8130023: API java.util.stream: explicitly specify guaranteed execution of the pipeline
Reviewed-by: briangoetz, redestad
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/**
* Classes to support functional-style operations on streams of elements, such
* as map-reduce transformations on collections. For example:
*
* <pre>{@code
* int sum = widgets.stream()
* .filter(b -> b.getColor() == RED)
* .mapToInt(b -> b.getWeight())
* .sum();
* }</pre>
*
* <p>Here we use {@code widgets}, a {@code Collection<Widget>},
* as a source for a stream, and then perform a filter-map-reduce on the stream
* to obtain the sum of the weights of the red widgets. (Summation is an
* example of a <a href="package-summary.html#Reduction">reduction</a>
* operation.)
*
* <p>The key abstraction introduced in this package is <em>stream</em>. The
* classes {@link java.util.stream.Stream}, {@link java.util.stream.IntStream},
* {@link java.util.stream.LongStream}, and {@link java.util.stream.DoubleStream}
* are streams over objects and the primitive {@code int}, {@code long} and
* {@code double} types. Streams differ from collections in several ways:
*
* <ul>
* <li>No storage. A stream is not a data structure that stores elements;
* instead, it conveys elements from a source such as a data structure,
* an array, a generator function, or an I/O channel, through a pipeline of
* computational operations.</li>
* <li>Functional in nature. An operation on a stream produces a result,
* but does not modify its source. For example, filtering a {@code Stream}
* obtained from a collection produces a new {@code Stream} without the
* filtered elements, rather than removing elements from the source
* collection.</li>
* <li>Laziness-seeking. Many stream operations, such as filtering, mapping,
* or duplicate removal, can be implemented lazily, exposing opportunities
* for optimization. For example, "find the first {@code String} with
* three consecutive vowels" need not examine all the input strings.
* Stream operations are divided into intermediate ({@code Stream}-producing)
* operations and terminal (value- or side-effect-producing) operations.
* Intermediate operations are always lazy.</li>
* <li>Possibly unbounded. While collections have a finite size, streams
* need not. Short-circuiting operations such as {@code limit(n)} or
* {@code findFirst()} can allow computations on infinite streams to
* complete in finite time.</li>
* <li>Consumable. The elements of a stream are only visited once during
* the life of a stream. Like an {@link java.util.Iterator}, a new stream
* must be generated to revisit the same elements of the source.
* </li>
* </ul>
*
* Streams can be obtained in a number of ways. Some examples include:
* <ul>
* <li>From a {@link java.util.Collection} via the {@code stream()} and
* {@code parallelStream()} methods;</li>
* <li>From an array via {@link java.util.Arrays#stream(Object[])};</li>
* <li>From static factory methods on the stream classes, such as
* {@link java.util.stream.Stream#of(Object[])},
* {@link java.util.stream.IntStream#range(int, int)}
* or {@link java.util.stream.Stream#iterate(Object, UnaryOperator)};</li>
* <li>The lines of a file can be obtained from {@link java.io.BufferedReader#lines()};</li>
* <li>Streams of file paths can be obtained from methods in {@link java.nio.file.Files};</li>
* <li>Streams of random numbers can be obtained from {@link java.util.Random#ints()};</li>
* <li>Numerous other stream-bearing methods in the JDK, including
* {@link java.util.BitSet#stream()},
* {@link java.util.regex.Pattern#splitAsStream(java.lang.CharSequence)},
* and {@link java.util.jar.JarFile#stream()}.</li>
* </ul>
*
* <p>Additional stream sources can be provided by third-party libraries using
* <a href="package-summary.html#StreamSources">these techniques</a>.
*
* <h2><a name="StreamOps">Stream operations and pipelines</a></h2>
*
* <p>Stream operations are divided into <em>intermediate</em> and
* <em>terminal</em> operations, and are combined to form <em>stream
* pipelines</em>. A stream pipeline consists of a source (such as a
* {@code Collection}, an array, a generator function, or an I/O channel);
* followed by zero or more intermediate operations such as
* {@code Stream.filter} or {@code Stream.map}; and a terminal operation such
* as {@code Stream.forEach} or {@code Stream.reduce}.
