Learn the difference between classes and interfaces once and for all, then get started declaring, implementing, and extending interfaces in your Java programs ...

BooleanSupplier, LongSupplier, DoubleSupplier LongUnaryOperator, DoubleUnaryOperator, IntUnaryOperator As you can see, the various Java functional interfaces simply provide a somewhat different spin ...

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 ...