【java】1.求数字的最大、最小、和、平均值 2.其他求平均值方法 3.Java.lang.util.stream.stream的其他方法

朱雀 2024-03-27 14:05 17阅读 0赞

#### 目录 ####

*  1.求数字的最大、最小、和、平均值
 *  2.其他求平均值方法
 *  3.Java.lang.util.stream.stream的其他方法

> 代码中遇到了list.stream().mapToDouble(UserInfo::getAge).sum();这种写法，不太熟悉，遂记录。

*  应用场景：求age字段的和。

Double sum = list.stream().mapToDouble(UserInfo::getAge).sum();

## 1.求数字的最大、最小、和、平均值 ##

list.stream().mapToDouble(UserInfo::getAge).max()	//最大
    list.stream().mapToDouble(UserInfo::getAge).min()	//最小
    list.stream().mapToDouble(UserInfo::getAge).sum()	//和
    list.stream().mapToDouble(UserInfo::getAge).average()//平均值

以上方法涉及到的工具包是rt.jar中的Java.lang.util.stream.stream工具包。有时间可以多研究下该工具包找中的各类方法，以提高编程效率。

## 2.其他求平均值方法 ##

*  应用场景：求age字段的平均值。

Double age = lstKM.stream().collect(Collectors.averagingDouble(p -> p.getAge()));

以上方法涉及到的工具包是rt.jar中的Java.lang.util.stream.Collectora工具包。有时间可以多研究下该工具包找中的各类方法，以提高编程效率。  
更多java.util.stream.Collectors的方法：[https://blog.csdn.net/sun2012930/article/details/106507179][https_blog.csdn.net_sun2012930_article_details_106507179]

## 3.Java.lang.util.stream.stream的其他方法 ##

Java.lang.util.stream.stream中的部分方法如下：

/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 
 */
 package java.util.stream;
 
 import java.nio.charset.Charset;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Spliterator;
 import java.util.Spliterators;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.BiConsumer;
 import java.util.function.BiFunction;
 import java.util.function.BinaryOperator;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.IntFunction;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
 import java.util.function.ToDoubleFunction;
 import java.util.function.ToIntFunction;
 import java.util.function.ToLongFunction;
 import java.util.function.UnaryOperator;
 