*
* <p>Intermediate operations return a new stream. They are always
* <em>lazy</em>; executing an intermediate operation such as
* {@code filter()} does not actually perform any filtering, but instead
* creates a new stream that, when traversed, contains the elements of
* the initial stream that match the given predicate. Traversal
* of the pipeline source does not begin until the terminal operation of the
* pipeline is executed.
*
* <p>Terminal operations, such as {@code Stream.forEach} or
* {@code IntStream.sum}, may traverse the stream to produce a result or a
* side-effect. After the terminal operation is performed, the stream pipeline
* is considered consumed, and can no longer be used; if you need to traverse
* the same data source again, you must return to the data source to get a new
* stream. In almost all cases, terminal operations are <em>eager</em>,
* completing their traversal of the data source and processing of the pipeline
* before returning. Only the terminal operations {@code iterator()} and
* {@code spliterator()} are not; these are provided as an "escape hatch" to enable
* arbitrary client-controlled pipeline traversals in the event that the
* existing operations are not sufficient to the task.
*
* <p> Processing streams lazily allows for significant efficiencies; in a
* pipeline such as the filter-map-sum example above, filtering, mapping, and
* summing can be fused into a single pass on the data, with minimal
* intermediate state. Laziness also allows avoiding examining all the data
* when it is not necessary; for operations such as "find the first string
* longer than 1000 characters", it is only necessary to examine just enough
* strings to find one that has the desired characteristics without examining
* all of the strings available from the source. (This behavior becomes even
* more important when the input stream is infinite and not merely large.)
*
* <p>Intermediate operations are further divided into <em>stateless</em>
* and <em>stateful</em> operations. Stateless operations, such as {@code filter}
* and {@code map}, retain no state from previously seen element when processing
* a new element -- each element can be processed
* independently of operations on other elements. Stateful operations, such as
* {@code distinct} and {@code sorted}, may incorporate state from previously
* seen elements when processing new elements.
*
* <p>Stateful operations may need to process the entire input
* before producing a result. For example, one cannot produce any results from
* sorting a stream until one has seen all elements of the stream. As a result,
* under parallel computation, some pipelines containing stateful intermediate
* operations may require multiple passes on the data or may need to buffer
* significant data. Pipelines containing exclusively stateless intermediate
* operations can be processed in a single pass, whether sequential or parallel,
* with minimal data buffering.
*
* <p>Further, some operations are deemed <em>short-circuiting</em> operations.
* An intermediate operation is short-circuiting if, when presented with
* infinite input, it may produce a finite stream as a result. A terminal
* operation is short-circuiting if, when presented with infinite input, it may
* terminate in finite time. Having a short-circuiting operation in the pipeline
* is a necessary, but not sufficient, condition for the processing of an infinite
* stream to terminate normally in finite time.
*
* <h3><a name="Parallelism">Parallelism</a></h3>
*
* <p>Processing elements with an explicit {@code for-}loop is inherently serial.
* Streams facilitate parallel execution by reframing the computation as a pipeline of
* aggregate operations, rather than as imperative operations on each individual
* element. All streams operations can execute either in serial or in parallel.
* The stream implementations in the JDK create serial streams unless parallelism is
* explicitly requested. For example, {@code Collection} has methods
* {@link java.util.Collection#stream} and {@link java.util.Collection#parallelStream},
* which produce sequential and parallel streams respectively; other
* stream-bearing methods such as {@link java.util.stream.IntStream#range(int, int)}
* produce sequential streams but these streams can be efficiently parallelized by
* invoking their {@link java.util.stream.BaseStream#parallel()} method.
* To execute the prior "sum of weights of widgets" query in parallel, we would
* do:
*
* <pre>{@code
* int sumOfWeights = widgets.}<code><b>parallelStream()</b></code>{@code
* .filter(b -> b.getColor() == RED)
* .mapToInt(b -> b.getWeight())
* .sum();
* }</pre>
*
* <p>The only difference between the serial and parallel versions of this
* example is the creation of the initial stream, using "{@code parallelStream()}"
* instead of "{@code stream()}". The stream pipeline is executed sequentially or
* in parallel depending on the mode of the stream on which the terminal operation
* is invoked. The sequential or parallel mode of a stream can be determined with the
* {@link java.util.stream.BaseStream#isParallel()} method, and the
* stream's mode can be modified with the
* {@link java.util.stream.BaseStream#sequential()} and
* {@link java.util.stream.BaseStream#parallel()} operations.
