Variables in Java

A variable is a data container that contains the data while the Java program is executed. A variable is assigned a single data type, such as String, Integer, Boolean, Float, etc. The data type defines the nature of the data value. A variable is the name of a memory location.

The variable name is a combination of two words: "vary" and "able" which means its value can be changed. Multiple times, different values can be assigned to the same variable.

All Java variables must be identified with unique names. These unique names are called Identifiers.

There are three types of variables in Java:

1) Local Variables

If we declare a variable inside a method, constructor, or block, that variable is known as a local variable in Java. A local variable is only accessible inside the method; outside the method, there is no identity for the local variable. A local variable cannot be static.

2) Instance Variables or Non-Static Variables

If we declare a non-static variable outside the method, constructor, and block, that variable is known as the instance variable in Java. Instance variables belong to the instance of the class. Instance variables are initialized while creating the object of the class and are accessed by the object only.

3) Static Variables

A variable declared with the static keyword is known as a static variable in Java. Static variables are initialized when a class is loaded into memory. Static variables are common for all the objects of the class, and they are accessed by the class name, which is why they are also known as class variables. Static variables cannot be declared as local variables.

Java variable naming conventions

While declaring a variable in Java, we must follow the below rules and conventions related to the naming of variables.

Variable names in Java are case-sensitive.
For example: employeeId, EMPLOYEEID, EmployeeId

A variable name should start with a letter (A to Z or a to z) and two special characters like a dollar sign ($) or an underscore (_).
For example: mentor, Mentor, $mentor, _myVar

After the first character in a Java variable name, the name can also contain numbers (in addition to letters, the $, and the _ character).
For example: mentor1, mentor_1

A keyword or reserved word cannot be used as a variable name.
For example: int, char, if, while, for, etc.

A variable must not contain white spaces or special characters like the astric sign (*) etc.
For example: java mentor, java*mentor

Example: Valid and Invalid variables declaration in Java

  public class ValidInvalidVariables {
    /**************
    * VALID NAMES
    **************/
    String mentor;
    String Mentor;
    String MENTOR;
    String mentor1;
    String $mentor;
    String men$tor;
    String men_tor;
    String _mentor;
    
    /****************
    * INVALID NAMES
    ****************/
    String while;
    String &mentor;
    String 1mentor;
    String men%tor;
    String java mentor;
    String java*mentor;
  }

Java Variable Declaration

Java variable initialization and assignment

To initialize a variable, according to the data type, we assign a specific value to the variable.

  class Student {
    // Instance Variable
    String studentName; // --> Variable declaration
    
    // Static Variable
    static String schoolName = "ABC School"; // Variable declaration + assignment
    
    void getTotalMarks(int english, int science, int math) {
      // Local Variable
      int totalmarks;
      
      // variable initialization and assignment
      totalmarks = english + science + math;
      System.out.println("Total Marks: " + totalmarks);
    }
    // Cannot access local variable outside the method
    // System.out.println("Total Marks: " + totalmarks);
  }
  
  public class VariableExample {
    public static void main(String[] args) {
      Student student = new Student();
      student.studentName = "John";
      
      // Accessing Instance variable using object
      System.out.println("Student Name : " + student.studentName);
      // Accessing Static Variable with Class Name
      System.out.println("School Name : " + Student.schoolName);
      
      student.getTotalMarks(78, 92, 83);
    }
  }