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readme.md

☕ Java Lesson – Exception, throw, throws & Custom Exception (Complete Guide)

Is lesson me hum cover karenge:

  • Exception kya hota hai
  • Built-in exceptions
  • throw keyword
  • throws keyword
  • Custom (User-defined) exception
  • Exception flow aur real programs

1️⃣ Exception Kya Hota Hai?

Exception ek runtime problem hota hai jo program ke normal flow ko disturb karta hai.

Example:

10 / 0 → ArithmeticException

Agar exception handle na kiya jaye:

program terminate ho jata hai

2️⃣ Built-in Exceptions

Java already bahut saare exceptions provide karta hai.

Exception Cause
ArithmeticException divide by zero
NullPointerException null object
ArrayIndexOutOfBoundsException invalid index
NumberFormatException invalid number

Example:

int a = 10 / 0;  // ArithmeticException

3️⃣ Exception Handling Basic Syntax

try {
    risky code
} catch(Exception e) {
    handling code
}

Example:

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

4️⃣ throw Keyword

throw ka use hota hai:

manually exception generate karne ke liye

Syntax:

throw new ExceptionType("message");

Example:

int age = 15;

if(age < 18){
    throw new ArithmeticException("Not eligible");
}

5️⃣ throws Keyword

throws ka use hota hai:

method declaration me exception ko forward karne ke liye

Example:

void readFile() throws IOException {
}

6️⃣ throw vs throws

Feature throw throws
Use exception throw karna exception declare karna
Location method body method signature
Object required ✔ Yes ❌ No

7️⃣ Custom Exception Kya Hoti Hai?

Aap khud ki exception class bana sakte ho jab built-in exception sufficient na ho.

Example:

class InvalidAgeException extends Exception {

    InvalidAgeException(String msg){
        super(msg);
    }
}

8️⃣ Custom Exception Throw Karna

static void checkAge(int age) throws InvalidAgeException {

    if(age < 18){
        throw new InvalidAgeException("Under age");
    }
}

9️⃣ Custom Exception Handle Karna

public static void main(String[] args) {

    try {
        checkAge(15);
    } catch(InvalidAgeException e){
        System.out.println(e.getMessage());
    }
}

Output:

Under age

🔟 Exception Flow

method → throw → throws → try-catch → handled

1️⃣1️⃣ Complete Example Program

class InvalidAgeException extends Exception {

    InvalidAgeException(String msg){
        super(msg);
    }
}

class Test {

    static void checkAge(int age) throws InvalidAgeException {

        if(age < 18){
            throw new InvalidAgeException("Not eligible to vote");
        }
    }

    public static void main(String[] args) {

        try {
            checkAge(16);
        } catch(InvalidAgeException e){
            System.out.println(e.getMessage());
        }
    }
}

1️⃣2️⃣ Checked vs Unchecked Exception

Type Example
Checked IOException
Unchecked ArithmeticException

Custom exception default:

checked hoti hai

Agar RuntimeException extend kare:

unchecked ban jati hai

1️⃣3️⃣ Custom Unchecked Exception

class MyException extends RuntimeException {

    MyException(String msg){
        super(msg);
    }
}

1️⃣4️⃣ Important Rules

✔ throw ek exception object throw karta hai
✔ throws multiple exceptions declare kar sakta hai
✔ custom exception class Exception ya RuntimeException extend karti hai

🎯 Interview Questions

  1. throw aur throws me difference kya hai?
  2. Custom exception kab banani chahiye?
  3. Checked aur unchecked custom exception me difference kya hai?
  4. Exception hierarchy kya hoti hai?

🧾 Lesson Summary

Is lesson me humne seekha:

✔ Exception handling basics
✔ Built-in exceptions
✔ throw keyword
✔ throws keyword
✔ Custom exception creation aur handling

Java me exception handling robust aur error-safe programs banane ke liye essential hai.