* The most recent sequential or parallel mode setting applies to the
* execution of the entire stream pipeline.
*
* <p>Except for operations identified as explicitly nondeterministic, such
* as {@code findAny()}, whether a stream executes sequentially or in parallel
* should not change the result of the computation.
*
* <p>Most stream operations accept parameters that describe user-specified
* behavior, which are often lambda expressions. To preserve correct behavior,
* these <em>behavioral parameters</em> must be <em>non-interfering</em>, and in
* most cases must be <em>stateless</em>. Such parameters are always instances
* of a <a href="../function/package-summary.html">functional interface</a> such
* as {@link java.util.function.Function}, and are often lambda expressions or
* method references.
*
* <h3><a name="NonInterference">Non-interference</a></h3>
*
* Streams enable you to execute possibly-parallel aggregate operations over a
* variety of data sources, including even non-thread-safe collections such as
* {@code ArrayList}. This is possible only if we can prevent
* <em>interference</em> with the data source during the execution of a stream
* pipeline. Except for the escape-hatch operations {@code iterator()} and
* {@code spliterator()}, execution begins when the terminal operation is
* invoked, and ends when the terminal operation completes. For most data
* sources, preventing interference means ensuring that the data source is
* <em>not modified at all</em> during the execution of the stream pipeline.
* The notable exception to this are streams whose sources are concurrent
* collections, which are specifically designed to handle concurrent modification.
* Concurrent stream sources are those whose {@code Spliterator} reports the
* {@code CONCURRENT} characteristic.
*
* <p>Accordingly, behavioral parameters in stream pipelines whose source might
* not be concurrent should never modify the stream's data source.
* A behavioral parameter is said to <em>interfere</em> with a non-concurrent
* data source if it modifies, or causes to be
* modified, the stream's data source. The need for non-interference applies
* to all pipelines, not just parallel ones. Unless the stream source is
* concurrent, modifying a stream's data source during execution of a stream
* pipeline can cause exceptions, incorrect answers, or nonconformant behavior.
*
* For well-behaved stream sources, the source can be modified before the
* terminal operation commences and those modifications will be reflected in
* the covered elements. For example, consider the following code:
*
* <pre>{@code
* List<String> l = new ArrayList(Arrays.asList("one", "two"));
* Stream<String> sl = l.stream();
* l.add("three");
* String s = sl.collect(joining(" "));
* }</pre>
*
* First a list is created consisting of two strings: "one"; and "two". Then a
* stream is created from that list. Next the list is modified by adding a third
* string: "three". Finally the elements of the stream are collected and joined
* together. Since the list was modified before the terminal {@code collect}
* operation commenced the result will be a string of "one two three". All the
* streams returned from JDK collections, and most other JDK classes,
* are well-behaved in this manner; for streams generated by other libraries, see
* <a href="package-summary.html#StreamSources">Low-level stream
* construction</a> for requirements for building well-behaved streams.
*
* <h3><a name="Statelessness">Stateless behaviors</a></h3>
*
* Stream pipeline results may be nondeterministic or incorrect if the behavioral
* parameters to the stream operations are <em>stateful</em>. A stateful lambda
* (or other object implementing the appropriate functional interface) is one
* whose result depends on any state which might change during the execution
* of the stream pipeline. An example of a stateful lambda is the parameter
* to {@code map()} in:
*
* <pre>{@code
* Set<Integer> seen = Collections.synchronizedSet(new HashSet<>());
* stream.parallel().map(e -> { if (seen.add(e)) return 0; else return e; })...
* }</pre>
*
* Here, if the mapping operation is performed in parallel, the results for the
* same input could vary from run to run, due to thread scheduling differences,
* whereas, with a stateless lambda expression the results would always be the
* same.
*
* <p>Note also that attempting to access mutable state from behavioral parameters
* presents you with a bad choice with respect to safety and performance; if
* you do not synchronize access to that state, you have a data race and
* therefore your code is broken, but if you do synchronize access to that
* state, you risk having contention undermine the parallelism you are seeking
* to benefit from. The best approach is to avoid stateful behavioral
* parameters to stream operations entirely; there is usually a way to
* restructure the stream pipeline to avoid statefulness.
