javac -d option

Using "-d" option, you can create a class in a hierarchical directory. "HelloA.java" has "package a.b", with the "-d" option, the class is generated in "testa/build/a/b" directory.

javac -d testa/build -g testa/src/a/b/HelloA.java 
java -cp testa/build a.b.HelloA