If you want to know where it is, type:
whereis java
[The standard "I'm running OS X 10.5.8, so this may not be relevant" caveat applies here.]
On my machine, the bulk of the parts and pieces constituting "Java" are found in
/System/Library/Frameworks/JavaVM.framework/. The "java" returned by typing
whereis java in Terminal is an alias, residing in
/usr/bin, to the Unix Executable file
/System/Library/Frameworks/JavaVM.framework/Versions/A/Commands/java—one of 32 commands in that folder which are so aliased. (I assume that's to facilitate Java programming from the command line.)