*
* <h3><a name="SideEffects">Side-effects</a></h3>
*
* Side-effects in behavioral parameters to stream operations are, in general,
* discouraged, as they can often lead to unwitting violations of the
* statelessness requirement, as well as other thread-safety hazards.
*
* <p>If the behavioral parameters do have side-effects, unless explicitly
* stated, there are no guarantees as to:
* <ul>
* <li>the <a href="../concurrent/package-summary.html#MemoryVisibility">
* <i>visibility</i></a> of those side-effects to other threads;</li>
* <li>that different operations on the "same" element within the same stream
* pipeline are executed in the same thread; and</li>
* <li>that behavioral parameters are always invoked, since a stream
* implementation is free to elide operations (or entire stages) from a
* stream pipeline if it can prove that it would not affect the result of the
* computation.
* </li>
* </ul>
* <p>The ordering of side-effects may be surprising. Even when a pipeline is
* constrained to produce a <em>result</em> that is consistent with the
* encounter order of the stream source (for example,
* {@code IntStream.range(0,5).parallel().map(x -> x*2).toArray()}
* must produce {@code [0, 2, 4, 6, 8]}), no guarantees are made as to the order
* in which the mapper function is applied to individual elements, or in what
* thread any behavioral parameter is executed for a given element.
*
* <p>The eliding of side-effects may also be surprising. With the exception of
* terminal operations {@link java.util.stream.Stream#forEach forEach} and
* {@link java.util.stream.Stream#forEachOrdered forEachOrdered}, side-effects
* of behavioral parameters may not always be executed when the stream
* implementation can optimize away the execution of behavioral parameters
* without affecting the result of the computation. (For a specific example
* see the API note documented on the {@link java.util.stream.Stream#count count}
* operation.)
*
* <p>Many computations where one might be tempted to use side effects can be more
* safely and efficiently expressed without side-effects, such as using
* <a href="package-summary.html#Reduction">reduction</a> instead of mutable
* accumulators. However, side-effects such as using {@code println()} for debugging
* purposes are usually harmless. A small number of stream operations, such as
* {@code forEach()} and {@code peek()}, can operate only via side-effects;
* these should be used with care.
*
* <p>As an example of how to transform a stream pipeline that inappropriately
* uses side-effects to one that does not, the following code searches a stream
* of strings for those matching a given regular expression, and puts the
* matches in a list.
*
* <pre>{@code
* ArrayList<String> results = new ArrayList<>();
* stream.filter(s -> pattern.matcher(s).matches())
* .forEach(s -> results.add(s)); // Unnecessary use of side-effects!
* }</pre>
*
* This code unnecessarily uses side-effects. If executed in parallel, the
* non-thread-safety of {@code ArrayList} would cause incorrect results, and
* adding needed synchronization would cause contention, undermining the
* benefit of parallelism. Furthermore, using side-effects here is completely
* unnecessary; the {@code forEach()} can simply be replaced with a reduction
* operation that is safer, more efficient, and more amenable to
* parallelization:
*
* <pre>{@code
* List<String>results =
* stream.filter(s -> pattern.matcher(s).matches())
* .collect(Collectors.toList()); // No side-effects!
* }</pre>
*
* <h3><a name="Ordering">Ordering</a></h3>
*
* <p>Streams may or may not have a defined <em>encounter order</em>. Whether
* or not a stream has an encounter order depends on the source and the
* intermediate operations. Certain stream sources (such as {@code List} or
* arrays) are intrinsically ordered, whereas others (such as {@code HashSet})
* are not. Some intermediate operations, such as {@code sorted()}, may impose
* an encounter order on an otherwise unordered stream, and others may render an
* ordered stream unordered, such as {@link java.util.stream.BaseStream#unordered()}.
* Further, some terminal operations may ignore encounter order, such as
* {@code forEach()}.
*
* <p>If a stream is ordered, most operations are constrained to operate on the
* elements in their encounter order; if the source of a stream is a {@code List}
* containing {@code [1, 2, 3]}, then the result of executing {@code map(x -> x*2)}
* must be {@code [2, 4, 6]}. However, if the source has no defined encounter
* order, then any permutation of the values {@code [2, 4, 6]} would be a valid
* result.
*
* <p>For sequential streams, the presence or absence of an encounter order does
* not affect performance, only determinism. If a stream is ordered, repeated
* execution of identical stream pipelines on an identical source will produce
* an identical result; if it is not ordered, repeated execution might produce
* different results.