 /**
 * A sequence of elements supporting sequential and parallel aggregate
 * operations. The following example illustrates an aggregate operation using
 * {@link Stream} and {@link IntStream}:
 *
 * <pre>{@code
 * int sum = widgets.stream()
 * .filter(w -> w.getColor() == RED)
 * .mapToInt(w -> w.getWeight())
 * .sum();
 * }</pre>
 *
 * In this example, {@code widgets} is a {@code Collection<Widget>}. We create
 * a stream of {@code Widget} objects via {@link Collection#stream Collection.stream()},
 * filter it to produce a stream containing only the red widgets, and then
 * transform it into a stream of {@code int} values representing the weight of
 * each red widget. Then this stream is summed to produce a total weight.
 *
 * In addition to {@code Stream}, which is a stream of object references,
 * there are primitive specializations for {@link IntStream}, {@link LongStream},
 * and {@link DoubleStream}, all of which are referred to as "streams" and
 * conform to the characteristics and restrictions described here.
 *
 * To perform a computation, stream
 * <a href="package-summary.html#StreamOps">operations</a> are composed into a
 * stream pipeline. A stream pipeline consists of a source (which
 * might be an array, a collection, a generator function, an I/O channel,
 * etc), zero or more intermediate operations (which transform a
 * stream into another stream, such as {@link Stream#filter(Predicate)}), and a
 * terminal operation (which produces a result or side-effect, such
 * as {@link Stream#count()} or {@link Stream#forEach(Consumer)}).
 * Streams are lazy; computation on the source data is only performed when the
 * terminal operation is initiated, and source elements are consumed only
 * as needed.
 *
 * Collections and streams, while bearing some superficial similarities,
 * have different goals. Collections are primarily concerned with the efficient
 * management of, and access to, their elements. By contrast, streams do not
 * provide a means to directly access or manipulate their elements, and are
 * instead concerned with declaratively describing their source and the
 * computational operations which will be performed in aggregate on that source.
 * However, if the provided stream operations do not offer the desired
 * functionality, the {@link #iterator()} and {@link #spliterator()} operations
 * can be used to perform a controlled traversal.
 *
 * A stream pipeline, like the "widgets" example above, can be viewed as
 * a query on the stream source. Unless the source was explicitly
 * designed for concurrent modification (such as a {@link ConcurrentHashMap}),
 * unpredictable or erroneous behavior may result from modifying the stream
 * source while it is being queried.
 *
 * Most stream operations accept parameters that describe user-specified
 * behavior, such as the lambda expression {@code w -> w.getWeight()} passed to
 * {@code mapToInt} in the example above. To preserve correct behavior,
 * these behavioral parameters:
 * <ul>
 * <li>must be <a href="package-summary.html#NonInterference">non-interfering</a>
 * (they do not modify the stream source); and</li>
 * <li>in most cases must be <a href="package-summary.html#Statelessness">stateless</a>
 * (their result should not depend on any state that might change during execution
 * of the stream pipeline).</li>
 * </ul>
 *
 * 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. Unless otherwise specified these parameters must be
 * non-null.
 *
 * A stream should be operated on (invoking an intermediate or terminal stream
 * operation) only once. This rules out, for example, "forked" streams, where
 * the same source feeds two or more pipelines, or multiple traversals of the
 * same stream. A stream implementation may throw {@link IllegalStateException}
 * if it detects that the stream is being reused. However, since some stream
 * operations may return their receiver rather than a new stream object, it may
 * not be possible to detect reuse in all cases.
 *
 * Streams have a {@link #close()} method and implement {@link AutoCloseable},
 * but nearly all stream instances do not actually need to be closed after use.
 * Generally, only streams whose source is an IO channel (such as those returned
 * by {@link Files#lines(Path, Charset)}) will require closing. Most streams
 * are backed by collections, arrays, or generating functions, which require no
 * special resource management. (If a stream does require closing, it can be
 * declared as a resource in a {@code try}-with-resources statement.)
 *
 * Stream pipelines may execute either sequentially or in
 * <a href="package-summary.html#Parallelism">parallel</a>. This
 * execution mode is a property of the stream. Streams are created
 * with an initial choice of sequential or parallel execution. (For example,
 * {@link Collection#stream() Collection.stream()} creates a sequential stream,
 * and {@link Collection#parallelStream() Collection.parallelStream()} creates
 * a parallel one.) This choice of execution mode may be modified by the
 * {@link #sequential()} or {@link #parallel()} methods, and may be queried with
 * the {@link #isParallel()} method.
 *
 * @param <T> the type of the stream elements
 * @since 1.8
 * @see IntStream
 * @see LongStream
 * @see DoubleStream
 * @see <a href="package-summary.html">java.util.stream</a>
 */
 public interface Stream<T> extends BaseStream<T, Stream<T>> {
 
 
 /**
 * Returns a stream consisting of the elements of this stream that match
 * the given predicate.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * predicate to apply to each element to determine if it
 * should be included
 * @return the new stream
 */
 Stream<T> filter(Predicate<? super T> predicate);
 
 /**
 * Returns a stream consisting of the results of applying the given
 * function to the elements of this stream.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param <R> The element type of the new stream
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element
 * @return the new stream
 */
 <R> Stream<R> map(Function<? super T, ? extends R> mapper);
 
 /**
 * Returns an {@code IntStream} consisting of the results of applying the
 * given function to the elements of this stream.
 *
 * This is an <a href="package-summary.html#StreamOps">
 * intermediate operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element
 * @return the new stream
 */
 IntStream mapToInt(ToIntFunction<? super T> mapper);
 
 /**
 * Returns a {@code LongStream} consisting of the results of applying the
 * given function to the elements of this stream.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element
 * @return the new stream
 */
 LongStream mapToLong(ToLongFunction<? super T> mapper);
 
 /**
 * Returns a {@code DoubleStream} consisting of the results of applying the
 * given function to the elements of this stream.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element
 * @return the new stream
 */
 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);
 
 /**
 * Returns a stream consisting of the results of replacing each element of
 * this stream with the contents of a mapped stream produced by applying
 * the provided mapping function to each element. Each mapped stream is
 * {@link java.util.stream.BaseStream#close() closed} after its contents
 * have been placed into this stream. (If a mapped stream is {@code null}
 * an empty stream is used, instead.)
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @apiNote
 * The {@code flatMap()} operation has the effect of applying a one-to-many
 * transformation to the elements of the stream, and then flattening the
 * resulting elements into a new stream.
 *
 * Examples.
 *
 * If {@code orders} is a stream of purchase orders, and each purchase
 * order contains a collection of line items, then the following produces a
 * stream containing all the line items in all the orders:
 * <pre>{@code
 * orders.flatMap(order -> order.getLineItems().stream())...
 * }</pre>
 *
 * If {@code path} is the path to a file, then the following produces a
 * stream of the {@code words} contained in that file:
 * <pre>{@code
 * Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
 * Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
 * }</pre>
 * The {@code mapper} function passed to {@code flatMap} splits a line,
 * using a simple regular expression, into an array of words, and then
 * creates a stream of words from that array.
 *
 * @param <R> The element type of the new stream
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element which produces a stream
 * of new values
 * @return the new stream
 */
 <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
 
