Java - Functional Interface

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A functional interface is is a interface that contains only one abstract method.

The interface is so simple that java defines several standard (built-in) functional interfaces

Functional interfaces represent abstract concepts like:

A functional interface is is a interface that contains only one abstract method.

The single abstract method is called the functional method.

It may contain one or more:

The FunctionalInterface annotation is an aid to capture design intent not a requirement. Compilers may generate an error.

The single abstract method in a function interface is called the functional method.

Implementation of functional interfaces can be created with:

lambda expressions, Because a functional interface contains only one abstract method, you omit the name of that method when you implement it with a lambda expression.
method references,
or constructor references.

The predefined/standard/built-in function interface can find in the package java.util.function package

All standard interface follows the following naming convention.

Supplier (nilary function to R).
Predicate (unary function from T to boolean), abstract method that returns a boolean (Example: filtering)

interface Predicate<T> {
    boolean test(T t);
}

Consumer (unary function from T to void), abstract method that returns no value (Example: print)

interface Consumer<T> {
    void accept(T t);
}

Function (unary function from T to R), - abstract method that returns a value (Example: retrieve information or validation)

interface Function<T> {
   R apply(T t);
}

UnaryOperator (unary function from T to T), - abstract method that return always returns its input argument (handy for chaining)

interface UnaryOperator<T> {
   T apply(T t);
}