In this Java 101 tutorial, I’ve explained how interfaces differ from classes, and showed you how to declare, implement, and extend interfaces in your Java programs.

Furthermore, the Java lambda syntax separates these two elements with an arrow. So to learn Java lambda syntax, you need to be familiar with its three component parts: The argument list; The arrow; ...

Consternation over Java's lambda syntax is the perfect example of how Java's evolution and incremental improvements have created a community of spoiled programmers. ... It's not quite equivalent - ...

For an introduction to Java interfaces, including the difference between classes and interfaces, see my Java 101 tutorial Working with interfaces in Java. Nesting interfaces in classes.