Each C source file undergoes a lexical preprocessing stage which serves several purposes, including conditional compilation and macro expansion. The main purpose is to allow declarations of commonly used types, global data and functions to be conveniently and consistently made available to modules which need to access them. In general, the preprocessor is able to insert, remove or replace text from the source code as it is supplied to the compiler. (The original source code doesn't change.)

There is no equivalent of preprocessing in Java, but its purposes don't usually apply to Java anyway.