Basics of Java | Bsc | Btech | Exams

Java is a powerful and flexible programming language used widely by developers. Starting with Java requires an understanding of its fundamental components, including syntax, comments, identifiers, keywords, class structure, input/output handling, and writing your first program. Let’s explore each of these basics to build a solid foundation in Java programming.

Table of Contents

1. Java Basic Syntax

Java’s syntax is a set of rules that define how code should be written and organized. Each statement in Java ends with a semicolon (;), and Java is case-sensitive, so Example and example would be treated as different identifiers. Curly braces { } are used to group blocks of code, like methods and loops. These rules are essential for all Java code.

Example:

public class MyProgram 
{
  public static void main(String[] args) 
  {
     System.out.println("Hello, Java!");    
     }
     }

In this example:

• public class MyProgram: Defines a class named MyProgram.
• public static void main(String[] args): The main method, where the program starts.
• System.out.println("Hello, Java!");: Prints “Hello, Java!” to the console.

2. Comments

Comments make Java code easier to understand. They’re ignored by the compiler and don’t affect the program’s execution. Java supports three types of comments:

Single-Line Comments: Start with // and go to the end of the line. These are useful for short explanations of the code.

// Print a message to the console 
System.out.println("Hello, world!");

Multi-Line Comments: Start with /* and end with */. They’re helpful for longer explanations or temporarily disabling code sections.

/* This is a multi-line comment that spans multiple lines */ System.out.println("Welcome to Java!");

Documentation Comments: Start with /** and end with */. These are used for generating structured documentation with Javadoc

/** * This method prints a greeting. */ 
public void greet() { System.out.println("Hello!"); }

– Using comments keeps your code organized and easy to read.

3. Identifiers

Identifiers are names used for variables, methods, classes, and other program elements. They must follow specific rules:

1. Begin with a letter (A-Z or a-z), an underscore (_), or a dollar sign ($).
2. After the first character, they may include numbers.
3. Keywords like int or class cannot be used as identifiers.

Good identifiers make code clear and readable.

Example:

int age = 25;        // 'age' is an identifier for a variable
String name = "Alice"; // 'name' is an identifier for a variable

4. Keywords

Java uses keywords that have special meanings and functions within the language. They cannot be used as identifiers. Each keyword allows Java to perform different operations and define the program structure.

Some common keywords:

• public: Controls visibility.
• class: Defines a class.
• int: Declares an integer variable.
• if / else: Used for conditional statements.
• return: Ends a method and returns a value if specified.

Example:

public class Example {
    public static void main(String[] args) 
    {
        int number = 10;
        if (number > 5) 
        {
            System.out.println("Number is greater than 5");
        }
    }
}

In this example:

• int declares an integer variable.
• if is used for a conditional statement.

5. Class Syntax

In Java, everything exists within classes. A class serves as a blueprint for objects and contains variables, methods, and constructors. Each Java program must contain at least one class, which includes the entry point of the program, the main method.

Example of Class Syntax:

public class MyClass {
    int myVariable;  // Class variable

    public MyClass() {  // Constructor
        myVariable = 0;
    }

    public void display() {  // Method
        System.out.println("Hello from Java!");
    }

    public static void main(String[] args) {  // Entry point of the program
        MyClass obj = new MyClass();
        obj.display();
    }
}

In this example:

• public class MyClass: Declares a class named MyClass.
• int myVariable;: A variable declared within the class.
• display(): A method that prints a message.
• public static void main(String[] args): The main method, where Java starts running the program.

Classes are the foundation of Java programs, making them essential to learn.

6. Input/Output in Java

Java handles input and output (I/O) operations to make programs interactive. The System.out.println() method displays messages on the console, and the Scanner class reads user input.

Output Example:

System.out.println("Enter your name:");

Input Example:

import java.util.Scanner;

public class UserInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);  // Create a Scanner object
        System.out.print("Enter your age: ");
        int age = scanner.nextInt();  // Reads an integer from user input
        System.out.println("You are " + age + " years old.");
    }
}

In this example:

• System.out.println("Enter your name:");: Prints a message to the console.
• Scanner scanner = new Scanner(System.in);: Creates a Scanner object to read input.
• scanner.nextInt();: Reads an integer from user input.

Input and output are key for creating interactive applications.

7. First Java Program (Hello World)

The “Hello World” program is often the first program in Java. This simple program shows Java’s structure and syntax and outputs “Hello, World!” to the console. It’s a good starting point for understanding Java’s setup.

Hello World Example:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Explanation:

• public class HelloWorld: Declares a class named HelloWorld.
• public static void main(String[] args): The main method where Java begins program execution.
• System.out.println("Hello, World!");: Prints “Hello, World!” to the console.

This first program introduces Java’s basic structure and syntax, making it an essential starting point.

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The basics of Java include understanding its syntax, comments, identifiers, keywords, and class structure. Java’s syntax provides the rules for writing code, while comments improve readability. Identifiers are names for variables, methods, and classes, and keywords define specific functions. Java programs revolve around classes, each containing methods like main for execution. These essentials form the foundation of Java programming, enabling developers to write efficient, organized code.