The this keyword refers to the current object and is only valid within member functions of a class. It allows member functions like constructors to access variables and functions of the current object. The example code shows a class with two constructors that use the this keyword - the no-argument constructor calls the parameterized constructor as its first statement to initialize an instance variable, and the parameterized constructor uses this to set the instance variable to the passed parameter value.

1. What is the meaning of the keyword this? Explain why it is valid only in the implementation of a
member function
Solution
This Keyword : It refers to the current object.Used to invoke local and global variables and
member functions like constructors.
Here is the usage of this keyword in java.
In java class  we are having member functions .we can implement those member functions
using this keyword only.Here member functions are nothing but methods in java programming.
Let us consider sample java program to implementation of a member function using this
keyword.Here in below java program we are taking member functions as constructors.
Program:
import java.io.*;
class Thiskeyword
{
public int i;
public Thiskeyword() // member function of Thiskeyword
{
this(20); //must be the first statement , calling the constructor
System.out.println(" t THIS() Constructor is called");
i=10;
System.out.println(" t the value of i= "+i);
}
public Thiskeyword(int i) //member function with parameters
{
System.out.println(" t THIS(int j) constructor is called");
this.i=i; //pointing to the current class member
System.out.println(" t the value of i= "+i);
}
}

2. class This // main class
{
public static void main(String a[])throws IOException
{
System.out.println(" t start of main()");
Thiskeyword t=new Thiskeyword();
System.out.println(" t end of main()");
}
}
Output:
start of main()
THIS(int j) constructor is called
the value of i=20
THIS() constructor is called
the value of i=10
end of main()