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/**
* <em>Functional interfaces</em> provide target types for lambda expressions
* and method references. Each functional interface has a single abstract
* method, called the <em>functional method</em> for that functional interface,
* to which the lambda expression's parameter and return types are matched or
* adapted. Functional interfaces can provide a target type in multiple
* contexts, such as assignment context, method invocation, or cast context:
* <pre>{@code
* // Assignment context
* Predicate<String> p = String::isEmpty;
* // Method invocation context
* stream.filter(e -> e.getSize() > 10)...
* // Cast context
* stream.map((ToIntFunction) e -> e.getSize())...
* }</pre>
* <p>The interfaces in this package are general purpose functional interfaces
* used by the JDK, and are available to be used by user code as well. While
* they do not identify a complete set of function shapes to which lambda
* expressions might be adapted, they provide enough to cover common
* requirements. Other functional interfaces provided for specific purposes,
* such as {@link java.io.FileFilter}, are defined in the packages where they
* are used.
* <p>The interfaces in this package are annotated with
* {@link java.lang.FunctionalInterface}. This annotation is not a requirement
* for the compiler to recognize an interface as a functional interface, but
* merely an aid to capture design intent and enlist the help of the compiler in
* identifying accidental violations of design intent.
* <p>Functional interfaces often represent abstract concepts like functions,
* actions, or predicates. In documenting functional interfaces, or referring
* to variables typed as functional interfaces, it is common to refer directly
* to those abstract concepts, for example using "this function" instead of
* "the function represented by this object". When an API method is said to
* accept or return a functional interface in this manner, such as "applies the
* provided function to...", this is understood to mean a <i>non-null</i>
* reference to an object implementing the appropriate functional interface,
* unless potential nullity is explicitly specified.
* <p>The functional interfaces in this package follow an extensible naming
* convention, as follows:
* <ul>
* <li>There are several basic function shapes, including
* {@link java.util.function.Function} (unary function from {@code T} to {@code R}),
* {@link java.util.function.Consumer} (unary function from {@code T} to {@code void}),
* {@link java.util.function.Predicate} (unary function from {@code T} to {@code boolean}),
* and {@link java.util.function.Supplier} (nilary function to {@code R}).
* </li>
* <li>Function shapes have a natural arity based on how they are most
* commonly used. The basic shapes can be modified by an arity prefix to
* indicate a different arity, such as
* {@link java.util.function.BiFunction} (binary function from {@code T} and
* {@code U} to {@code R}).
* <li>There are additional derived function shapes which extend the basic
* function shapes, including {@link java.util.function.UnaryOperator}
* (extends {@code Function}) and {@link java.util.function.BinaryOperator}
* (extends {@code BiFunction}).
* <li>Type parameters of functional interfaces can be specialized to
* primitives with additional type prefixes. To specialize the return type
* for a type that has both generic return type and generic arguments, we
* prefix {@code ToXxx}, as in {@link java.util.function.ToIntFunction}.
* Otherwise, type arguments are specialized left-to-right, as in
* {@link java.util.function.DoubleConsumer}
* or {@link java.util.function.ObjIntConsumer}.
* (The type prefix {@code Obj} is used to indicate that we don't want to
* specialize this parameter, but want to move on to the next parameter,
* as in {@link java.util.function.ObjIntConsumer}.)
* These schemes can be combined, as in {@code IntToDoubleFunction}.
* <li>If there are specialization prefixes for all arguments, the arity
* prefix may be left out (as in {@link java.util.function.ObjIntConsumer}).
* </ul>
* @see java.lang.FunctionalInterface
* @since 1.8
package java.util.function;