Exception handling: OO vs conventional

Exception and error handling

An error is a wrong condition or result in a program (for example a wrong value, a logical mistake, resource failure). An exception is an unexpected event during the run that disrupts the normal flow of instructions (division by zero, file not found, null reference, network down). Error handling is the set of techniques that detect these situations and respond to them: recover, report or stop in a safe state, without losing data or crashing.

Conventional method (procedural: C, Pascal)

• Functions return error codes (-1, NULL, errno) or set global flags.
• The caller must check after every call with if statements.
• Normal logic and error logic are mixed, so the code is long and hard to read.
• Easy to forget the check; the error is then silently ignored.
• Error cannot easily be passed up through several levels.

FILE *f = fopen("a.txt", "r");
if (f == NULL) { printf("error\n"); return -1; }

Object-oriented method (Java, C++, C#)

• Errors are objects of exception classes (organised in an inheritance hierarchy, e.g. Throwable -> Exception -> IOException).
• The code that detects the problem throws an exception; the one that knows how to handle it catches it with try/catch/finally.
• Exceptions propagate up the call stack automatically.
• Resources are cleaned up in finally or by destructors/try-with-resources.

try {
    FileReader f = new FileReader("a.txt");
} catch (FileNotFoundException e) {
    System.out.println("File missing");
} finally { System.out.println("done"); }

The OO method is therefore cleaner, safer, and more informative.