 /**
 * Returns an {@code IntStream} consisting of the results of replacing each
 * element of this stream with the contents of a mapped stream produced by
 * applying the provided mapping function to each element. Each mapped
 * stream is {@link java.util.stream.BaseStream#close() closed} after its
 * contents have been placed into this stream. (If a mapped stream is
 * {@code null} an empty stream is used, instead.)
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element which produces a stream
 * of new values
 * @return the new stream
 * @see #flatMap(Function)
 */
 IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);
 
 /**
 * Returns an {@code LongStream} consisting of the results of replacing each
 * element of this stream with the contents of a mapped stream produced by
 * applying the provided mapping function to each element. Each mapped
 * stream is {@link java.util.stream.BaseStream#close() closed} after its
 * contents have been placed into this stream. (If a mapped stream is
 * {@code null} an empty stream is used, instead.)
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element which produces a stream
 * of new values
 * @return the new stream
 * @see #flatMap(Function)
 */
 LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);
 
 /**
 * Returns an {@code DoubleStream} consisting of the results of replacing
 * each element of this stream with the contents of a mapped stream produced
 * by applying the provided mapping function to each element. Each mapped
 * stream is {@link java.util.stream.BaseStream#close() closed} after its
 * contents have placed been into this stream. (If a mapped stream is
 * {@code null} an empty stream is used, instead.)
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function to apply to each element which produces a stream
 * of new values
 * @return the new stream
 * @see #flatMap(Function)
 */
 DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);
 
 /**
 * Returns a stream consisting of the distinct elements (according to
 * {@link Object#equals(Object)}) of this stream.
 *
 * For ordered streams, the selection of distinct elements is stable
 * (for duplicated elements, the element appearing first in the encounter
 * order is preserved.) For unordered streams, no stability guarantees
 * are made.
 *
 * This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @apiNote
 * Preserving stability for {@code distinct()} in parallel pipelines is
 * relatively expensive (requires that the operation act as a full barrier,
 * with substantial buffering overhead), and stability is often not needed.
 * Using an unordered stream source (such as {@link #generate(Supplier)})
 * or removing the ordering constraint with {@link #unordered()} may result
 * in significantly more efficient execution for {@code distinct()} in parallel
 * pipelines, if the semantics of your situation permit. If consistency
 * with encounter order is required, and you are experiencing poor performance
 * or memory utilization with {@code distinct()} in parallel pipelines,
 * switching to sequential execution with {@link #sequential()} may improve
 * performance.
 *
 * @return the new stream
 */
 Stream<T> distinct();
 
 /**
 * Returns a stream consisting of the elements of this stream, sorted
 * according to natural order. If the elements of this stream are not
 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
 * when the terminal operation is executed.
 *
 * For ordered streams, the sort is stable. For unordered streams, no
 * stability guarantees are made.
 *
 * This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @return the new stream
 */
 Stream<T> sorted();
 
 /**
 * Returns a stream consisting of the elements of this stream, sorted
 * according to the provided {@code Comparator}.
 *
 * For ordered streams, the sort is stable. For unordered streams, no
 * stability guarantees are made.
 *
 * This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * {@code Comparator} to be used to compare stream elements
 * @return the new stream
 */
 Stream<T> sorted(Comparator<? super T> comparator);
 
 /**
 * Returns a stream consisting of the elements of this stream, additionally
 * performing the provided action on each element as elements are consumed
 * from the resulting stream.
 *
 * This is an <a href="package-summary.html#StreamOps">intermediate
 * operation</a>.
 *
 * For parallel stream pipelines, the action may be called at
 * whatever time and in whatever thread the element is made available by the
 * upstream operation. If the action modifies shared state,
 * it is responsible for providing the required synchronization.
 *
 * @apiNote This method exists mainly to support debugging, where you want
 * to see the elements as they flow past a certain point in a pipeline:
 * <pre>{@code
 * Stream.of("one", "two", "three", "four")
 * .filter(e -> e.length() > 3)
 * .peek(e -> System.out.println("Filtered value: " + e))
 * .map(String::toUpperCase)
 * .peek(e -> System.out.println("Mapped value: " + e))
 * .collect(Collectors.toList());
 * }</pre>
 *
 * @param action a <a href="package-summary.html#NonInterference">
 * non-interfering</a> action to perform on the elements as
 * they are consumed from the stream
 * @return the new stream
 */
 Stream<T> peek(Consumer<? super T> action);
 
