Exception Handling in Java.

What are Exceptions?

Exception ka matlab hai program ke run hone ke dauraan aayi koi error ya unusual situation. Jaise:

• File ka nhi milne
• Divide by zero karna
• Null value pe operation karna

Java mein exceptions runtime pe aate hain aur agar handle nahi kiya to program crash ho sakta hai.

int a = 5 / 0; // ArithmeticException

Checked vs Unchecked Exceptions

Java exceptions two types ke hote hain:

1. Checked Exceptions:

• Ye compile time pe check ki jaati hain.
• Jaise: IOException, SQLException
• Tumhe ya toh inhe handle karna padta hai try-catch se, ya throw karna padna hai throws keyword lagana padta hai.

public void readFile() throws IOException {
    FileReader fr = new FileReader("data.txt");
}

2. Unchecked Exceptions:

• Ye runtime pe aate hain.
• Jaise: NullPointerException, ArithmeticException
• Inhe compiler check nahi karta.

String s = null;
System.out.println(s.length()); // NullPointerException

try-catch-finally Block

Java mein exception handle karne ke liye try-catch-finally ka use hota hai.

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Can't divide by zero!");
} finally {
    System.out.println("This block always executes.");
}

• try mein likhte hain risky code.
• catch exception handle karta hai.
• finally block hamesha run hota hai (chahe exception aaye ya na aaye).

throw and throws Keywords

throw aur throws similar lagte hain, par dono ka kaam alag hai:

throw:

• Ye kisi specific exception ko manually throw karta hai.

throw new ArithmeticException("Custom error message");

throws:

• Ye method ke signature mein use hota hai jab method koi checked exception throw karta hai.

public void readFile() throws IOException {
    // file handling code
}

Custom Exceptions

Kabhi-kabhi tumhe apni specific situation ke liye custom exception banana padta hai.

class AgeException extends Exception {
    AgeException(String msg) {
        super(msg);
    }
}

public class Test {
    public void checkAge(int age) throws AgeException {
        if (age < 18) {
            throw new AgeException("You must be 18+ to vote");
        }
    }
}

Use karte time:

Test t = new Test();
try {
    t.checkAge(16);
} catch (AgeException e) {
    System.out.println(e.getMessage());
}

Entire Code:

// Step 1: Custom Exception class banate hain
class AgeException extends Exception {
    public AgeException(String message) {
        super(message); // super = Exception class ka constructor
    }
}


// Step 2: Main class jahan exception handle hoga
public class CustomExceptionDemo {


    // Method to check age
    public static void checkAge(int age) throws AgeException {
        if (age < 18) {
            throw new AgeException("You must be at least 18 years old.");
        } else {
            System.out.println("Age valid! You are eligible.");
        }
    }

    // Step 3: main() method to test
    public static void main(String[] args) {
        int userAge = 16; // Tum isko change karke test kar sakte ho

        try {
            checkAge(userAge);
        } catch (AgeException e) {
            System.out.println("Custom Exception Caught:");
            System.out.println(e.getMessage());
        }

        System.out.println("Program continues...");
    }
}

Custom Exception Caught:

You must be at least 18 years old.

Program continues...

Try-with-Resources (Java 7+)

Agar tum file, database ya kisi bhi resource ke saath kaam kar rahe ho jo close karna padta hai, to try-with-resources ka use karo.

try (BufferedReader br = new BufferedReader(new FileReader("data.txt"))) {
    System.out.println(br.readLine());
} catch (IOException e) {
    e.printStackTrace();
}

Yahan resource (file reader) automatically close ho jati hai — koi finally block ki zarurat nahi hoti.

Developer Tips:

• Exception handling code hamesha clean aur minimal rakho.
• Har exception ko catch karne ki bajaye specific exception pakdo.
• Don’t ignore exceptions silently – 'log' ya handle zaroor karo.

Summary

• Exception = error jo runtime pe aata hai
• Checked = compile time pe handle karna padta hai
• Unchecked = runtime errors hote hain
• try-catch-finally se exception handle hota hai
• throw/throws keywords se exceptions manage hote hain
• Custom exceptions se tum apne use-case ke errors define kar sakte ho
• Try-with-resources se tumhara resource auto-close ho jata hai