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📜 WAYPOINT LESSON

Three classes of errors

⭐ beginner⏳ 10 min read📍 Lesson 61 of 256

Compile-time, runtime exceptions, logical errors — and where each one hides.

Every Java failure belongs to one of three classes, and each has a different repair strategy:

  • Compile-time error — the compiler rejects the source: missing semicolon, wrong type, unknown name. Nothing runs. Read the message: it names the file, line, and what it expected.
  • Runtime error (exception) — compiles, then dies mid-run: ArithmeticException (integer divide by zero), ArrayIndexOutOfBoundsException, NullPointerException. The stack trace names the line that threw.
  • Logical error — compiles, runs, produces a wrong answer. No message ever appears. Only testing finds these, which is why they're the most dangerous.
int avg = total / n;      // compiles; throws at runtime if n == 0;
                          // silently wrong if total or n is wrong

The exam's "code does not compile" questions are all compile-class; its "what is output" questions are trace questions guarding against your logical errors. Classify first, then debug — the class tells you where to look.