Java is one of the most popular object-oriented programming languages used for developing desktop applications, web applications, mobile apps, and enterprise software. To write efficient Java programs, it is essential to understand the basic concepts such as creating a class, using different data types, and working with static variables. This article explains these concepts in a simple and easy-to-understand manner.

Java Rules for Creating a Class

A class is a blueprint for creating objects. It defines the properties (variables) and behaviors (methods) of an object.

Rules for Creating a Java Class

1. Class Name Should Start with a Letter

A Java class name must begin with an alphabet (A-Z or a-z), underscore (_), or dollar sign ($). However, it is recommended to start with a letter.

Valid Examples:

class Student
class Employee
class _Demo

Invalid Example:

class 123Student

2. Use Pascal Case Naming Convention

Java follows the Pascal Case naming convention for class names, where the first letter of every word is capitalized.

Examples:

• Student
• EmployeeDetails
• BankAccount

3. Class Name Should Not Be a Java Keyword

Reserved keywords cannot be used as class names.

Invalid Examples:

class int
class public
class static

4. Only One Public Class Per File

If a class is declared as public, the filename must match the class name.

Example:

File Name: Student.java

public class Student {
}

5. Curly Braces Define the Class Body

Everything related to the class, such as variables, constructors, and methods, must be enclosed within curly braces { }.

Example:

public class Student {

    int id;
    String name;

    void display() {
        System.out.println(name);
    }

}

6. A Class Can Contain

• Variables
• Methods
• Constructors
• Nested Classes
• Static Members

Example of a Java Class

public class Student {

    int id;
    String name;

    void display() {
        System.out.println("ID: " + id);
        System.out.println("Name: " + name);
    }

    public static void main(String[] args) {

        Student s = new Student();

        s.id = 101;
        s.name = "Rahul";

        s.display();
    }
}

Output:

ID: 101
Name: Rahul

Java Data Types

A data type specifies the type of value that a variable can store.

Java data types are divided into two categories:

1. Primitive Data Types
2. Non-Primitive (Reference) Data Types

1. Primitive Data Types

Primitive data types are predefined by Java and store simple values.

Integer Example

public class IntegerExample {

    public static void main(String[] args) {

        int age = 25;

        System.out.println(age);
    }
}

Output

25

Double Example

public class DoubleExample {

    public static void main(String[] args) {

        double salary = 45000.75;

        System.out.println(salary);
    }
}

Output

45000.75

Character Example

public class CharacterExample {

    public static void main(String[] args) {

        char grade = 'A';

        System.out.println(grade);
    }
}

Output

A

Boolean Example

public class BooleanExample {

    public static void main(String[] args) {

        boolean isPassed = true;

        System.out.println(isPassed);
    }
}

Output

true

2. Non-Primitive Data Types

Non-primitive data types store references to objects rather than actual values.

Examples include:

• String
• Arrays
• Classes
• Interfaces
• Objects

Example:

public class StringExample {

    public static void main(String[] args) {

        String name = "John";

        System.out.println(name);
    }
}

Output

John

Static Variable in Java

A static variable is a variable that belongs to the class rather than to individual objects. Only one copy of the static variable is created, and it is shared among all objects of the class.

Static variables are declared using the static keyword.

Syntax

class ClassName {

    static dataType variableName;

}

Characteristics of Static Variables

• Belong to the class instead of objects.
• Memory is allocated only once.
• Shared among all objects.
• Can be accessed using the class name.
• Initialized when the class is loaded.

Example of a Static Variable

public class Student {

    int id;
    String name;

    static String college = "ABC College";

    Student(int i, String n) {
        id = i;
        name = n;
    }

    void display() {
        System.out.println(id + " " + name + " " + college);
    }

    public static void main(String[] args) {

        Student s1 = new Student(101, "Rahul");
        Student s2 = new Student(102, "Priya");

        s1.display();
        s2.display();
    }
}

Output

101 Rahul ABC College
102 Priya ABC College

Notice that both students share the same value for the college variable because it is declared as static.

Changing the Value of a Static Variable

If the value of a static variable changes, the updated value is reflected for all objects.

public class Student {

    static String college = "ABC College";

    public static void main(String[] args) {

        System.out.println(college);

        college = "XYZ University";

        System.out.println(college);
    }
}

Output

ABC College
XYZ University

Difference Between Static and Instance Variables