Question

Package structure should be same as directory structure in Java it seems in Eclipse.
I tried to browse through the formal documentation on Oracle Java docs but nowhere I could find this rule

Does this rule extend to other IDEs such as Netbeans or is this a rule of Java language itself?
How strict is this rule, can it be deviated from and why was it made?

Answer 1

It’s specified in the JLS as a rule that compilers may require:

Packages that are stored in a file system may have certain constraints on the organization of their compilation units to allow a simple implementation to find classes easily.

For example, if you specify all the source filenames on the command line for javac, you don’t have to follow those rules… but it’s pretty much universally used, and I wouldn’t dream of violating this rule for anything other than tiny tests.

Note that by default, javac will generate the class file as a peer of the source file, regardless of package structure; if you specify -d it will generate the appropriate output structure even if it doesn’t match the source structure.