What is Java? Computer notes by home academy

 

What is Java?

• Java is a high-level, object-oriented, class-based programming language.

• Developed by James Gosling at Sun Microsystems in 1995.

• Java follows the principle: “Write Once, Run Anywhere” (WORA).

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📌 Key Features of Java

Feature	Description
Platform Independent	Runs on JVM (Java Virtual Machine)
Object-Oriented	Based on objects, classes, and inheritance
Simple & Secure	Easy to learn with secure memory management
Robust & Reliable	Strong exception handling
Multithreaded	Supports concurrent execution
Portable	Runs on any OS with JVM
Compiled + Interpreted	Uses both compiler and interpreter

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🧱 Structure of a Java Program

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

🔍 Explanation:

• class Hello – Declares a class

• main() – Entry point of any Java program

• System.out.println() – Prints output to console

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📚 Basic Concepts

1. Data Types

java
int age = 25;
float marks = 87.5f;
char grade = 'A';
boolean isPassed = true;
String name = "Afzal";

2. Operators

java
+  -  *  /  %       // Arithmetic
== != > < >= <=     // Relational
&& || !             // Logical
= += -= *= /=       // Assignment

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🔁 Control Statements

If-else

java
if (age >= 18) {
    System.out.println("Adult");
} else {
    System.out.println("Minor");
}

Switch

java
switch (day) {
    case 1: System.out.println("Monday"); break;
    default: System.out.println("Other");
}

Loops

java
for(int i = 0; i < 5; i++) {
    System.out.println(i);
}

int i = 0;
while(i < 5) {
    System.out.println(i);
    i++;
}

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🔤 Functions (Methods)

java
public static int add(int a, int b) {
    return a + b;
}

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🧠 Object-Oriented Concepts

Concept	Description
Class	Blueprint for creating objects
Object	Instance of a class
Inheritance	One class inherits another
Polymorphism	Same name, different behaviors
Encapsulation	Wrapping data and code together
Abstraction	Hiding internal implementation

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Example of Class & Object:

java
class Car {
    String brand = "BMW";
    void show() {
        System.out.println(brand);
    }
}

public class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.show();  // Output: BMW
    }
}

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📦 Packages and Import

java
import java.util.Scanner;

Scanner sc = new Scanner(System.in);

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🔐 Exception Handling

java
try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
}

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🧩 Arrays and Strings

Arrays

java
int[] nums = {1, 2, 3};
System.out.println(nums[0]);

Strings

java
String name = "Java";
System.out.println(name.length());

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🌍 Practical Applications of Java

Area	Examples
Web Applications	Spring, JSP
Mobile Apps	Android development
Desktop Software	Swing, JavaFX
Enterprise Apps	Banking systems
Game Development	2D/3D games
Embedded Systems	Smart devices

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❓ Frequently Asked MCQs

1. Who developed Java?
   a) Brendan Eich
   b) Bjarne Stroustrup
   c) James Gosling ✅
   d) Dennis Ritchie

2. Which of these is not a Java keyword?
   a) class
   b) public
   c) integer ✅
   d) static

3. Java is:
   a) Compiled only
   b) Interpreted only
   c) Both compiled and interpreted ✅
   d) None

4. Which method is the entry point of a Java program?
   a) start()
   b) init()
   c) main() ✅
   d) run()

5. Which keyword is used to inherit a class in Java?
   a) implements
   b) extends ✅
   c) include
   d) inherit

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📌 Important Points for Exams

• .java is the source file, .class is the compiled bytecode.

• Java uses automatic garbage collection.

• JDK = Java Development Kit (contains JRE + compiler)

• JRE = Java Runtime Environment

• JVM = Java Virtual Machine (executes bytecode)

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📝 Conclusion

Java is one of the most powerful, secure, and scalable languages, widely used for enterprise, Android, and web development. It’s essential for OOP, backend programming, and job interviews.