Introduction to Java

What is Java?

Java is a high-level, object-oriented, class-based programming language designed to have as few implementation dependencies as possible. It was created by James Gosling at Sun Microsystems and released in 1995. Today it is owned and maintained by Oracle Corporation.

The most important principle behind Java is "Write Once, Run Anywhere" (WORA). This means Java code compiled on one platform can run on any other platform that has a Java Virtual Machine (JVM) installed — without recompiling.

A Brief History of Java

How Java Works: Compilation & Execution

Unlike C or C++ which compile directly to machine code, Java uses a two-step process:

1. Compilation: The Java compiler (javac) converts your .java source file into bytecode — a platform-neutral .class file.
2. Execution: The Java Virtual Machine (JVM) on the target machine reads the bytecode and translates it into native machine instructions at runtime.

  YourProgram.java  ?  [javac compiler]  ?  YourProgram.class (bytecode)
  YourProgram.class ?  [JVM on any OS]   ?  Runs on Windows / Mac / Linux
      

This two-step design is why Java is platform-independent. The bytecode is the same everywhere; only the JVM differs per operating system.

Source codeHelloWorld.java
Written by you

→

Compilerjavac checks code
and creates bytecode

→

JVMReads bytecode
on your operating system

→

OutputProgram result
on the console

JDK vs JRE vs JVM

These three terms confuse most beginners. Here's a clear breakdown:

Variables in Java

A variable is a named location used to store a value while a program runs. Every Java variable has a data type, a name, and a current value. Java is statically typed, so the type is checked when the code is compiled.

int age = 25;
String name = "Asha";
boolean isLearning = true;

The general declaration format is:

dataType variableName = value;

In the example above, int, String, and boolean are data types. age, name, and isLearning are variable names.

Declaration, initialization, and reassignment

int score;       // declaration
score = 95;      // initialization
score = 100;     // reassignment

System.out.println(score); // 100

A local variable must be assigned a value before it is read. Java does not automatically give local variables a default value.

Common types of variables

class Student {
    String name = "Ravi";        // instance variable
    static int studentCount = 0; // static variable

    void display() {
        int marks = 85;           // local variable
        System.out.println(name + ": " + marks);
    }
}

Constants with final

Use final when a variable should be assigned only once. Constants are usually written in uppercase with underscores.

final double PI = 3.14159;
// PI = 3.14; // Compile-time error: PI cannot be changed

Type inference with var

Since Java 10, var can infer the type of a local variable from its initializer. The type is still fixed after compilation.

var language = "Java"; // inferred as String
var version = 21;       // inferred as int

// var empty;            // Compile-time error: an initializer is required

Variable naming rules

• Names may contain letters, digits, _, and $, but cannot start with a digit.
• Names are case-sensitive: total and Total are different.
• Java keywords such as class, int, and static cannot be used as names.
• Use camelCase for ordinary variables, such as totalMarks.
• Choose descriptive names instead of unclear names like x or data1.

Key Features of Java

Your First Java Program

Let's break down a simple Java program line by line.

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

Output

Line-by-line explanation

Print Output in Java

Java writes console output through System.out. Use print() when the cursor should stay on the same line and println() when the cursor should move to a new line. Use printf() when you need formatted output.

System.out.print("Hello ");
System.out.println("Java");
System.out.printf("Score: %d%%", 95);

• Join text and values with +: "Age: " + age.
• Use %d for integers, %f for decimals, and %s for strings with printf().
• Escape special characters with sequences such as \n for a new line and \t for a tab.

Taking Input with Scanner

The Scanner class reads input from the keyboard. Create one Scanner for System.in, then use a method that matches the expected type.

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.print("Enter your age: ");
int age = scanner.nextInt();

System.out.println(name + " is " + age + " years old.");
scanner.close();

Identifiers

Identifiers are the names given to classes, methods, variables, and other program elements. Good identifiers make code easier to understand.

class StudentProfile {       // class identifier
    String studentName;      // variable identifier

    void printProfile() {    // method identifier
        System.out.println(studentName);
    }
}

• Use letters, digits, underscores, and dollar signs; never begin with a digit.
• Identifiers are case-sensitive: total and Total differ.
• Do not use spaces or Java keywords.
• Prefer camelCase for variables and methods, and PascalCase for classes.