 /**
 * Returns a stream consisting of the elements of this stream, truncated
 * to be no longer than {@code maxSize} in length.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * stateful intermediate operation</a>.
 *
 * @apiNote
 * While {@code limit()} is generally a cheap operation on sequential
 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 * especially for large values of {@code maxSize}, since {@code limit(n)}
 * is constrained to return not just any n elements, but the
 * first n elements in the encounter order. Using an unordered
 * stream source (such as {@link #generate(Supplier)}) or removing the
 * ordering constraint with {@link #unordered()} may result in significant
 * speedups of {@code limit()} in parallel pipelines, if the semantics of
 * your situation permit. If consistency with encounter order is required,
 * and you are experiencing poor performance or memory utilization with
 * {@code limit()} in parallel pipelines, switching to sequential execution
 * with {@link #sequential()} may improve performance.
 *
 * @param maxSize the number of elements the stream should be limited to
 * @return the new stream
 * @throws IllegalArgumentException if {@code maxSize} is negative
 */
 Stream<T> limit(long maxSize);
 
 /**
 * Returns a stream consisting of the remaining elements of this stream
 * after discarding the first {@code n} elements of the stream.
 * If this stream contains fewer than {@code n} elements then an
 * empty stream will be returned.
 *
 * This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @apiNote
 * While {@code skip()} is generally a cheap operation on sequential
 * stream pipelines, it can be quite expensive on ordered parallel pipelines,
 * especially for large values of {@code n}, since {@code skip(n)}
 * is constrained to skip not just any n elements, but the
 * first n elements in the encounter order. Using an unordered
 * stream source (such as {@link #generate(Supplier)}) or removing the
 * ordering constraint with {@link #unordered()} may result in significant
 * speedups of {@code skip()} in parallel pipelines, if the semantics of
 * your situation permit. If consistency with encounter order is required,
 * and you are experiencing poor performance or memory utilization with
 * {@code skip()} in parallel pipelines, switching to sequential execution
 * with {@link #sequential()} may improve performance.
 *
 * @param n the number of leading elements to skip
 * @return the new stream
 * @throws IllegalArgumentException if {@code n} is negative
 */
 Stream<T> skip(long n);
 
 /**
 * Performs an action for each element of this stream.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * The behavior of this operation is explicitly nondeterministic.
 * For parallel stream pipelines, this operation does not
 * guarantee to respect the encounter order of the stream, as doing so
 * would sacrifice the benefit of parallelism. For any given element, the
 * action may be performed at whatever time and in whatever thread the
 * library chooses. If the action accesses shared state, it is
 * responsible for providing the required synchronization.
 *
 * @param action a <a href="package-summary.html#NonInterference">
 * non-interfering</a> action to perform on the elements
 */
 void forEach(Consumer<? super T> action);
 
 /**
 * Performs an action for each element of this stream, in the encounter
 * order of the stream if the stream has a defined encounter order.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * This operation processes the elements one at a time, in encounter
 * order if one exists. Performing the action for one element
 * <a href="../concurrent/package-summary.html#MemoryVisibility">happens-before</a>
 * performing the action for subsequent elements, but for any given element,
 * the action may be performed in whatever thread the library chooses.
 *
 * @param action a <a href="package-summary.html#NonInterference">
 * non-interfering</a> action to perform on the elements
 * @see #forEach(Consumer)
 */
 void forEachOrdered(Consumer<? super T> action);
 
 /**
 * Returns an array containing the elements of this stream.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @return an array containing the elements of this stream
 */
 Object[] toArray();
 
 /**
 * Returns an array containing the elements of this stream, using the
 * provided {@code generator} function to allocate the returned array, as
 * well as any additional arrays that might be required for a partitioned
 * execution or for resizing.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @apiNote
 * The generator function takes an integer, which is the size of the
 * desired array, and produces an array of the desired size. This can be
 * concisely expressed with an array constructor reference:
 * <pre>{@code
 * Person[] men = people.stream()
 * .filter(p -> p.getGender() == MALE)
 * .toArray(Person[]::new);
 * }</pre>
 *
 * @param <A> the element type of the resulting array
 * @param generator a function which produces a new array of the desired
 * type and the provided length
 * @return an array containing the elements in this stream
 * @throws ArrayStoreException if the runtime type of the array returned
 * from the array generator is not a supertype of the runtime type of every
 * element in this stream
 */
 <A> A[] toArray(IntFunction<A[]> generator);
 
