Exception Handling in Java: A Complete Tutorial

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Last Updated on: 23rd Nov 2025 16:13:26 PM

In real-world applications, errors can happen anytime — wrong user input, network failure, file not found, division by zero, etc.
If these errors are not handled properly, the program may crash.

 

To prevent this, Java provides a powerful mechanism called Exception Handling.

 

Exception Handling allows a program to:

✔ Detect errors
✔ Handle them gracefully
✔ Continue program execution without crashing

It improves:

• Program stability

• Error reporting

• User experience

• Security

Java uses a structured model with try–catch–finally, throw, throws, and custom exceptions to handle different types of errors efficiently.

 

What is an Exception?

An exception is an unwanted event that occurs during program execution and disrupts the normal flow of the program.

Example:

• Dividing a number by zero

• Accessing an invalid array index

• File not found

• Null object access

 

Types of Errors in Java

1. Compile-time Errors

Errors detected by the compiler. Example: missing semicolon, wrong syntax.

 

2. Runtime Errors (Exceptions)

Errors that happen while the program is running. Example: ArithmeticException, NullPointerException.

 

3. Logical Errors

Program runs but produces wrong output.

 

Types of Exceptions in Java

Java divides exceptions into two categories:

 

1️⃣ Checked Exceptions

Caught by the compiler . These are checked at compile-time. Must be handled using try–catch or throws.

Examples:

• IOException

• SQLException

• FileNotFoundException

 

2️⃣ Unchecked Exceptions

These occur at runtime and are not checked by the compiler. They usually result from programming errors. Occur due to programming mistakes

Examples:

• ArithmeticException

• NullPointerException

• ArrayIndexOutOfBoundsException

 

Exception Hierarchy Diagram

Throwable
   |
   |-- Exception
   |       |-- RuntimeException
   |       |-- IOException
   |       |-- SQLException
   |
   |-- Error
           |-- StackOverflowError
           |-- OutOfMemoryError

 

Java Exception Handling Keywords

Keyword	Meaning
try	Code that may throw an exception
catch	Handle the exception
finally	Always executes (cleanup code)
throw	Throw an exception manually
throws	Declare exception to JVM
custom exception	User-defined exception class

 

1. try–catch Example

public class Main {
    public static void main(String[] args) {
        try {
            int result = 10 / 0;
            System.out.println(result);
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero!");
        }
    }
}

 

OUTPUT

Cannot divide by zero!

 

2. Using Multiple catch Blocks

public class Main {
    public static void main(String[] args) {
        try {
            String s = null;
            System.out.println(s.length());
        } catch (NullPointerException e) {
            System.out.println("Null value found!");
        } catch (Exception e) {
            System.out.println("Other exception occurred.");
        }
    }
}

 

3. finally Block Example

The finally block always executes.

try {
    int a = 5 / 0;
} catch (Exception e) {
    System.out.println("Exception handled.");
} finally {
    System.out.println("Finally block executed.");
}

 

4. throw Keyword Example (Manual Exception)

public class Main {
    static void checkAge(int age) {
        if (age < 18) {
            throw new ArithmeticException("Access denied");
        } else {
            System.out.println("Access granted");
        }
    }

    public static void main(String[] args) {
        checkAge(15);
    }
}

 

5. throws Keyword Example

Used when a method may throw an exception.

import java.io.*;

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

    public static void main(String[] args) {
        Test t = new Test();
        try {
            t.readFile();
        } catch (IOException e) {
            System.out.println("File not found!");
        }
    }
}

6. Custom Exception Example

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

public class Bank {
    void withdraw(int amount) throws InvalidAmountException {
        if (amount < 0) {
            throw new InvalidAmountException("Amount cannot be negative!");
        } else {
            System.out.println("Withdrawal successful: " + amount);
        }
    }

    public static void main(String[] args) {
        Bank b = new Bank();
        try {
            b.withdraw(-100);
        } catch (InvalidAmountException e) {
            System.out.println(e.getMessage());
        }
    }
}

 

Real-Life Example: Online Payment Failure

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

class PaymentGateway {
    void processPayment(boolean status) throws PaymentFailedException {
        if (!status) {
            throw new PaymentFailedException("Payment Failed! Try Again.");
        } else {
            System.out.println("Payment Successful");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        PaymentGateway pg = new PaymentGateway();
        try {
            pg.processPayment(false);
        } catch (PaymentFailedException e) {
            System.out.println(e.getMessage());
        }
    }
}

 

Output

Payment Failed! Try Again.

 

Final Summary

• Exception Handling avoids program crashes

• Java provides try, catch, finally, throw, throws

• Two types: Checked and Unchecked

• We can create Custom Exceptions

• Helps build robust, error-free applications

 

Keep practicing — you're doing amazing!

Happy Coding!    

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