Stream流与Lambda表达式（一）杂谈

刺骨的言语ヽ痛彻心扉 2022-03-07 10:20 175阅读 0赞

## 一、流 转换为数组、集合 ##

package com.java.design.java8.Stream;
    
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.test.context.junit4.SpringRunner;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Stream;
    
    /**
     * @author 陈杨
     */
    
    @SpringBootTest
    @RunWith(SpringRunner.class)
    public class ListChange {
    
        private Stream<String> stream = Stream.of("Kirito", "Asuna", "Illyasviel", "Sakura");
    
        @Test
        public void testListChange() {
    
    
            //   将流转换为数组
            //   System.out.println("-------------将流转换为数组---------------");
            //   String[] array = stream.toArray(len -> new String[len]);
            //   String[] array = stream.toArray(String[]::new);
            //   Arrays.asList(array).stream().forEach(System.out::println);
    
    
            //    将流转换为集合
            //    System.out.println("-------------将流转换为集合---------------");
            //    System.out.println("-------Collectors.toList()解析-----------");
            /*    public static <T>
             *               Collector<T, ?, List<T>> toList() {
             *          return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
             *                 (left, right) -> { left.addAll(right); return left; },
             *                 CH_ID);
            }*/
    
            //    List<String> list = stream.collect(Collectors.toList());
            //    List<String> linkedList = stream.collect(LinkedList::new,LinkedList::add,LinkedList::addAll);
            List<String> list = stream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
            list.forEach(System.out::println);
            System.out.println(list.getClass());
    
    
            //    System.out.println("-------Collectors.toCollection()解析-----");
    
            /*    public static <T, C extends Collection<T>>
             *        Collector<T, ?, C> toCollection(Supplier<C> collectionFactory) {
             *              return new CollectorImpl<>(collectionFactory, Collection<T>::add,
             *             (r1, r2) -> { r1.addAll(r2); return r1; },
             *              CH_ID);
            }*/
    
            //    List<String> list =stream.collect(Collectors.toCollection(ArrayList::new));
            //    List<String> linkedList =stream.collect(Collectors.toCollection(ArrayList::new));
            //    Set<String> treeSet =stream.collect(Collectors.toCollection(TreeSet::new));
            //    Set<String> hashSet =stream.collect(Collectors.toCollection(HashSet::new));
    
    
        }
    
    }

## 二、集合排序 ##

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.1.2.RELEASE)
    
    2019-02-20 15:58:39.959  INFO 4588 --- [           main] c.j.d.java8.Stream.ComparatorDetail      : Starting ComparatorDetail on DESKTOP-87RMBG4 with PID 4588 (started by 46250 in E:\IdeaProjects\design)
    2019-02-20 15:58:39.962  INFO 4588 --- [           main] c.j.d.java8.Stream.ComparatorDetail      : No active profile set, falling back to default profiles: default
    2019-02-20 15:58:40.459  INFO 4588 --- [           main] c.j.d.java8.Stream.ComparatorDetail      : Started ComparatorDetail in 0.729 seconds (JVM running for 1.462)
    [Alice, Asuna, Kirito, Sinon, Yuuki]
    -----------------------------------------
    
    [Yuuki, Sinon, Kirito, Asuna, Alice]
    -----------------------------------------
    
    [Alice, Asuna, Sinon, Yuuki, Kirito]
    -----------------------------------------
    
    [Kirito, Yuuki, Sinon, Asuna, Alice]
    -----------------------------------------
    
    [Alice, Asuna, Sinon, Yuuki, Kirito]
    -----------------------------------------

## 三、Stream之map(Lambda) ##

package com.java.design.java8.Stream;
    
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.test.context.junit4.SpringRunner;
    
    import java.util.Arrays;
    import java.util.Collections;
    import java.util.List;
    import java.util.stream.Collectors;
    
    
    /**
     * @author 陈杨
     */
    
    @SpringBootTest
    @RunWith(SpringRunner.class)
    public class StringOperation {
    
        private List<String> list = Arrays.asList("Kirito", "Asuna", "Illyasviel", "Sakura");
    
        private List<List<String>> listMap =
                Arrays.asList(Collections.singletonList("Kirito"), Collections.singletonList("Asuna"),
                        Collections.singletonList("Illyasviel"), Collections.singletonList("Sakura"));
    
        private List<List<String>> listFlatMap =
                Arrays.asList(Collections.singletonList("Kirito"), Collections.singletonList("Asuna"),
                        Collections.singletonList("Illyasviel"), Collections.singletonList("Sakura"));
    