 /**
 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 * elements of this stream, using the provided identity value and an
 * <a href="package-summary.html#Associativity">associative</a>
 * accumulation function, and returns the reduced value. This is equivalent
 * to:
 * <pre>{@code
 * T result = identity;
 * for (T element : this stream)
 * result = accumulator.apply(result, element)
 * return result;
 * }</pre>
 *
 * but is not constrained to execute sequentially.
 *
 * The {@code identity} value must be an identity for the accumulator
 * function. This means that for all {@code t},
 * {@code accumulator.apply(identity, t)} is equal to {@code t}.
 * The {@code accumulator} function must be an
 * <a href="package-summary.html#Associativity">associative</a> function.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @apiNote Sum, min, max, average, and string concatenation are all special
 * cases of reduction. Summing a stream of numbers can be expressed as:
 *
 * <pre>{@code
 * Integer sum = integers.reduce(0, (a, b) -> a+b);
 * }</pre>
 *
 * or:
 *
 * <pre>{@code
 * Integer sum = integers.reduce(0, Integer::sum);
 * }</pre>
 *
 * While this may seem a more roundabout way to perform an aggregation
 * compared to simply mutating a running total in a loop, reduction
 * operations parallelize more gracefully, without needing additional
 * synchronization and with greatly reduced risk of data races.
 *
 * @param identity the identity value for the accumulating function
 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for combining two values
 * @return the result of the reduction
 */
 T reduce(T identity, BinaryOperator<T> accumulator);
 
 /**
 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 * elements of this stream, using an
 * <a href="package-summary.html#Associativity">associative</a> accumulation
 * function, and returns an {@code Optional} describing the reduced value,
 * if any. This is equivalent to:
 * <pre>{@code
 * boolean foundAny = false;
 * T result = null;
 * for (T element : this stream) {
 * if (!foundAny) {
 * foundAny = true;
 * result = element;
 * }
 * else
 * result = accumulator.apply(result, element);
 * }
 * return foundAny ? Optional.of(result) : Optional.empty();
 * }</pre>
 *
 * but is not constrained to execute sequentially.
 *
 * The {@code accumulator} function must be an
 * <a href="package-summary.html#Associativity">associative</a> function.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for combining two values
 * @return an {@link Optional} describing the result of the reduction
 * @throws NullPointerException if the result of the reduction is null
 * @see #reduce(Object, BinaryOperator)
 * @see #min(Comparator)
 * @see #max(Comparator)
 */
 Optional<T> reduce(BinaryOperator<T> accumulator);
 
 /**
 * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
 * elements of this stream, using the provided identity, accumulation and
 * combining functions. This is equivalent to:
 * <pre>{@code
 * U result = identity;
 * for (T element : this stream)
 * result = accumulator.apply(result, element)
 * return result;
 * }</pre>
 *
 * but is not constrained to execute sequentially.
 *
 * The {@code identity} value must be an identity for the combiner
 * function. This means that for all {@code u}, {@code combiner(identity, u)}
 * is equal to {@code u}. Additionally, the {@code combiner} function
 * must be compatible with the {@code accumulator} function; for all
 * {@code u} and {@code t}, the following must hold:
 * <pre>{@code
 * combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
 * }</pre>
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @apiNote Many reductions using this form can be represented more simply
 * by an explicit combination of {@code map} and {@code reduce} operations.
 * The {@code accumulator} function acts as a fused mapper and accumulator,
 * which can sometimes be more efficient than separate mapping and reduction,
 * such as when knowing the previously reduced value allows you to avoid
 * some computation.
 *
 * @param The type of the result
 * @param identity the identity value for the combiner function
 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for incorporating an additional element into a result
 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for combining two values, which must be
 * compatible with the accumulator function
 * @return the result of the reduction
 * @see #reduce(BinaryOperator)
 * @see #reduce(Object, BinaryOperator)
 */
 U reduce(U identity,
 BiFunction<U, ? super T, U> accumulator,
 BinaryOperator combiner);
 
