[Java] file layout: pad numbers in filenames

[vlkl-sap]

Problem statement

Directories are currently sorted like this by default:

69_pass_variable_between_java_files
6_callable
70_raw_types

Proposed changes

While it is possible to use ls -v, consider pading all numbers, i.e., 6 to 006, etc.

[compaluca]

Also this is a good point, similar to #15. Merging them together in #15.