JEP 280 ("Indify String Concatenation") was implemented in conjunction with JDK 9 and, according to its "Summary" section, "Change[s] the static String-concatenation bytecode sequence generated by javac to use invokedynamic calls to JDK library functions." The impact this has on string concatenation in Java is most easily seen by looking at the javap output of classes using string concatenation that is compiled in pre-JDK 9 and post-JDK 9 JDKs.
The following simple Java class named "HelloWorldStringConcat" will be used for the first demonstration.