*
* <p>For parallel streams, relaxing the ordering constraint can sometimes enable
* more efficient execution. Certain aggregate operations,
* such as filtering duplicates ({@code distinct()}) or grouped reductions
* ({@code Collectors.groupingBy()}) can be implemented more efficiently if ordering of elements
* is not relevant. Similarly, operations that are intrinsically tied to encounter order,
* such as {@code limit()}, may require
* buffering to ensure proper ordering, undermining the benefit of parallelism.
* In cases where the stream has an encounter order, but the user does not
* particularly <em>care</em> about that encounter order, explicitly de-ordering
* the stream with {@link java.util.stream.BaseStream#unordered() unordered()} may
* improve parallel performance for some stateful or terminal operations.
* However, most stream pipelines, such as the "sum of weight of blocks" example
* above, still parallelize efficiently even under ordering constraints.
*
* <h2><a name="Reduction">Reduction operations</a></h2>
*
* A <em>reduction</em> operation (also called a <em>fold</em>) takes a sequence
* of input elements and combines them into a single summary result by repeated
* application of a combining operation, such as finding the sum or maximum of
* a set of numbers, or accumulating elements into a list. The streams classes have
* multiple forms of general reduction operations, called
* {@link java.util.stream.Stream#reduce(java.util.function.BinaryOperator) reduce()}
* and {@link java.util.stream.Stream#collect(java.util.stream.Collector) collect()},
* as well as multiple specialized reduction forms such as
* {@link java.util.stream.IntStream#sum() sum()}, {@link java.util.stream.IntStream#max() max()},
* or {@link java.util.stream.IntStream#count() count()}.
*
* <p>Of course, such operations can be readily implemented as simple sequential
* loops, as in:
* <pre>{@code
* int sum = 0;
* for (int x : numbers) {
* sum += x;
* }
* }</pre>
* However, there are good reasons to prefer a reduce operation
* over a mutative accumulation such as the above. Not only is a reduction
* "more abstract" -- it operates on the stream as a whole rather than individual
* elements -- but a properly constructed reduce operation is inherently
* parallelizable, so long as the function(s) used to process the elements
* are <a href="package-summary.html#Associativity">associative</a> and
* <a href="package-summary.html#NonInterfering">stateless</a>.
* For example, given a stream of numbers for which we want to find the sum, we
* can write:
* <pre>{@code
* int sum = numbers.stream().reduce(0, (x,y) -> x+y);
* }</pre>
* or:
* <pre>{@code
* int sum = numbers.stream().reduce(0, Integer::sum);
* }</pre>
*
* <p>These reduction operations can run safely in parallel with almost no
* modification:
* <pre>{@code
* int sum = numbers.parallelStream().reduce(0, Integer::sum);
* }</pre>
*
* <p>Reduction parallellizes well because the implementation
* can operate on subsets of the data in parallel, and then combine the
* intermediate results to get the final correct answer. (Even if the language
* had a "parallel for-each" construct, the mutative accumulation approach would
* still required the developer to provide
* thread-safe updates to the shared accumulating variable {@code sum}, and
* the required synchronization would then likely eliminate any performance gain from
* parallelism.) Using {@code reduce()} instead removes all of the
* burden of parallelizing the reduction operation, and the library can provide
* an efficient parallel implementation with no additional synchronization
* required.
*
* <p>The "widgets" examples shown earlier shows how reduction combines with
* other operations to replace for loops with bulk operations. If {@code widgets}
* is a collection of {@code Widget} objects, which have a {@code getWeight} method,
* we can find the heaviest widget with:
* <pre>{@code
* OptionalInt heaviest = widgets.parallelStream()
* .mapToInt(Widget::getWeight)
* .max();
* }</pre>
*
* <p>In its more general form, a {@code reduce} operation on elements of type
* {@code <T>} yielding a result of type {@code <U>} requires three parameters:
* <pre>{@code
* <U> U reduce(U identity,
* BiFunction<U, ? super T, U> accumulator,
* BinaryOperator<U> combiner);
* }</pre>
* Here, the <em>identity</em> element is both an initial seed value for the reduction
* and a default result if there are no input elements. The <em>accumulator</em>
* function takes a partial result and the next element, and produces a new
* partial result. The <em>combiner</em> function combines two partial results
* to produce a new partial result. (The combiner is necessary in parallel
* reductions, where the input is partitioned, a partial accumulation computed
* for each partition, and then the partial results are combined to produce a
* final result.)
