JavaOPPs

1. Classes and Objects in Java:
class Student {
    String name;
    int rollNo;

    void display() {
        System.out.println("Name: " + name + ", Roll No: " + rollNo);
    }
    
    public static void main(String[] args) {
        Student s1 = new Student();
        s1.name = "Pratik";
        s1.rollNo = 43;
        s1.display();
    }
}

Constructors, Destructors and Overloading:
class Box {
    int width, height;

    Box() {
        width = height = 0;
        System.out.println("Default Constructor");
    }

    Box(int w, int h) {
        width = w;
        height = h;
        System.out.println("Parameterized Constructor");
    }

    void display() {
        System.out.println("Width: " + width + ", Height: " + height);
    }

    protected void finalize() {
        System.out.println("Object is destroyed");
    }

    public static void main(String[] args) {
        Box b1 = new Box();
        Box b2 = new Box(1, 20);
        b1.display();
        b2.display();
        b1 = null;
        b2 = null;
        System.gc();
    }
}

3. Inheritance:
class University {
    void student() {
        System.out.println("In the University");
    }
}

class Teacher extends University {
    void student() {
        System.out.println("In the class");
    }

    public static void main(String[] args) {
        Teacher d = new Teacher();
        d.student();
    }
}

4. Interface:
interface Playing {
    void play();
}

class Ground implements Playing {
    public void play() {
        System.out.println("Students are Playing");
    }

    public static void main(String[] args) {
        Playing d = new Ground();
        d.play();
    }
}


5. Packages:
File 1 (inside package folder mypack) - MyClass.java:
package mypack;

public class MyClass {
    public void show() {
        System.out.println("Hello from MyClass in mypack");
    }
}
File 2 - MainClass.java:
import mypack.MyClass;

public class MainClass {
    public static void main(String[] args) {
        MyClass obj = new MyClass();
        obj.show();
    }
}

6. Exception Handling:
public class Yash {
    public static void main(String[] args) {
        try {
            int a = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        } finally {
            System.out.println("Finally block executed");
        }
    }
}

7. Generics (Class and Method):
class GenericClass<T> {
    T value;

    GenericClass(T val) {
        value = val;
    }

    void display() {
        System.out.println("Value: " + value);
    }

    static <E> void genericMethod(E element) {
        System.out.println("Element: " + element);
    }

    public static void main(String[] args) {
        GenericClass<Integer> obj1 = new GenericClass<>(123);
        obj1.display();
        genericMethod("Hello Generics");
    }
}

8. File Handling:
import java.io.*;

public class FileHandlingDemo {
    public static void main(String[] args) {
        try {
            FileWriter writer = new FileWriter("output.txt");
            writer.write("Hello File Handling!");
            writer.close();

            BufferedReader reader = new BufferedReader(new FileReader("output.txt"));
            String line;
            while ((line = reader.readLine()) != null)
                System.out.println("File Content: " + line);
            reader.close();

        } catch (IOException e) {
            System.out.println("File error: " + e.getMessage());
        }
    }
}