Introduction

• An Exception is an unwanted event that occurs during the execution of a program and disrupts the normal flow of instructions.
• Exceptions usually happen due to problems in the program logic.
• For Example :
  • IOException
    • Occurs when an input/output operation fails.
    • Syntax:

      FileReader fr = new FileReader("file.txt");  // May throw IOException

  • ArithmeticException
    • Occurs when dividing a number by zero.
    • Syntax:

      int result = 10 / 0;  // Throws ArithmeticException

  • NullPointerException
    • Occurs when calling a method on a null object.
    • Syntax:

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

• Unlike Errors, exceptions are within the control of the programmer and can be handled in code.

Types of Exceptions in Java

There are two types of Exceptions in Java

1. Checked Exceptions
   • Checked Exceptions are those which are checked at compile time.
   • Program won't compile unless they are handled with try-catch or declared using throws.
   • Examples :
     • IOException
       • Input/output operation fails (e.g., file not found).

       • FileReader fr = new FileReader("abc.txt");  // May throw IOException

     • SQLException
       • Error in database access.

       • Connection con = DriverManager.getConnection(url, user, pass);  // May throw SQLException

2. Unchecked Exceptions
   • Unchecked exceptions are those which occur during runtime, not checked by compiler.
   • Usually caused by programming mistakes like invalid index, null access, or divide by zero.
   • Examples :
     • ArithmeticException
       • Divide by zero error.

       • int result = 10 / 0;  // Throws ArithmeticException

     • ArrayIndexOutOfBoundsException
       • Accessing array index outside valid range.

       • int[] arr = {10, 20, 30};  
         System.out.println(arr[5]);  // Throws ArrayIndexOutOfBoundsException

Exception Class Hierarchy

• Exception is the pre-defined class in Java which inherits the Throwable class.
• Below is the hierarchy of Exception class in Java.

• Points to remember:
  • Object class is the parent class of all the classes in Java
  • Throwable class is the parent class of Exception class in Java.
  • Exception class itself is a checked exception, because it is not a subclass of RuntimeException.

What is Exception Handling?

• Exception Handling is the mechanism to handle the exceptions (or runtime errors) so that the normal flow of the program is not disrupted.
• Need for Exception Handling:
  • Prevents program crashes.
  • Provides meaningful error messages.
  • Separates normal logic from error-handling logic.
  • Makes applications more robust and user-friendly.
• Keywords Used in Exception Handling:
  • try → Block of code that may throw an exception.
  • catch → Block to handle the exception.
  • finally → Block that always executes (used for cleanup code).
  • throw → Used to explicitly throw an exception.
  • throws → Declares exceptions that a method can throw.

• Points to remember:
  • Technically, the catch keyword is used to handle exceptions in Java.
    • Other keywords (try, finally, throw, throws) have different functionalities and do not directly handle exceptions.
  • It is compulsory to handle Checked Exceptions in Java.
    • It is not compulsory to handle Unchecked Exceptions, but it is a best practice to handle both (Checked and Unchecked) for making applications more stable and user-friendly.