Java - When is it a compiler error and when is it a runtime exception?

java exception compilation scjp
Michael picture Michael · Jul 5, 2010 · Viewed 27.2k times · Source

I am currently studying for the SCJP certification using the Sierra and Bates Study Guide and in many of the self tests (mock exam questions) I keep running into the same problem - I can't tell whether a particular error will be at runtime (an exception) or at compile (compile error). I know this is a bit of a vague question and that it might not be possible to answer but, how can I tell if an error will be found at compile or at runtime? Would you be able to send me some website links that might be able to help me?

Answer

Andreas Dolk picture Andreas Dolk · Jul 5, 2010

Compile time error - the java compiler can't compile the code, often because of syntax errors. Typical candidates:

  • missing brackets
  • missing semicolons
  • access to private fields in other classes
  • missing classes on the classpath (at compile time)

Runtime error - the code did compile, can be executed but crashes at some point, like you have a division by zero.

  • using variable that are actually null (may cause NullPointerException)
  • using illegal indexes on arrays
  • accessing resources that are currently unavailable (missing files, ...)
  • missing classes on the classpath (at runtime)

('Crashes' is really not the correct term and is only used to illustrate what happens)

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