Java Keywords

Keywords are reserved words with special meaning to the compiler. They cannot be used as identifiers.

For example, class defines a class and return sends a value back from a method. Java also has contextual keywords such as var, whose meaning depends on where it is used.

Data Types

A data type defines what kind of value a variable can hold and which operations are valid for it. Java types are divided into primitive types and reference types.

Reference types store a reference to an object, such as String, arrays, and custom classes. They can hold null; primitives cannot.

Primitive variableint age = 25;
Stores the value directly

 or 

Reference variableString name = "Asha";
Points to an object

Literals

A literal is a fixed value written directly in source code. Java supports several literal forms.

int decimal = 42;
int binary = 0b1010;
int hexadecimal = 0x2A;
long largeNumber = 1_000_000L;
double temperature = 36.5;
char initial = 'J';
String message = "Hello, Java";
boolean ready = true;
String nothing = null;

Use underscores to make long numbers easier to read. The underscores are ignored by the compiler and cannot appear at the beginning or end of a number.

Wrapper Classes

Wrapper classes represent primitive values as objects. They are useful with collections and APIs that require objects.

Integer boxed = 42;        // autoboxing: int to Integer
int number = boxed;         // unboxing: Integer to int
int value = Integer.parseInt("123");

Operators

Operators combine values and produce a result.

Decision Making

Decision statements run different code depending on whether a condition is true or false.

Start with a value, for example marks = 76

↓

Is marks >= 90 true?

No ↓

Is marks >= 60 true?

Yes ↓

Print Grade B

int marks = 76;

if (marks >= 90) {
    System.out.println("Grade A");
} else if (marks >= 60) {
    System.out.println("Grade B");
} else {
    System.out.println("Needs improvement");
}

Use switch when comparing one value with several fixed options.

int day = 2;
switch (day) {
    case 1 -> System.out.println("Monday");
    case 2 -> System.out.println("Tuesday");
    default -> System.out.println("Another day");
}

Loops and Jump Statements

Loops repeat a block of code. Use for when the number of repetitions is known, while when a condition controls repetition, and do-while when the body must run at least once.

for (int i = 1; i <= 5; i++) {
    if (i == 3) continue; // skip this iteration
    if (i == 5) break;    // stop the loop
    System.out.println(i);
}

int attempts = 0;
while (attempts < 3) {
    attempts++;
}

• break exits the nearest loop or switch.
• continue skips the current iteration and starts the next one.
• return exits the current method, optionally returning a value.

Project: Number Guessing Game

Use input, variables, operators, decisions, and loops together in this beginner project. The program chooses a number from 1 to 100 and gives the player hints until the answer is found.

import java.util.Random;
import java.util.Scanner;

public class NumberGuessingGame {
    public static void main(String[] args) {
        Random random = new Random();
        Scanner scanner = new Scanner(System.in);
        int target = random.nextInt(100) + 1;
        int attempts = 0;
        int guess;

        System.out.println("Guess a number from 1 to 100.");
        do {
            System.out.print("Your guess: ");
            guess = scanner.nextInt();
            attempts++;

            if (guess < target) {
                System.out.println("Too low!");
            } else if (guess > target) {
                System.out.println("Too high!");
            } else {
                System.out.println("Correct in " + attempts + " attempts!");
            }
        } while (guess != target);

        scanner.close();
    }
}

How to Compile & Run

Using the command line (after installing JDK):

# Step 1 — Save your file as HelloWorld.java
# Step 2 — Compile it
javac HelloWorld.java

# Step 3 — Run it
java HelloWorld

Where is Java Used?

• Android Development — Android apps were historically written in Java (now also Kotlin).
• Enterprise Backend — Large-scale server-side applications using Spring Boot, Hibernate, Jakarta EE.
• Web Applications — Java Servlets, JSP, and modern REST APIs.
• Big Data — Apache Hadoop, Apache Spark, and Kafka are written in Java.
• Cloud & Microservices — Popular with AWS, GCP, and Azure backend services.
• Embedded Systems & IoT — Smart cards, set-top boxes, and industrial controllers.
• Scientific Computing — Used in research tools and simulations.

Java vs Other Languages