*
* <p>More formally, the {@code identity} value must be an <em>identity</em> for
* the combiner function. This means that for all {@code u},
* {@code combiner.apply(identity, u)} is equal to {@code u}. Additionally, the
* {@code combiner} function must be <a href="package-summary.html#Associativity">associative</a> and
* must be compatible with the {@code accumulator} function: for all {@code u}
* and {@code t}, {@code combiner.apply(u, accumulator.apply(identity, t))} must
* be {@code equals()} to {@code accumulator.apply(u, t)}.
*
* <p>The three-argument form is a generalization of the two-argument form,
* incorporating a mapping step into the accumulation step. We could
* re-cast the simple sum-of-weights example using the more general form as
* follows:
* <pre>{@code
* int sumOfWeights = widgets.stream()
* .reduce(0,
* (sum, b) -> sum + b.getWeight(),
* Integer::sum);
* }</pre>
* though the explicit map-reduce form is more readable and therefore should
* usually be preferred. The generalized form is provided for cases where
* significant work can be optimized away by combining mapping and reducing
* into a single function.
*
* <h3><a name="MutableReduction">Mutable reduction</a></h3>
*
* A <em>mutable reduction operation</em> accumulates input elements into a
* mutable result container, such as a {@code Collection} or {@code StringBuilder},
* as it processes the elements in the stream.
*
* <p>If we wanted to take a stream of strings and concatenate them into a
* single long string, we <em>could</em> achieve this with ordinary reduction:
* <pre>{@code
* String concatenated = strings.reduce("", String::concat)
* }</pre>
*
* <p>We would get the desired result, and it would even work in parallel. However,
* we might not be happy about the performance! Such an implementation would do
* a great deal of string copying, and the run time would be <em>O(n^2)</em> in
* the number of characters. A more performant approach would be to accumulate
* the results into a {@link java.lang.StringBuilder}, which is a mutable
* container for accumulating strings. We can use the same technique to
* parallelize mutable reduction as we do with ordinary reduction.
*
* <p>The mutable reduction operation is called
* {@link java.util.stream.Stream#collect(Collector) collect()},
* as it collects together the desired results into a result container such
* as a {@code Collection}.
* A {@code collect} operation requires three functions:
* a supplier function to construct new instances of the result container, an
* accumulator function to incorporate an input element into a result
* container, and a combining function to merge the contents of one result
* container into another. The form of this is very similar to the general
* form of ordinary reduction:
* <pre>{@code
* <R> R collect(Supplier<R> supplier,
* BiConsumer<R, ? super T> accumulator,
* BiConsumer<R, R> combiner);
* }</pre>
* <p>As with {@code reduce()}, a benefit of expressing {@code collect} in this
* abstract way is that it is directly amenable to parallelization: we can
* accumulate partial results in parallel and then combine them, so long as the
* accumulation and combining functions satisfy the appropriate requirements.
* For example, to collect the String representations of the elements in a
* stream into an {@code ArrayList}, we could write the obvious sequential
* for-each form:
* <pre>{@code
* ArrayList<String> strings = new ArrayList<>();
* for (T element : stream) {
* strings.add(element.toString());
* }
* }</pre>
* Or we could use a parallelizable collect form:
* <pre>{@code
* ArrayList<String> strings = stream.collect(() -> new ArrayList<>(),
* (c, e) -> c.add(e.toString()),
* (c1, c2) -> c1.addAll(c2));
* }</pre>
* or, pulling the mapping operation out of the accumulator function, we could
* express it more succinctly as:
* <pre>{@code
* List<String> strings = stream.map(Object::toString)
* .collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
* }</pre>
* Here, our supplier is just the {@link java.util.ArrayList#ArrayList()
* ArrayList constructor}, the accumulator adds the stringified element to an
* {@code ArrayList}, and the combiner simply uses {@link java.util.ArrayList#addAll addAll}
* to copy the strings from one container into the other.
