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.

There are over 40 functional interfaces listed in the java.util.functions package, which can make the prospect of functional programming in Java ... This restriction doesn’t apply to the Function ...

java.util.function.UnaryOperator @FunctionalInterface public interface UnaryOperator<T> extends Function<T,T> T apply(T t) Applies this function to the given argument. Parameter Types: T - the input ...

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.