By convention, a Java class' physical location (source directories) and its logical representation (packages) should be kept in sync. Thus a Java file located at "src/org/bar/Foo.java" should have a package of "org.bar".

Unfortunately, this convention is not enforced by Java compilers, and nothing prevents a developer from making the "Foo.java" class part of the "com.apple" package, which could degrade the maintainability of both the class and its application.

Similarly, source placed in a folder with dots in its name instead of having the equivalent folder structure will compile but cause problems at run time. For instance, code with a package declaration of org.foo.bar that is placed in org/foo.bar will compile, but the classloader will always search for the class into the folder based on package structure, and will consequently expect sources to be in org/foo/bar folder. foo.bar is therefore not a proper folder name for sources.