this Keyword

In this article, we will look at the uses of this keyword in Java programs along with example Java code.

Introduction to this Keyword

this keyword in Java is used to refer current object on which a method is invoked. Using such property, we can refer the fields and other methods inside the class of that object. The this keyword has two main uses which are listed below:

1. It is used to eliminate ambiguity between fields and method parameters having the same name.
2. It is used for chaining constructors.

To explain the first use, let’s consider the following program, which creates a square and displays it:

class Square
{
	int side;
	Square(int  s)
	{
		side = s;
		//Code to display a square with side s
	}
}
class SquareDemo
{
	Square s1 = new Square(4);
}

In the above code segment, there are no errors as the field name side and parameter name s are different. Now, let’s modify our Square class as shown below:

class Square
{
	int side;
	Square(int side)
	{
		side = side;
		//Code to display a square with side side
	}
}
class SquareDemo
{
	Square s1 = new Square(4);
}

In the above code segment, observe line no: 6. At this line, JVM will be in a ambiguous situation to decide whether to initialize the field side or the parameter side. To eliminate such ambiguity, the left side variables can be preceded with this keyword as shown below:

class Square
{
	int side;
	Square(int side)
	{
		this.side = side;
		//Code to display a square with side side
	}
}
class SquareDemo
{
	Square s1 = new Square(4);
}

Now, JVM will be able to decide that the left hand side variable is the field side, and the right hand side variable is the parameter side.

Some programmers prefer to declare fields and parameters with different names and other programmers prefer the same names for both fields and parameters. Which one to practice, is left to you.

The second use of this keyword is constructor chaining, which refers to the invocation of one constructor from another constructor. We can call a constructor from another constructor using this keyword as shown below:

this(param1, param2, …, paramn);

While using the above syntax, the constructor whose signature (number of and type of parameters) matches, will be invoked.

Note: While invoking another constructor using the this keyword, it should be the first line inside the constructor’s definition. For clarity regarding this, look at the example given below:

Example on constructor chaining:

class Square
{
	int side;
	Square( )
	{
		this(4);  //demonstrates constructor chaining
		//Code to display a square
	}
	Square(int  s)
	{
		side = s;
	}
}
class SquareDemo
{
	Sqaure s1 = new Sqaure(4);
}

In the above program, at line number 6, the second constructor is invoked as it is having a single integer parameter and 4 is passed to the parameter s which will be assigned to the field, side.

Suryateja Pericherla

Suryateja Pericherla, at present is a Research Scholar (full-time Ph.D.) in the Dept. of Computer Science & Systems Engineering at Andhra University, Visakhapatnam. Previously worked as an Associate Professor in the Dept. of CSE at Vishnu Institute of Technology, India.

He has 11+ years of teaching experience and is an individual researcher whose research interests are Cloud Computing, Internet of Things, Computer Security, Network Security and Blockchain.

He is a member of professional societies like IEEE, ACM, CSI and ISCA. He published several research papers which are indexed by SCIE, WoS, Scopus, Springer and others.