The big language features for Java SE 8 are lambda expressions (a.k.a. closures) and default methods (a.k.a. virtual extension methods). Adding closures to the Java language opens up a host of new expressive opportunities for applications and libraries, but how are they implemented? You might assume that lambda expressions are simply a compact syntax for inner classes, but, in fact, the implementation of lambda expressions is substantially different and builds on the invokedynamic feature added in Java SE 7.