rJava is a simple R-to-Java interface. It is comparable to the .C/.Call C
interface. rJava provides a low-level bridge between R and Java (via JNI). It
allows to create objects, call methods and access fields of Java objects from R.

In a sense the inverse of rJava is JRI (Java/R Interface) which provides the
opposite direction - calling R from Java. JRI is now shipped as a part of the
rJava package, although it still can be used as a separate entity (especially
for development).

See also: https://rforge.net/rJava/