    
        @Test
        public void testStringOperation() {
    
            //    集合中 每个字符串  按照排列先后顺序拼接 形成一个长字符串
            //    String concat =
            //    list.stream().collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();
    
            //    String concat = list.stream().collect(Collectors.joining());
            //    System.out.println(concat);
    
    
            //    集合中 对每个字符串元素  将所有字母变成大写字母
            System.out.println("-----------------------------------------\n");
            List<String> upperCase = list.stream().map(String::toUpperCase).collect(Collectors.toList());
            upperCase.forEach(System.out::println);
    
    
            //    集合中 对每个字符串元素  将所有字母变成小写字母
            System.out.println("-----------------------------------------\n");
            List<String> lowerCase = list.stream().map(String::toLowerCase).collect(Collectors.toList());
            lowerCase.forEach(System.out::println);
    
    
            System.out.println("-----------------------------------------\n");
            System.out.println("FlatMap与Map的区别:\n");
    
            //    map:  对多个list 分别map Fuction 映射  形成多个list
            System.out.println("-----------------------------------------\n");
            System.out.println("进行map映射:");
    
    
            List<List<String>> upperMap = listMap.stream()
                    .map(list -> list.stream().map(String::toUpperCase)
                            .collect(Collectors.toList())).collect(Collectors.toList());
            upperMap.forEach(System.out::println);
    
    
            //    FlatMap： 对多个list 进行flat扁平化 后再进行map Fuction 映射  形成一个list
            System.out.println("-----------------------------------------\n");
            System.out.println("FlatMap扁平化进行map映射:");
    
    
            List<String> upperFlatMap = listFlatMap.stream()
                    .flatMap(list -> list.stream().map(String::toUpperCase)).collect(Collectors.toList());
            upperFlatMap.forEach(System.out::println);
    
    
        }
    }

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.1.2.RELEASE)
    
    2019-02-20 15:50:07.423  INFO 8208 --- [           main] c.j.design.java8.Stream.StringOperation  : Starting StringOperation on DESKTOP-87RMBG4 with PID 8208 (started by 46250 in E:\IdeaProjects\design)
    2019-02-20 15:50:07.424  INFO 8208 --- [           main] c.j.design.java8.Stream.StringOperation  : No active profile set, falling back to default profiles: default
    2019-02-20 15:50:07.917  INFO 8208 --- [           main] c.j.design.java8.Stream.StringOperation  : Started StringOperation in 0.717 seconds (JVM running for 1.5)
    -----------------------------------------
    
    KIRITO
    ASUNA
    ILLYASVIEL
    SAKURA
    -----------------------------------------
    
    kirito
    asuna
    illyasviel
    sakura
    -----------------------------------------
    
    FlatMap与Map的区别:
    
    -----------------------------------------
    
    进行map映射:
    [KIRITO]
    [ASUNA]
    [ILLYASVIEL]
    [SAKURA]
    -----------------------------------------
    
    FlatMap扁平化进行map映射:
    KIRITO
    ASUNA
    ILLYASVIEL
    SAKURA

## 四、内部迭代与外部迭代 ##

package com.java.design.java8.Stream;
    
    
    import com.java.design.java8.entity.Student;
    import com.java.design.java8.entity.Students;
    import org.junit.Before;
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.test.context.junit4.SpringRunner;
    
    import java.util.*;
    
    
    /**
     * @author 陈杨
     */
    
    @SpringBootTest
    @RunWith(SpringRunner.class)
    //迭代本质
    public class IterativeEssence {
    
        private List<Student> students;
    
    
        @Before
        public void init() {
            students = new Students().init();
        }
    
    
        @Test
        public void testIterativeEssence() {
    
            //    需求： select name from students where  age > 14 and addr ="Sword Art Online"  order by id desc ;
    
    
            //    外部迭代
            System.out.println("-----------------------------------------\n");
            System.out.println("外部迭代");
            List<Student> list = new ArrayList<>();
    
            for (Student student : students) {
                if (student.getAge() > 14 && student.getAddr().equals("Sword Art Online")) {
                    list.add(student);
                }
            }
    
            list.sort(Comparator.comparingInt(Student::getId));
    
            for (Student student : list) {
                System.out.println(student.getName());
            }
    
    
            //    内部迭代
            System.out.println("-----------------------------------------\n");
            System.out.println("内部迭代");
            students.stream()
                    .filter(student -> student.getAge() > 14)
                    .filter(student -> student.getAddr().equals("Sword Art Online"))
                    .sorted(Comparator.comparingInt(Student::getId)).
                    forEach(student -> System.out.println(student.getName()));
    
