Using JRI to simulate R console

I need to build an application with JRI that works similarly to the R console. The issue is that sometimes the console will respond displaying a line (for example, after entering x below) and sometimes it doesn't (for example, after entering x = 1)

I tried to analyze eng.parseAndEval(statement) (where statement is the command entered on the console) but cannot tell when to display the response on the console, and when not. Any ideas?

What you are seeing is the visibility flag maintained by R which tells the regular event loop when to print and when not. See withVisible in R for details. However, you really don't want to get into re-implementing the R event loop. Instead, JRI allows you to have R do all the work for you - you just implement the console callbacks RMainLoopCallbacks to handle all your console I/O and then start the event loop. You may want to have a look at JGR - it is a Java console implemented using JRI.