Difference Between this and this() in Java

In Java, both this and this() are completely different from each other.

• this keyword is used to refer to the current object, i.e. through which the method is called.
• this() is used to call one constructor from the other of the same class.

The below table shows the point-to-point difference between both this keyword and this().

this	this()
this keyword is used with the objects only.	this() is used with constructors only.
It refers to the current object.	It refers to the constructor of the same class whose parameters matches with the parameters passed to this(parameters).
Dot(.) operator is used to access the members.  For example, this.variableName;	No Dot(.) operator is used. Only the matching parameters are passed.
It is used to differentiate between the local variable and the instance variable in the method.	It is used to refer to the constructor belonging to the same class.

See the code below, which describes the utilization of this keyword.

Java

import java.io.*;

public class Student {

    private String name;
    private int age;

    // Note that in the Constructor below "this keyword" is
    // used to differentiate between the local variable and
    // the instance variable.

    public Student(String name, int age)
    {
        // Assigns the value of local name variable
        // to the name(instance variable).
        this.name = name;
        // Assigns the value of local Age variable
        // to the Age(instance variable).
        this.age = age;
    }

    public void show()
    {
        System.out.println("Name = " + this.name);
        System.out.println("Age = " + this.age);
    }

    public static void main(String[] args)
    {
        // Creating new instance of Student Class
        Student student = new Student("Geeks", 20);
        student.show();
    }
}

Output

Name = Geeks
Age = 20

Now take a look at the code below which describes the use of this().

Java

import java.io.*;

public class Student {

    private String name;
    private int age;

    // Constructor 1 with String as parameter.
    public Student(String name)
    {
        // This line of code calls the second constructor.
        this(20);
        System.out.println("Name of Student : " + name);
    }

    // Constructor 2 with int in parameter.
    public Student(int age)
    {
        System.out.println("Age of student = " + age);
    }

    // Constructor 3 with no parameters.
    public Student()
    {
        // This line calls the first constructor.
        this("Geeks");
    }

    public static void main(String[] args)
    {
        // This calls the third constructor.
        Student student = new Student();
    }
}

Output

Age of student = 20
Name of Student : Geeks

Please note that this() should always be the first executable statement in the constructor. Otherwise, the program will give compile time error.