create_jar

Open a command prompt.
Go to the directory where you have your .java files
Create a directory "build"
Run java compilation from the command line

javac -d ./build *.java  
#if there are multiple files to be jarred or


javac -d ./build file_name.java
#for jarring a single file

to compile in the current directory:
javac -d . *.java  

to use another jar for compiling java file:
javac -d -classpath ~/Downloads/hadoop-core1.2.1.jar ./build *.java  


now to crete jar of compiled classes:
jar cvf YourJar.jar kmeans*.class
(mention kmeans*class instead of just * as jar needs to know all the files added.)