 /**
 * Performs a <a href="package-summary.html#MutableReduction">mutable
 * reduction</a> operation on the elements of this stream. A mutable
 * reduction is one in which the reduced value is a mutable result container,
 * such as an {@code ArrayList}, and elements are incorporated by updating
 * the state of the result rather than by replacing the result. This
 * produces a result equivalent to:
 * <pre>{@code
 * R result = supplier.get();
 * for (T element : this stream)
 * accumulator.accept(result, element);
 * return result;
 * }</pre>
 *
 * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations
 * can be parallelized without requiring additional synchronization.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @apiNote There are many existing classes in the JDK whose signatures are
 * well-suited for use with method references as arguments to {@code collect()}.
 * For example, the following will accumulate strings into an {@code ArrayList}:
 * <pre>{@code
 * List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
 * ArrayList::addAll);
 * }</pre>
 *
 * The following will take a stream of strings and concatenates them into a
 * single string:
 * <pre>{@code
 * String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
 * StringBuilder::append)
 * .toString();
 * }</pre>
 *
 * @param <R> type of the result
 * @param supplier a function that creates a new result container. For a
 * parallel execution, this function may be called
 * multiple times and must return a fresh value each time.
 * @param accumulator an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for incorporating an additional element into a result
 * @param combiner an <a href="package-summary.html#Associativity">associative</a>,
 * <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * function for combining two values, which must be
 * compatible with the accumulator function
 * @return the result of the reduction
 */
 <R> R collect(Supplier<R> supplier,
 BiConsumer<R, ? super T> accumulator,
 BiConsumer<R, R> combiner);
 
 /**
 * Performs a <a href="package-summary.html#MutableReduction">mutable
 * reduction</a> operation on the elements of this stream using a
 * {@code Collector}. A {@code Collector}
 * encapsulates the functions used as arguments to
 * {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for reuse of
 * collection strategies and composition of collect operations such as
 * multiple-level grouping or partitioning.
 *
 * If the stream is parallel, and the {@code Collector}
 * is {@link Collector.Characteristics#CONCURRENT concurrent}, and
 * either the stream is unordered or the collector is
 * {@link Collector.Characteristics#UNORDERED unordered},
 * then a concurrent reduction will be performed (see {@link Collector} for
 * details on concurrent reduction.)
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * When executed in parallel, multiple intermediate results may be
 * instantiated, populated, and merged so as to maintain isolation of
 * mutable data structures. Therefore, even when executed in parallel
 * with non-thread-safe data structures (such as {@code ArrayList}), no
 * additional synchronization is needed for a parallel reduction.
 *
 * @apiNote
 * The following will accumulate strings into an ArrayList:
 * <pre>{@code
 * List<String> asList = stringStream.collect(Collectors.toList());
 * }</pre>
 *
 * The following will classify {@code Person} objects by city:
 * <pre>{@code
 * Map<String, List<Person>> peopleByCity
 * = personStream.collect(Collectors.groupingBy(Person::getCity));
 * }</pre>
 *
 * The following will classify {@code Person} objects by state and city,
 * cascading two {@code Collector}s together:
 * <pre>{@code
 * Map<String, Map<String, List<Person>>> peopleByStateAndCity
 * = personStream.collect(Collectors.groupingBy(Person::getState,
 * Collectors.groupingBy(Person::getCity)));
 * }</pre>
 *
 * @param <R> the type of the result
 * @param <A> the intermediate accumulation type of the {@code Collector}
 * @param collector the {@code Collector} describing the reduction
 * @return the result of the reduction
 * @see #collect(Supplier, BiConsumer, BiConsumer)
 * @see Collectors
 */
 <R, A> R collect(Collector<? super T, A, R> collector);
 
 /**
 * Returns the minimum element of this stream according to the provided
 * {@code Comparator}. This is a special case of a
 * <a href="package-summary.html#Reduction">reduction</a>.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
 *
 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * {@code Comparator} to compare elements of this stream
 * @return an {@code Optional} describing the minimum element of this stream,
 * or an empty {@code Optional} if the stream is empty
 * @throws NullPointerException if the minimum element is null
 */
 Optional<T> min(Comparator<? super T> comparator);
 
 /**
 * Returns the maximum element of this stream according to the provided
 * {@code Comparator}. This is a special case of a
 * <a href="package-summary.html#Reduction">reduction</a>.
 *
 * This is a <a href="package-summary.html#StreamOps">terminal
 * operation</a>.
 *
 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * {@code Comparator} to compare elements of this stream
 * @return an {@code Optional} describing the maximum element of this stream,
 * or an empty {@code Optional} if the stream is empty
 * @throws NullPointerException if the maximum element is null
 */
 Optional<T> max(Comparator<? super T> comparator);
 
 /**
 * Returns the count of elements in this stream. This is a special case of
 * a <a href="package-summary.html#Reduction">reduction</a> and is
 * equivalent to:
 * <pre>{@code
 * return mapToLong(e -> 1L).sum();
 * }</pre>
 *
 * This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
 *
 * @return the count of elements in this stream
 */
 long count();
 
