Exceptions in Java

In Java, an exception is an event that disrupts the normal flow of execution in a program. Exceptions occur during runtime due to various reasons like invalid input, hardware failure, or logical errors in code.

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Exception Hierarchy in Java

The root class of all exceptions in Java is Throwable, which is a subclass of Object. It has two direct subclasses:

1. Exception (java.lang.Exception) – Used for handling conditions that a program should catch and handle.
2. Error (java.lang.Error) – Represents serious system failures that a program should not attempt to catch (e.g., OutOfMemoryError).

Java Exception Hierarchy Diagram

                Object

                   │

               Throwable

                   │

       ┌──────────┴───────────┐

       │                      │

     Error                Exception

       │                      │

       │             ┌────────┴──────────┐

  (System-related)  IOException       RuntimeException

                          │                    │

                    FileNotFoundException    ArithmeticException

                    EOFException             NullPointerException

                    etc.                     ArrayIndexOutOfBoundsException

                                              etc.

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Types of Exceptions in Java

1. Checked Exceptions (Compile-time Exceptions)

• These exceptions are checked at compile time.
• The compiler forces the programmer to handle them using try-catch or throws.
• Example: IOException, SQLException, FileNotFoundException.

import java.io.*;

 

public class CheckedExceptionExample {

    public static void main(String[] args) {

        try {

            FileReader file = new FileReader("non_existent_file.txt");

            BufferedReader br = new BufferedReader(file);

        } catch (IOException e) {

            System.out.println("File not found: " + e.getMessage());

        }

    }

}

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2. Unchecked Exceptions (Runtime Exceptions)

• These exceptions occur at runtime and are not checked at compile time.
• They occur due to programming logic errors, such as division by zero or accessing an invalid index.
• Example: NullPointerException, ArithmeticException, ArrayIndexOutOfBoundsException.

public class UncheckedExceptionExample {

    public static void main(String[] args) {

        int[] numbers = {1, 2, 3};

        System.out.println(numbers[5]);  // ArrayIndexOutOfBoundsException

    }

}

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3. Errors (Serious System Failures)

• Errors are serious failures that usually cannot be handled in a normal program.
• Example: OutOfMemoryError, StackOverflowError.

public class ErrorExample {

    public static void recursiveMethod() {

        recursiveMethod();  // Infinite recursion leading to StackOverflowError

    }

 

    public static void main(String[] args) {

        recursiveMethod();

    }

}