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 ...

Java lets you declare interfaces inside of classes. Once declared, an interface is automatically a static member of the class. There is no need to declare the interface with the static keyword.