 /**
 * Returns whether any elements of this stream match the provided
 * predicate. May not evaluate the predicate on all elements if not
 * necessary for determining the result. If the stream is empty then
 * {@code false} is returned and the predicate is not evaluated.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * terminal operation</a>.
 *
 * @apiNote
 * This method evaluates the existential quantification of the
 * predicate over the elements of the stream (for some x P(x)).
 *
 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * predicate to apply to elements of this stream
 * @return {@code true} if any elements of the stream match the provided
 * predicate, otherwise {@code false}
 */
 boolean anyMatch(Predicate<? super T> predicate);
 
 /**
 * Returns whether all elements of this stream match the provided predicate.
 * May not evaluate the predicate on all elements if not necessary for
 * determining the result. If the stream is empty then {@code true} is
 * returned and the predicate is not evaluated.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * terminal operation</a>.
 *
 * @apiNote
 * This method evaluates the universal quantification of the
 * predicate over the elements of the stream (for all x P(x)). If the
 * stream is empty, the quantification is said to be vacuously
 * satisfied and is always {@code true} (regardless of P(x)).
 *
 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * predicate to apply to elements of this stream
 * @return {@code true} if either all elements of the stream match the
 * provided predicate or the stream is empty, otherwise {@code false}
 */
 boolean allMatch(Predicate<? super T> predicate);
 
 /**
 * Returns whether no elements of this stream match the provided predicate.
 * May not evaluate the predicate on all elements if not necessary for
 * determining the result. If the stream is empty then {@code true} is
 * returned and the predicate is not evaluated.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * terminal operation</a>.
 *
 * @apiNote
 * This method evaluates the universal quantification of the
 * negated predicate over the elements of the stream (for all x ~P(x)). If
 * the stream is empty, the quantification is said to be vacuously satisfied
 * and is always {@code true}, regardless of P(x).
 *
 * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,
 * <a href="package-summary.html#Statelessness">stateless</a>
 * predicate to apply to elements of this stream
 * @return {@code true} if either no elements of the stream match the
 * provided predicate or the stream is empty, otherwise {@code false}
 */
 boolean noneMatch(Predicate<? super T> predicate);
 
 /**
 * Returns an {@link Optional} describing the first element of this stream,
 * or an empty {@code Optional} if the stream is empty. If the stream has
 * no encounter order, then any element may be returned.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * terminal operation</a>.
 *
 * @return an {@code Optional} describing the first element of this stream,
 * or an empty {@code Optional} if the stream is empty
 * @throws NullPointerException if the element selected is null
 */
 Optional<T> findFirst();
 
 /**
 * Returns an {@link Optional} describing some element of the stream, or an
 * empty {@code Optional} if the stream is empty.
 *
 * This is a <a href="package-summary.html#StreamOps">short-circuiting
 * terminal operation</a>.
 *
 * The behavior of this operation is explicitly nondeterministic; it is
 * free to select any element in the stream. This is to allow for maximal
 * performance in parallel operations; the cost is that multiple invocations
 * on the same source may not return the same result. (If a stable result
 * is desired, use {@link #findFirst()} instead.)
 *
 * @return an {@code Optional} describing some element of this stream, or an
 * empty {@code Optional} if the stream is empty
 * @throws NullPointerException if the element selected is null
 * @see #findFirst()
 */
 Optional<T> findAny();
 
 // Static factories
 
 /**
 * Returns a builder for a {@code Stream}.
 *
 * @param <T> type of elements
 * @return a stream builder
 */
 public static<T> Builder<T> builder() {
 
 return new Streams.StreamBuilderImpl<>();
 }
 
 /**
 * Returns an empty sequential {@code Stream}.
 *
 * @param <T> the type of stream elements
 * @return an empty sequential stream
 */
 public static<T> Stream<T> empty() {
 
 return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
 }
 
 /**
 * Returns a sequential {@code Stream} containing a single element.
 *
 * @param t the single element
 * @param <T> the type of stream elements
 * @return a singleton sequential stream
 */
 public static<T> Stream<T> of(T t) {
 
 return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
 }
 
 /**
 * Returns a sequential ordered stream whose elements are the specified values.
 *
 * @param <T> the type of stream elements
 * @param values the elements of the new stream
 * @return the new stream
 */
 @SafeVarargs
 @SuppressWarnings("varargs") // Creating a stream from an array is safe
 public static<T> Stream<T> of(T... values) {
 
