Basic Concept of Identifiers and Variables in Java

Background of this article

 

• Distinguish between valid and invalid identifiers
• List of eight primitives types
• Define literal values for numeric and textual types
• Define the terms variables and reference variable

 

Identifiers

 

“An identifier is a name given to a variable, class, or method.”

  

Identifiers have the following characteristics:

 

Can start with a Unicode letter, underscore (_), or a dollar sign ($).

 

Are case-sensitive and have no maximum length.

 

Examples of valid identifiers:

 

• identifier
• username
• user_name
• _sys_var1
• $chnage

The Java programming language supports the following two basic data types:

1. Primitive types
2. Class types

“Primitive data types are simple values, not objects.”

• The Java programing language defines eight primitive data types, that can be considered in the following four categories:
   
  Class types are used for more complex types, including all the types that you declare yourself.” They are used to create objects.

  Logical: Boolean
  Textual: char
  Integral: byte, short, int and long
  Floating: double and float

  How to declare variables, Declarations and Assignments

  Public class Assign {
   Public static void main(String[] args) {
    Integer variables int x = 6, y = 1000;
    Float z = 3.414 f;
    Boolean truth = true;
    Char c = ’A’;
   }
  }

• In Java programming, beyond primitive types, all other types are reference types.
   
• A reference variable contains a handle to an object.
   
  Example:

  Public class MyDate {
   Private int day = 9;
   Private int month = 11;
   Private int year = 1991;
   Public MyDate(int day, int month, int year) {
    …………..
   }
   Public String tostring() {
    …………}
  }
  Public class TestMyDate {
   Public static void main(String[] args) {
    MyDate today = new MyDate(9, 11, 1991);
   }
  }

• Variable “today” is a reference variable holding one object of “MyDate” class. 
   
  Calling new xyz() performs the following actions:
   
  • Memory is allocated for the object
  • Explicit attribute initialization is done
  • A constructor is executed
  • The object reference is returned by the new operator
  • The reference to the object is assigned to a variable
   
  For example:

  MyDate my_birth = new MyDate(9, 11, 1991);

  In a single Java Virtual Machine, the Java programming language only passes arguments by value.
   
  When an object instance is passed as an argument to a method, the value of the argument is a reference to the object.
   
  The contents of the object can be changed in the called method, but the original object reference is never changed.
   
  Two uses of this keyword are:
   
  1. To resolve ambiguity between instance variables and parameters
   
  2. To pass the current object as a parameter to another method or constructor
  Let's see how to define a reference type instance variable in a Java class and manipulate the object referenced by this variable.

   

  Examples of coding conventions

   

  The following are examples of coding conventions.

  Packages: com.example.domain;
  Classes, interfaces and enum types: SavingsAccount
  Methods: getAccount()
  Variables: CurrentCustomer
  Constants: HEAD_COUNT

  Variables defined inside a method are called local variables, also referred to as automatic, temporary, or stack variables.  Local variables must be initialized before the first use.

   

  Variables defined outside a method, are created when the object is constructed using the new xxx() call.  They are the following two types:

   

  1. Static variables: They are created when the class is loaded and continue to exist for as long as the class is loaded.

   

  2. Instance variables: They are declared without using the static keyword.  They continue to exist for as long as the object exists.
  So, friends, these are a few articles that will help you with the basic concepts of identifiers and variables.  The next time I will provide more commands that are interesting.  Thank you!  I hope this is helpful for you... enjoy :).KR