Assuming rJava is installed and works, it is possible to take advantage of its magic to get the path where java is installed:

$ Rscript --default-packages="methods,rJava" -e ".jinit(); .jcall( 'java/lang/System', 'S', 'getProperty', 'java.home' ) "
[1] "/opt/jdk/jre"

This is useful when you develop scripts that need to call a java program without assuming that java is on the path, or the JAVA_HOME environment variable is set, etc ...