Java Concepts

1. Introduction to Java

Java Basics

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

2. Data Types and Variables

Primitive Data Types

int num = 5;
char letter = 'A';
boolean isTrue = true;
double decimal = 5.99;

Reference Data Types

String text = "Hello, Java";
int[] numbers = {1, 2, 3, 4, 5};

3. Operators

Arithmetic Operators

int sum = 5 + 3;
int difference = 5 - 3;
int product = 5 * 3;
int quotient = 5 / 3;
int remainder = 5 % 3;

4. Control Flow Statements

If-Else Statement

int num = 10;
if (num > 5) {
    System.out.println("Number is greater than 5");
} else {
    System.out.println("Number is 5 or less");
}

Switch Statement

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other day");
}

For Loop

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

While Loop

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

5. Methods

Defining and Calling Methods

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

6. Object-Oriented Programming

Classes and Objects

class Dog {
    String name;
    int age;
    
    void bark() {
        System.out.println("Woof!");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.name = "Buddy";
        myDog.age = 3;
        myDog.bark();
    }
}

Inheritance

class Animal {
    void eat() {
        System.out.println("Eating...");
    }
}
 
class Dog extends Animal {
    void bark() {
        System.out.println("Woof!");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.eat();
        myDog.bark();
    }
}

Polymorphism

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}
 
class Dog extends Animal {
    void sound() {
        System.out.println("Woof");
    }
}
 
public class Main {
    public static void main(String[] args) {
        Animal myAnimal = new Dog();
        myAnimal.sound();
    }
}

Encapsulation

class Person {
    private String name;
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
}
 
public class Main {
    public static void main(String[] args) {
        Person person = new Person();
        person.setName("John");
        System.out.println(person.getName());
    }
}

7. Arrays

One-dimensional Arrays

int[] numbers = {1, 2, 3, 4, 5};
for (int number : numbers) {
    System.out.println(number);
}

Two-dimensional Arrays

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.print(matrix[i][j] + " ");
    }
    System.out.println();
}

8. Exception Handling

Try-Catch Block

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

9. Collections Framework

ArrayList

import java.util.ArrayList;
 
public class Main {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");
        
        for (String fruit : list) {
            System.out.println(fruit);
        }
    }
}

HashMap

import java.util.HashMap;
 
public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Cherry", 3);
        
        for (String key : map.keySet()) {
            System.out.println(key + ": " + map.get(key));
        }
    }
}

10. File Handling

Reading from a File

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
 
public class Main {
    public static void main(String[] args) {
        try {
            File myFile = new File("filename.txt");
            Scanner reader = new Scanner(myFile);
            while (reader.hasNextLine()) {
                String data = reader.nextLine();
                System.out.println(data);
            }
            reader.close();
        } catch (FileNotFoundException e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        }
    }
}

11. Multithreading

Creating a Thread by Extending Thread Class

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}
 
public class Main {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start();
    }
}

Creating a Thread by Implementing Runnable Interface

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Thread is running");
    }
}
 
public class Main {
    public static void main(String[] args) {
        MyRunnable myRunnable = new MyRunnable();
        Thread t1 = new Thread(myRunnable);
        t1.start();
    }
}

12. Advanced Concepts

Lambda Expressions

interface MyFunctionalInterface {
    void myMethod();
}
 
public class Main {
    public static void main(String[] args) {
        MyFunctionalInterface myFunc = () -> System.out.println("Lambda Expression Example");
        myFunc.myMethod();
    }
}

Streams API

import java.util.Arrays;
import java.util.List;
 
public class Main {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("John", "Jane", "Jack", "Doe");
        names.stream()
             .filter(name -> name.startsWith("J"))
             .forEach(System.out::println);
    }
}

Java Theory Questions

1. What is Java?

Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is intended to let application developers write once, run anywhere (WORA).

2. What are the main features of Java?

Object-Oriented: Java follows the OOP principles.
Platform Independent: Java code can run on any device that has a Java Virtual Machine (JVM).
Simple and Secure: Java provides a robust and secure environment.
Multithreading: Java supports concurrent execution of two or more threads for maximum CPU utilization.

3. What is the difference between JDK, JRE, and JVM?

JDK (Java Development Kit): A software development kit that includes tools for developing Java applications, including the JRE.
JRE (Java Runtime Environment): A part of the JDK that allows you to run Java applications. It includes the JVM and libraries.
JVM (Java Virtual Machine): An abstract machine that enables a computer to run Java programs. It translates Java bytecode into machine code.

4. Explain the concept of Object-Oriented Programming (OOP) and its principles.

Object-Oriented Programming is a programming paradigm that uses "objects" to represent data and methods. The main principles of OOP are:

Encapsulation: Bundling the data and methods that operate on the data within one unit, or class.
Inheritance: Mechanism by which one class can inherit properties and methods from another class.
Polymorphism: The ability of different classes to respond to the same method call in different ways.

5. What is the role of the main method in Java?

The main method is the entry point of any Java program. It is always defined as:

public static void main(String[] args) {
    // code to be executed
}