this Keyword in Java

Two uses of the this keyword are

To resolve ambiguity between instance variables and parameters
To pass the current object as a parameter to another method or constructor
this() can be used to invoked current class constructor.
The this keyword can be used to return current class instance.

Example

class Student
{
//instance variable name
String name;

Student(String name) //parameter variable name
{
  name=name;    //wrong , creating ambiguity
this.name=name;  //Correct, this.name is instance variable
		// and name is local parameter variable

}

public static void main(String[] args)
{
Student ob=new Student("Rohit");
System.out.println("Name Is :"+ob.name);
} 
}

To pass the current object as a parameter to another method or constructor

Example

 public class MyDate {
 private int day = 1;
 private int month = 1;
 private int year = 2000;

 public MyDate(int day, int month, int year) {
 this.day = day;
 this.month = month;
 this.year = year;
 }
 public MyDate(MyDate date) {
 this.day = date.day;
 this.month = date.month;
 this.year = date.year;
 }

 public MyDate addDays(int moreDays) {
 MyDate newDate = new MyDate(this);
 newDate.day = newDate.day + moreDays;
 // Not Yet Implemented: wrap around code...
 return newDate;
 }
 public String toString() {
 return "" + day + "-" + month + "-" + year;
 }
 }

The this() constructor call can be used to invoke the current class constructor (constructor chaining). This approach is better if you have many constructors in the class and want to reuse that constructor.

Example

  
class Student{  
    int id;  
    String name;  
    Student()
    {
    System.out.println("default constructor is invoked");
     }  
      
    Student(int id,String name)
    {  
    this ();//it is used to invoked current class constructor.  
    this.id = id;  
    this.name = name;  
    }  
    void display()
    {
     System.out.println(id+" "+name); 
    }  
      
    public static void main(String args[])
    {  
    Student ob1 = new Student(101,"Rahul");  
    Student ob2 = new Student(102,"Rohit");  
    ob1.display();  
    ob2.display();  
   }  
}

Output:
       default constructor is invoked
       default constructor is invoked
       101 Rahul
       102 Rohit

this keyword that you return as a statement from the method

Example

  

class Demo{  
Demo  getDemo(){  
return this;  
}  
void show()
{
System.out.println("Welcome");
}  
}  
  
class Demo1
{  
public static void main(String args[])
{  
new Demo().getDemo().show();  
}  
}