Exception handling allows a program to continue running even after an error occurs, instead of abruptly terminating.

public class ExceptionExample {
    public static void main(String[] args) {
        try {
            int result = 10 / 0; // ArithmeticException
        } catch (ArithmeticException e) {
            System.out.println("Error: Division by zero!");
        } finally {
            System.out.println("Finally block executed.");
        }
    }
}


Error: Division by zero!
Finally block executed.


// Checked Exception
import java.io.*;

public class CheckedExample {
    public static void main(String[] args) throws IOException {
        FileReader fr = new FileReader("data.txt"); // must handle or declare
    }
}

// Unchecked Exception
public class UncheckedExample {
    public static void main(String[] args) {
        int num = 5 / 0; // no compile error, but runtime exception
    }
}


User-defined exceptions created by extending the 

class AgeNotVerifiedException extends Exception {
    public AgeNotVerifiedException(String msg) {
        super(msg);
    }
}

public class AgeChecker {
    public static void validateAge(int age) throws AgeNotVerifiedException {
        if (age < 18)
            throw new AgeNotVerifiedException("Underage! Access denied.");
        else
            System.out.println("Access granted.");
    }

    public static void main(String[] args) {
        try {
            validateAge(15);
        } catch (AgeNotVerifiedException e) {
            System.out.println("Exception caught: " + e.getMessage());
        }
    }
}


It can have both abstract (no body) and concrete methods.

abstract class Animal {
    abstract void sound(); // abstract method

    void eat() {
        System.out.println("Animals can eat");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();
        d.eat();
    }
}


final class Vehicle {
    final int wheels = 4;

    final void showType() {
        System.out.println("This is a vehicle");
    }
}

// class Car extends Vehicle {} // Error: cannot inherit final class

public class Main {
    public static void main(String[] args) {
        Vehicle v = new Vehicle();
        System.out.println("Wheels: " + v.wheels);
        v.showType();
    }
}


Used to read from or write to files using classes from 

import java.io.FileWriter;
import java.io.IOException;

public class WriteExample {
    public static void main(String[] args) {
        try {
            FileWriter writer = new FileWriter("output.txt");
            writer.write("Hello, this is a test file!");
            writer.close();
            System.out.println("Successfully wrote to the file.");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


import java.io.FileReader;
import java.io.IOException;

public class ReadExample {
    public static void main(String[] args) {
        try {
            FileReader reader = new FileReader("output.txt");
            int data;
            while ((data = reader.read()) != -1) {
                System.out.print((char) data);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


An interface is a collection of abstract methods (implicitly 

interface Animal {
    void sound();
}

class Dog implements Animal {
    public void sound() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();
    }
}


Packages group related classes and interfaces.

They help organize code and avoid name conflicts.

package mypack;

public class Message {
    public void display() {
        System.out.println("Hello from package!");
    }
}

// Another file
import mypack.Message;

public class Test {
    public static void main(String[] args) {
        Message msg = new Message();
        msg.display();
    }
}
