Some operating systems come with a JRE and some not. Of course YOU will have no idea what VERSION the user has. Many application folks include a version of the JRE with their application build just to be sure that
1. It exists on the machine
2. It is the version that the application was written to.
There are tools out there that can assist in deciding such thing. For instance InstallSheild an InstallAnywhere are tools to create executables for the purpose of deploying your application. They can be configured to go LOOK to see if the correct JRE is there and if not prompt the user to have it installed for them.
But where I work we usually just include a copy of the JRE with our build anyway. Insurance you know.
