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

• identifier
• username
• user_name
• _sys_var1
• $chnage

1. Primitive types
2. Class types

• 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.
  1. Logical: Boolean
  2. Textual: char
  3. Integral: byte, short, int and long
  4. Floating: double and float
  How to declare variables, Declarations and Assignments
  1. Public class Assign {
  2. Public static void main(String[] args) {
  3. Integer variables int x = 6, y = 1000;
  4. Float z = 3.414 f;
  5. Boolean truth = true;
  6. Char c = ’A’;
  7. }
  8. }
• In Java programming, beyond primitive types, all other types are reference types.
• A reference variable contains a handle to an object.
  Example:
  1. Public class MyDate {
  2. Private int day = 9;
  3. Private int month = 11;
  4. Private int year = 1991;
  5. Public MyDate(int day, int month, int year) {
  6. …………..
  7. }
  8. Public String tostring() {
  9. …………}
  10. }
  11. Public class TestMyDate {
  12. Public static void main(String[] args) {
  13. MyDate today = new MyDate(9, 11, 1991);
  14. }
  15. }
• 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:
  1. 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.

  1. Packages: com.example.domain;
  2. Classes, interfaces and enum types: SavingsAccount
  3. Methods: getAccount()
  4. Variables: CurrentCustomer
  5. 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