*
* <p>The three aspects of {@code collect} -- supplier, accumulator, and
* combiner -- are tightly coupled. We can use the abstraction of a
* {@link java.util.stream.Collector} to capture all three aspects. The
* above example for collecting strings into a {@code List} can be rewritten
* using a standard {@code Collector} as:
* <pre>{@code
* List<String> strings = stream.map(Object::toString)
* .collect(Collectors.toList());
* }</pre>
*
* <p>Packaging mutable reductions into a Collector has another advantage:
* composability. The class {@link java.util.stream.Collectors} contains a
* number of predefined factories for collectors, including combinators
* that transform one collector into another. For example, suppose we have a
* collector that computes the sum of the salaries of a stream of
* employees, as follows:
*
* <pre>{@code
* Collector<Employee, ?, Integer> summingSalaries
* = Collectors.summingInt(Employee::getSalary);
* }</pre>
*
* (The {@code ?} for the second type parameter merely indicates that we don't
* care about the intermediate representation used by this collector.)
* If we wanted to create a collector to tabulate the sum of salaries by
* department, we could reuse {@code summingSalaries} using
* {@link java.util.stream.Collectors#groupingBy(java.util.function.Function, java.util.stream.Collector) groupingBy}:
*
* <pre>{@code
* Map<Department, Integer> salariesByDept
* = employees.stream().collect(Collectors.groupingBy(Employee::getDepartment,
* summingSalaries));
* }</pre>
*
* <p>As with the regular reduction operation, {@code collect()} operations can
* only be parallelized if appropriate conditions are met. For any partially
* accumulated result, combining it with an empty result container must
* produce an equivalent result. That is, for a partially accumulated result
* {@code p} that is the result of any series of accumulator and combiner
* invocations, {@code p} must be equivalent to
* {@code combiner.apply(p, supplier.get())}.
*
* <p>Further, however the computation is split, it must produce an equivalent
* result. For any input elements {@code t1} and {@code t2}, the results
* {@code r1} and {@code r2} in the computation below must be equivalent:
* <pre>{@code
* A a1 = supplier.get();
* accumulator.accept(a1, t1);
* accumulator.accept(a1, t2);
* R r1 = finisher.apply(a1); // result without splitting
*
* A a2 = supplier.get();
* accumulator.accept(a2, t1);
* A a3 = supplier.get();
* accumulator.accept(a3, t2);
* R r2 = finisher.apply(combiner.apply(a2, a3)); // result with splitting
* }</pre>
*
* <p>Here, equivalence generally means according to {@link java.lang.Object#equals(Object)}.
* but in some cases equivalence may be relaxed to account for differences in
* order.
*
* <h3><a name="ConcurrentReduction">Reduction, concurrency, and ordering</a></h3>
*
* With some complex reduction operations, for example a {@code collect()} that
* produces a {@code Map}, such as:
* <pre>{@code
* Map<Buyer, List<Transaction>> salesByBuyer
* = txns.parallelStream()
* .collect(Collectors.groupingBy(Transaction::getBuyer));
* }</pre>
* it may actually be counterproductive to perform the operation in parallel.
* This is because the combining step (merging one {@code Map} into another by
* key) can be expensive for some {@code Map} implementations.
*
* <p>Suppose, however, that the result container used in this reduction
* was a concurrently modifiable collection -- such as a
* {@link java.util.concurrent.ConcurrentHashMap}. In that case, the parallel
* invocations of the accumulator could actually deposit their results
* concurrently into the same shared result container, eliminating the need for
* the combiner to merge distinct result containers. This potentially provides
* a boost to the parallel execution performance. We call this a
* <em>concurrent</em> reduction.
*
* <p>A {@link java.util.stream.Collector} that supports concurrent reduction is
* marked with the {@link java.util.stream.Collector.Characteristics#CONCURRENT}
* characteristic. However, a concurrent collection also has a downside. If
* multiple threads are depositing results concurrently into a shared container,
* the order in which results are deposited is non-deterministic. Consequently,
* a concurrent reduction is only possible if ordering is not important for the
* stream being processed. The {@link java.util.stream.Stream#collect(Collector)}
* implementation will only perform a concurrent reduction if
* <ul>
* <li>The stream is parallel;</li>
* <li>The collector has the
* {@link java.util.stream.Collector.Characteristics#CONCURRENT} characteristic,
* and;</li>
* <li>Either the stream is unordered, or the collector has the
* {@link java.util.stream.Collector.Characteristics#UNORDERED} characteristic.
