You will probably use the System.in input stream so you will want to look at the following classes:
java.lang.System
java.io.InputStream
java.nio.InputStreamReader
java.io.BufferedReader
what I would do is wrap the System.in into a InputStreamReader, which I would then wrap in a BufferedReader eg
BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
then you can use BufferedReader's readLine() method to see when the user presses enter. Probably not as simple as you are looking for, but I think that is about as simple as it gets with the standard API