Interfaces
Functional interfaces only have a single method.
They can be annotated with @FunctionalInterface to ensure, during compile, that they fit this requirement. If the marked interface gets more than one method then it won't compile.
These interfaces are very useful when working with the functional paradigm, as with them it is easy making sure that operations have a single concern.
By default Java comes with several functional patterns.
Default Methods
These interfaces mix well with default methods, and the functional interfaces included in the JDK make use of them.
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