There are a few exceptions, mostly caused by changes during a major Java version upgrade.
- Java 1.4 introduced assertions. "assert" became a keyword. You can turn this off by using the compiler flags (-source 1.3) to use Java 1.3 source compatibility, thereby using the keyword "assert" as an identifier
- Java 5.0 introduced enumerations. "enum" became a keyword. See the above; -source 1.4 allows you to use enum as an identifier.
Both will still show a warning but it will no longer be an error.