In my recent programming adventures, I separately encountered two perplexing error messages:

• RuntimeException: Variable is used without definition!
• IllegalArgumentException: Duplicate field expected

Inspection of the underlying code brought clarity:

• throw new RuntimeException("Variable " + m + " used without definition!");
• throw new IllegalArgumentException("Duplicate field " + fieldName);

The former was reporting a problem with a variable declared as InputStream is, the latter a problem with JUnit’s @Test annotation which defines a parameter java.lang.Class<? extends java.lang.Throwable> expected.

Tip from the pros: always wrap identifiers in quotes when including them in error messages.