Question: how to install rjava package in r?
Answer: Install the Java Runtime Environment (JRE). sudo apt-get install -y default-jre.
Install the Java Development Kit (JDK). sudo apt-get install -y default-jdk.
Update where R expects to find various Java files. sudo R CMD javareconf.
Install the package. ... 
If you have a RStudio session open, then exit and restart it.