compilation - How to compile a single Java file

Question

I have searched this, but I could'n find or understand what I found.
Now I'm not a Java programmer, but I have the need to compile a single Java file into an existing (compiled) Java program. The source of this Java code is not available to me, therefore I cannot compile the entire project.
I'm not interested in decompiling the original project.

How to quickly do this using only the JDK and javac? (Through the command line is what I prefer.)

I understand that to do so error checking outside of the single java file will have to be disabled, because it can't read the dependencies.

Thanks in advance,
-Aidiakapi

EDIT: I do have the JAR file, thanks for the answer :)

Answer 1

As far as I can understand you want to re-compile a single java file and replace it in an existing jar file..

So you compile it..

cmd>javac -classpath jar1.jar;jar2.jar my.company.MyClassToReplace.java

and replace it in the jar.

cmd>jar uf myJarFile.jar my/company/MyClassToReplace.class