Debugging Basics: Compiler Errors vs. Exceptions

Java surfaces problems in two very different ways. A compiler error (from javac) means the code doesn't even compile — a typo, a type mismatch, a missing semicolon — and no .class file is produced at all. An exception happens while a successfully compiled program is running — dividing by zero, indexing past an array's end, calling a method on null.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:
    Index 3 out of bounds for length 3
    at Main.main(Main.java:4)

Read a stack trace from the top down: the exception type and message come first, then the exact file and line where it happened. Set a breakpoint on that line in your IDE's debugger to inspect variable values at the moment things went wrong.

Common mistakes

• Reading only the first line of a long stack trace and missing the actual line number where the failure occurred, further down in "Caused by:" sections.