* </ul>
* You can ensure the stream is unordered by using the
* {@link java.util.stream.BaseStream#unordered()} method. For example:
* <pre>{@code
* Map<Buyer, List<Transaction>> salesByBuyer
* = txns.parallelStream()
* .unordered()
* .collect(groupingByConcurrent(Transaction::getBuyer));
* }</pre>
* (where {@link java.util.stream.Collectors#groupingByConcurrent} is the
* concurrent equivalent of {@code groupingBy}).
*
* <p>Note that if it is important that the elements for a given key appear in
* the order they appear in the source, then we cannot use a concurrent
* reduction, as ordering is one of the casualties of concurrent insertion.
* We would then be constrained to implement either a sequential reduction or
* a merge-based parallel reduction.
*
* <h3><a name="Associativity">Associativity</a></h3>
*
* An operator or function {@code op} is <em>associative</em> if the following
* holds:
* <pre>{@code
* (a op b) op c == a op (b op c)
* }</pre>
* The importance of this to parallel evaluation can be seen if we expand this
* to four terms:
* <pre>{@code
* a op b op c op d == (a op b) op (c op d)
* }</pre>
* So we can evaluate {@code (a op b)} in parallel with {@code (c op d)}, and
* then invoke {@code op} on the results.
*
* <p>Examples of associative operations include numeric addition, min, and
* max, and string concatenation.
*
* <h2><a name="StreamSources">Low-level stream construction</a></h2>
*
* So far, all the stream examples have used methods like
* {@link java.util.Collection#stream()} or {@link java.util.Arrays#stream(Object[])}
* to obtain a stream. How are those stream-bearing methods implemented?
*
* <p>The class {@link java.util.stream.StreamSupport} has a number of
* low-level methods for creating a stream, all using some form of a
* {@link java.util.Spliterator}. A spliterator is the parallel analogue of an
* {@link java.util.Iterator}; it describes a (possibly infinite) collection of
* elements, with support for sequentially advancing, bulk traversal, and
* splitting off some portion of the input into another spliterator which can
* be processed in parallel. At the lowest level, all streams are driven by a
* spliterator.
*
* <p>There are a number of implementation choices in implementing a
* spliterator, nearly all of which are tradeoffs between simplicity of
* implementation and runtime performance of streams using that spliterator.
* The simplest, but least performant, way to create a spliterator is to
* create one from an iterator using
* {@link java.util.Spliterators#spliteratorUnknownSize(java.util.Iterator, int)}.
* While such a spliterator will work, it will likely offer poor parallel
* performance, since we have lost sizing information (how big is the
* underlying data set), as well as being constrained to a simplistic
* splitting algorithm.
*
* <p>A higher-quality spliterator will provide balanced and known-size
* splits, accurate sizing information, and a number of other
* {@link java.util.Spliterator#characteristics() characteristics} of the
* spliterator or data that can be used by implementations to optimize
* execution.
*
* <p>Spliterators for mutable data sources have an additional challenge;
* timing of binding to the data, since the data could change between the time
* the spliterator is created and the time the stream pipeline is executed.
* Ideally, a spliterator for a stream would report a characteristic of
* {@code IMMUTABLE} or {@code CONCURRENT}; if not it should be
* <a href="../Spliterator.html#binding"><em>late-binding</em></a>. If a source
* cannot directly supply a recommended spliterator, it may indirectly supply
* a spliterator using a {@code Supplier}, and construct a stream via the
* {@code Supplier}-accepting versions of
* {@link java.util.stream.StreamSupport#stream(Supplier, int, boolean) stream()}.
* The spliterator is obtained from the supplier only after the terminal
* operation of the stream pipeline commences.
*
* <p>These requirements significantly reduce the scope of potential
* interference between mutations of the stream source and execution of stream
* pipelines. Streams based on spliterators with the desired characteristics,
* or those using the Supplier-based factory forms, are immune to
* modifications of the data source prior to commencement of the terminal
* operation (provided the behavioral parameters to the stream operations meet
* the required criteria for non-interference and statelessness). See
* <a href="package-summary.html#NonInterference">Non-Interference</a>
* for more details.
*
* @since 1.8
*/
package java.util.stream;
import java.util.function.BinaryOperator;
import java.util.function.UnaryOperator;