this keyword in java

Let’s learn this keyword in java.

this keyword in java

this keyword is reference variable that refers to the current object. It is required when we have a local variable with same name as an instance variable of the class. Let me explain this keyword in java with an example.

class Demo
{
   int m;
   int n;
   public void setValue(int m, int n)
   {
      m = m;
      n = n;
   }
   public void showValue()
   {
      System.out.println("Value m = " + m);
      System.out.println("Value n = " + n);
   }
}
public class ThisKeywordDemo
{
   public static void main(String[] args)
   {
      Demo obj = new Demo();
      obj.setValue(5, 6);
      obj.showValue();
   }
}

In the above example I have created class “Demo” with instance variables “m” and “n” of type int and method setValue() to set data.

To print values of “m” and “n” we have created method showValue(). When you run the above program you will get output as shown below,

Value m = 0

Value n = 0

Our expected output for “m” and “n” should be 5 and 6. But the value is zero!!!?? Why?

Let’s analyse,

In setValue() method have local variables that are named as “m” and “n”. Meanwhile instance variables are also named as “m” and “n”.

During execution in setValue() method both variables (m = m and n = n) are considered as local variables not instance variables.

Hence it does not set the value of “m” and “n” when showValue() method is called. This problem is solved by using this keyword. Let’s see java program using ‘this’ keyword.

class Demo
{
   int m;
   int n;
   public void setValue(int m, int n)
   {
      // this keyword
      this.m = m;
      this.n = n;
   }
   public void showValue()
   {
      System.out.println("Value m = " + m);
      System.out.println("Value n = " + n);
   }
}
public class ThisExample
{
   public static void main(String[] args)
   {
      Demo obj = new Demo();
      obj.setValue(5,6);
      obj.showValue();
   }
}

In setValue() method instance variables “m” and “n” are appended with ‘this’ keyword followed by dot.

During code execution when object calls method setValue(), keyword ‘this’ is replaced by the object’s handler ‘obj’ like this,

obj.m = m;

obj.n = n;

Now the compiler know that “m” on the left hand side is the instance variable whereas “m” on the right hand side is the local variable.

The variables are initialized correctly and expected output is as shown below.

Value m = 5

Value n = 6

Use of this keyword

Let’s see each use with an example.

Example: this keyword is used to invoke current class instance variable

class Example
{
   // instance variable
   int a;
   void setValues(int a)
   {
      // if this keyword is not used, then output will be default  
      // value of instance variable 'a'
      this.a = a;
   }
   void printValues()
   {
      System.out.println(a);
   }
   public static void main(String[] args)
   {
      Example obj = new Example();
      obj.setValues(5);
      obj.printValues();
   }
}

Output:

Example: this keyword can be used to invoke current class method implicitly

class Example
{
   void print()
   {
      System.out.println("flower brackets");
   }
   void display()
   {
      // implicitly uses this keyword if not added.
      // is equal to this.print();
      print();
   }
   public static void main(String[] args)
   {
      Example obj = new Example();
      obj.display();
   }
}

Output:

flower brackets

Example: this() keyword used to invoke current class constructor

class Example
{
   Example()
   {
      System.out.println("no argument constructor.");
   }
   Example(int a)
   {
      // invoking current class constructor
      this();
      System.out.println("parameterized constructor.");
   }
   public static void main(String[] args)
   {
      Example obj = new Example(5);
   }
}

Output:

no argument constructor.
parameterized constructor.

Example: this keyword can be used to pass as an argument in method call

class Example
{
   void display(Example ex)
   {
      System.out.println("in display() method.");
   }
   void print()
   {
      // this keyword can be used to pass as an argument in method call
      display(this);
   }
   public static void main(String[] args)
   {
      Example obj = new Example();
      obj.print();
   }
}

Output:

in display() method.

Example: this keyword can be used to pass as an argument in constructor call

class Demo
{
   Demo(Example ex)
   {
      System.out.println("in Demo class constructor.");
   }
}
class Example
{
   void display()
   {
      // this keyword can be used to pass as an argument in constructor call
      Demo d = new Demo(this);
   }
   public static void main(String[] args)
   {
      Example obj = new Example();
      obj.display();
   }
}

Output:

in Demo class constructor.

Example: this keyword can be used to return current class instance from method

class Example
{
   Example display()
   {
      // this keyword can be used to return current class instance from method
      return this;
   }
   public static void main(String[] args)
   {
      Example obj = new Example();
      obj.display();
   }
}