 return Arrays.stream(values);
 }
 
 /**
 * Returns an infinite sequential ordered {@code Stream} produced by iterative
 * application of a function {@code f} to an initial element {@code seed},
 * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
 * {@code f(f(seed))}, etc.
 *
 * The first element (position {@code 0}) in the {@code Stream} will be
 * the provided {@code seed}. For {@code n > 0}, the element at position
 * {@code n}, will be the result of applying the function {@code f} to the
 * element at position {@code n - 1}.
 *
 * @param <T> the type of stream elements
 * @param seed the initial element
 * @param f a function to be applied to the previous element to produce
 * a new element
 * @return a new sequential {@code Stream}
 */
 public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
 
 Objects.requireNonNull(f);
 final Iterator<T> iterator = new Iterator<T>() {
 
 @SuppressWarnings("unchecked")
 T t = (T) Streams.NONE;
 
 @Override
 public boolean hasNext() {
 
 return true;
 }
 
 @Override
 public T next() {
 
 return t = (t == Streams.NONE) ? seed : f.apply(t);
 }
 };
 return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
 iterator,
 Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
 }
 
 /**
 * Returns an infinite sequential unordered stream where each element is
 * generated by the provided {@code Supplier}. This is suitable for
 * generating constant streams, streams of random elements, etc.
 *
 * @param <T> the type of stream elements
 * @param s the {@code Supplier} of generated elements
 * @return a new infinite sequential unordered {@code Stream}
 */
 public static<T> Stream<T> generate(Supplier<T> s) {
 
 Objects.requireNonNull(s);
 return StreamSupport.stream(
 new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
 }
 
 /**
 * Creates a lazily concatenated stream whose elements are all the
 * elements of the first stream followed by all the elements of the
 * second stream. The resulting stream is ordered if both
 * of the input streams are ordered, and parallel if either of the input
 * streams is parallel. When the resulting stream is closed, the close
 * handlers for both input streams are invoked.
 *
 * @implNote
 * Use caution when constructing streams from repeated concatenation.
 * Accessing an element of a deeply concatenated stream can result in deep
 * call chains, or even {@code StackOverflowException}.
 *
 * @param <T> The type of stream elements
 * @param a the first stream
 * @param b the second stream
 * @return the concatenation of the two input streams
 */
 public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
 
 Objects.requireNonNull(a);
 Objects.requireNonNull(b);
 
 @SuppressWarnings("unchecked")
 Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
 (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
 Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
 return stream.onClose(Streams.composedClose(a, b));
 }
 
 /**
 * A mutable builder for a {@code Stream}. This allows the creation of a
 * {@code Stream} by generating elements individually and adding them to the
 * {@code Builder} (without the copying overhead that comes from using
 * an {@code ArrayList} as a temporary buffer.)
 *
 * A stream builder has a lifecycle, which starts in a building
 * phase, during which elements can be added, and then transitions to a built
 * phase, after which elements may not be added. The built phase begins
 * when the {@link #build()} method is called, which creates an ordered
 * {@code Stream} whose elements are the elements that were added to the stream
 * builder, in the order they were added.
 *
 * @param <T> the type of stream elements
 * @see Stream#builder()
 * @since 1.8
 */
 public interface Builder<T> extends Consumer<T> {
 
 
 /**
 * Adds an element to the stream being built.
 *
 * @throws IllegalStateException if the builder has already transitioned to
 * the built state
 */
 @Override
 void accept(T t);
 
 /**
 * Adds an element to the stream being built.
 *
 * @implSpec
 * The default implementation behaves as if:
 * <pre>{@code
 * accept(t)
 * return this;
 * }</pre>
 *
 * @param t the element to add
 * @return {@code this} builder
 * @throws IllegalStateException if the builder has already transitioned to
 * the built state
 */
 default Builder<T> add(T t) {
 
 accept(t);
 return this;
 }
 
 /**
 * Builds the stream, transitioning this builder to the built state.
 * An {@code IllegalStateException} is thrown if there are further attempts
 * to operate on the builder after it has entered the built state.
 *
 * @return the built stream
 * @throws IllegalStateException if the builder has already transitioned to
 * the built state
 */
 Stream<T> build();
 
 }
 }

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![在这里插入图片描述][3333c47855574cb5bacc07e8f412a3eb.png]

[https_blog.csdn.net_sun2012930_article_details_106507179]: https://blog.csdn.net/sun2012930/article/details/106507179
[3333c47855574cb5bacc07e8f412a3eb.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/27/8a3b81d50f8b444ca21fe1323a55671a.png