name method by using the this keyword.

Example 3

The this keyword is used to invoke the constructors of the current class. Suppose a class has two constructors: no-arg and parametrized constructors.

Code:

Output:

Explanation:

In the above example, there are two constructors. The no argument constructor is called while creating the object, and the parameterized constructor is called within the no argument constructor by using the “this” keyword.

Example 4

The this keyword can be passed as an argument in a method representing an object of a class. At industry level it is used in event handlers or at places where we have to provide reference of one class to another class. Let us understand it practically.

Code:

Output:

Explanation:

In the above example, we are passing the this keyword as an argument to call the print method within the invoke method.

Example 5

The “this” keyword can be passed as an argument in the constructor call. By passing this as an argument in the constructor call we can exchange data from one instance to another. This makes exchange of data between multiple classes or objects a lot easier.

Output:

Explanation:

In the above example, we have passed the this keyword as an argument in the constructor call. Here A and B are two classes; parameterized constructor of A is called in class B by creating an object of class A and passing this as an argument.

Example 6

The this keyword is used to return the current instance of a class. Methods of the class can be called directly at the time of creating an object using this

Output:

Explanation:

In the above example, the getIllustration method returns the current class instance and through which we called the print method.

What is a super keyword in Java?

Concept of Immediate Parent:

Let us understand the concept of immediate parent. In the above code snippet of multi-level inheritance class B extends class A, it implies class A is immediate parent of class B. Similarly class C extends class B and now class C has two parents i.e., class A and class B, where class B is immediate parent of class C.

super is a special keyword in Java that is used to refer to the instance of the immediate parent class. Using the super keyword, we can refer to the immediate parent class's methods, constructor, instance and static variables, etc. We cannot use the super keyword as an identifier in Java as it is a reserved keyword.

Uses of super Keyword in Java

• It is used to refer to an instance variable of the immediate parent class.
• It is used to invoke a method of the immediate parent class.
• It is used to invoke a constructor of immediate parent class.

Examples of super Keyword in Java

Example 1

The super keyword is used to refer to an instance variable of an immediate parent class.

Output:

Explanation:

• In the above example, the Child class extends the Parent class. Both have two instance variables, a and s.
• In the Child class, the print method calls the instance variables of the parent class using the super keyword.
• In comparison, the instance variables of child class don't need one. It doesn't matter whether the instance variable of parent class and child class share the same name; they can hold different values and are not overridden.

Example 2

The super keyword is used to invoke an immediate parent class method.

Output:

Explanation:

• In the above example, the Child class extends Parent class. Both have a display method.
• The print method in the child class invokes the display method of the parent class by using the super keyword.
• The example says that to invoke the methods in the child class from the parent class, we must use the “super” keyword.

Example 3

The super keyword is used to invoke an immediate parent class constructor.

Output:

Explanation:

In the above example, the child class extends parent class. While creating an object of the child class, the constructor of the child class is called, where the parent class constructor is invoked using the “super” keyword.

Difference Between this and super Keyword in Java

Similarities Between this and super Keyword in Java

1. Both this and super are non-static, so they can't be used in static context. It means that we cannot use both the keywords in the main method in Java.

Example 1

Output:

Explanation:

In the above example, we are referencing the non-static variable "this" in the static context. This led to a compiler error.

2. Both super and this keywords in Java can be used in constructor chaining to call another constructor. this() calls the no-argument constructor of the current class, and super() calls the no-argument constructor of the parent class.

Example 2

Output:

Explanation:

In the above example, we first forward a call from the child class no-argument constructor to the child class parameterized constructor using the this keyword. We are forwarding a call to the parent class parameterized constructor using the super keyword from a child class parameterized constructor.

3. this and super must be the first statement if used inside the constructor. This means we cannot call both statements in a single constructor.

Example 3

Output:

Explanation:

In the above example, the error will be displayed as shown in output because the this() statement is not executed first.

Can we Use Both this() and super() in the Same Constructor?

No, we cannot use the this() and super() in the same constructor. If we want to use this() or super() inside a constructor, they must be written (executed) at first. Both this() and super() cannot be executed simultaneously, hence “this()” and “super()” in Java cannot be used in the same constructor.

Important Points About this() and super() in Java

• this() as well super() can be used exactly once inside the constructor.
• If we use this() followed by super() or super() followed by this() we will get compile-time error. It is due to either this() or super() can be the first statement inside the constructor, not both.
• Inside the constructor, we cannot call the this() and super() recursively. Example

Output

In the above example, we have recursively made calls for the constructor, which leads to a compile-time error.