Java, the language, is case sensitive. Capitalisation must be precise in class, method and variable names. avar is not the same variable as aVar. However, the underlying operating system may be case insensitive, e.g. Windows. To Java, MyApp.class and Myapp.class are totally different files, but to Windows they are the same. To avoid problems, never name two different classes (or any other entities for that matter) with names differing only in capitalisation. The one possible exception is to name a local variable the same as its class, with the leading cap change to lower case, e.g. MyApplet myApplet = new MyApplet(); The OS never sees these local variable names.

Resources, resource bundles, jar members generally and images all need the correct capitalisation. Again for safety, file names should be correct too, though Windows permits sloppiness.

Gotchas

• Your HTML will work fine locally, but stop working when you upload it to a Unix server. Your W2K/XP/W2K3/Vista development machine will work fine with links with incorrect capitalisation, but the Unix machine will fail.
• Your code will work fine on your machine, but when you give it to someone who has a case-sensitive OS (e.g. Linux), or you post it on a case-sensitive server, it won’t work for others.
• Your code works fine, until you bundle it up in jars to distribute. The problem is, jars are case sensitive where the OS, when looking for naked class files, is not.
• Maintaining strict case sensitivity in a sloppy, case-insensitive OS is much more difficult than maintaining it in a case-sensitive one like Unix. File explorer programs often lie about the true case of a file. Configure your directory viewer to be truthful or find a tool that is. Hint, Java’s File. getCanonicalPath and File. list will not lie to you, but File. getName will.
• Windows Explorer will usually ignore your attempts to rename a file to correct its case. To prove to Windows that you are serious, change the name to something else entirely, then change it back to the name with the proper case spelling.
• In Windows, if you say:
  copy C:\here\rabbits.txt E:\there
  and the file is actually called C:\here\Rabbits.txt, the result will be a file called E:\there\Rabbits.txt. This is actually a nice feature that lets you be sloppy with capitalisation except when initially creating files.
• In Windows, if you say:
  copy C:\here\Rabbits.txt E:\there
  and the file is indeed called C:\here\Rabbits.txt, but there is already a file called E:\there\rabbits.txt, the result will be a file called E:\there\rabbits.txt. This gotcha defeats you when you attempt to make a correction to capitalisation. Copy will not propagate the correction. You must seek and destroy all the bad copies before propagating, or correct the all individually either before or after copying.

Converting Case