Phase 5: Exception Handling & Robust Systems

Exception handling is tested directly in Theory Assignment Q2. Master the Throwable hierarchy, checked vs unchecked exceptions, and try-catch mechanics.

1. Exception Hierarchy & Checked vs Unchecked

                        Throwable
                        /       \
                  Exception     Error (OutOfMemory, StackOverflow - Fatal)
                   /     \
      Checked Exceptions  RuntimeException (Unchecked)
      (IOException,       (NullPointerException,
       SQLException)       NumberFormatException,
                           ArrayIndexOutOfBoundsException)
        

Type	Cause & Compiler Rule	Examples
Checked Exception	Caused by external factors (missing file, database offline). Compiler FORCES handling (`try-catch` or `throws`).	`IOException`, `SQLException`, `FileNotFoundException`
Unchecked Exception (`RuntimeException`)	Caused by flawed program logic. Compiler does NOT force handling. Should be fixed in code.	`NullPointerException`, `NumberFormatException`, `ArrayIndexOutOfBoundsException`

2. Exam Scenarios & Handling (Direct Assignment Solutions)

Exception	Scenario Where It Occurs	How to Handle / Prevent
`NullPointerException`	Attempting to invoke a method or access an attribute on an uninitialized (`null`) object reference.	Null checks (`if (obj != null)`), using `Optional` class.
`IOException`	Attempting to read/write a file that is missing, locked, or corrupt during I/O operations.	Wrap in `try-catch(IOException e)` block or declare `throws IOException`.
`NumberFormatException`	Attempting to parse an invalid string into a numeric value (e.g. `Integer.parseInt("abc")`).	Input validation, catching `NumberFormatException` and prompting user for valid numbers.
`ArrayIndexOutOfBoundsException`	Accessing an array index less than 0 or greater than or equal to `array.length`.	Check array bounds before access using `index < array.length`.

💡 Examiner Distinction Tip: throw vs throws

throw is used INSIDE a method body to explicitly instantiate and throw an exception object (e.g., throw new IllegalArgumentException();).
throws is declared in the METHOD HEADER to notify callers that this method may throw specified checked exceptions (e.g., public void readFile() throws IOException).

⚡ Quick Self-Test

Q: Does the finally block execute if an exception occurs inside the try block?

Answer: Yes!.
The finally block ALWAYS executes, regardless of whether an exception occurred or was caught (used for cleanup resources like closing database connections or files), unless System.exit(0) is called.