this keyword:

  this keyword is used within the class only. It must be used for instance members only. this keyword can be used:
  
 (a) at instance variable level
 (b) at instance method level
 (c) at constructor level

--------------------
Rules for this keyword at variable & method level:

1. It can be used inside the constructor &/OR method of the same class.

2. It can be used multiple times inside the constructor &/OR method of the same class.

3. It can be at any line inside the constructor &/OR method of the same class.


Note: this keyword can be used inside the instance members (constructor & instance methods) only. It can't be used inside the static members (static block & static methods)



Rules for this keyword at constructor level:

1. It can be used inside the constructor of the same class. Can't use inside the method.

2. It should be used only once inside the constructor of the same class.

3. It should be at first line inside the constructor of the same class.

Note: this keyword at constructor level is used to achieve constructor nesting.
constructor nesting: It is a process of calling one constructor inside the another constructor of the same class using this keyword.