            //    备注：
            //    内部迭代与SQL语句属于描述性语言
            //    集合关注的是数据与数据存储
            //    流关注的是数据的计算
            //    流中间操作返回的都是Stream对象 泛型取决于中间操作的类型
            //    流终止操作： 无返回值（forEach） 返回值是其他类型
        }
    
    
    }

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.1.2.RELEASE)
    
    2019-02-20 15:53:03.633  INFO 8864 --- [           main] c.j.d.java8.Stream.IterativeEssence      : Starting IterativeEssence on DESKTOP-87RMBG4 with PID 8864 (started by 46250 in E:\IdeaProjects\design)
    2019-02-20 15:53:03.640  INFO 8864 --- [           main] c.j.d.java8.Stream.IterativeEssence      : No active profile set, falling back to default profiles: default
    2019-02-20 15:53:04.167  INFO 8864 --- [           main] c.j.d.java8.Stream.IterativeEssence      : Started IterativeEssence in 0.746 seconds (JVM running for 1.455)
    -----------------------------------------
    
    外部迭代
    Kirito
    Asuna
    -----------------------------------------
    
    内部迭代
    Kirito
    Asuna

## 五、串行流与并行流 简单性能测试 ##

package com.java.design.java8.Stream;
    
    import org.junit.Before;
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.test.context.junit4.SpringRunner;
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    import java.util.UUID;
    import java.util.concurrent.TimeUnit;
    import java.util.function.Function;
    import java.util.stream.Collectors;
    import java.util.stream.IntStream;
    
    
    /**
     * @author 陈杨
     */
    
    @RunWith(SpringRunner.class)
    @SpringBootTest
    public class ErgodicString {
    
        private List<String> uuid;
        private long startTime;
        private long endTime;
        private long parallelEndTime;
    
        @Before
        public void init() {
    
            uuid = new ArrayList<>(10000000);
    
            IntStream.range(0, 10000000).forEach(i -> uuid.add(UUID.randomUUID().toString()));
    
        }
    
    
        public void testNormal() {
    
            startTime = System.nanoTime();
            uuid.stream().sorted().collect(Collectors.toList());
    
            endTime = System.nanoTime();
            long millis = TimeUnit.NANOSECONDS.toMillis(endTime - startTime);
            System.out.println("单线程" + millis);
        }
    
        public void testParallel() {
    
            startTime = System.nanoTime();
            uuid.parallelStream().sorted().collect(Collectors.toList());
            parallelEndTime = System.nanoTime();
            long millis = TimeUnit.NANOSECONDS.toMillis(parallelEndTime - startTime);
            System.out.println("多线程" + millis);
        }
    
    
        @Test
        public void testErgodicString() {
    
            List<String> list = Arrays.asList("Kirito", "Asuna", "Illyasviel", "Sakura");
    
    
            //  需求： 将数组中每个元素各个字母大写 并放入集合
    
            System.out.println("--------------------串行流stream---------------------");
            // spliterator 分割迭代器
            //  串行流stream() 单线程处理
            /*
             *  default Stream<E> stream() {
             *       return StreamSupport.stream(spliterator(), false);}
             */
    
            //  List<String> collect = list.stream().map(str -> str.toUpperCase()).collect(Collectors.toList());
    
            //  R apply(T t); toUpperCase() 没有参数作为输入
            //  但把调用toUpperCase的对象作为T作为输入 返回的R是return的对象结果
            //  List<String> collect = list.stream().map(String::toUpperCase).collect(Collectors.toList());
    
            Function<String, String> function = String::toUpperCase;
    
    
            //等价于 (String str)  -> str.toUpperCase()
            //方法引用 类的类型::实例方法 对应的lambda表达式 第一个输入参数 是调用此方法的对象
            List<String> collect = list.stream().map(function).collect(Collectors.toList());
    
            collect.forEach(System.out::println);
    
            this.testNormal();
    
            System.out.println("-----------------并行流parallelStream------------------");
            //  并行流parallelStream() 多线程处理
            /*
             *  default Stream<E> parallelStream() {
             *       return StreamSupport.stream(spliterator(), true);}
             */
    
            List<String> parallelCollect = list.parallelStream().map(str -> str.toUpperCase()).collect(Collectors.toList());
            parallelCollect.forEach(System.out::println);
    
            this.testParallel();
    
        }
    
    
    }

.   ____          _            __ _ _
     /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
    ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
     \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
      '  |____| .__|_| |_|_| |_\__, | / / / /
     =========|_|==============|___/=/_/_/_/
     :: Spring Boot ::        (v2.1.2.RELEASE)
    
    2019-02-20 15:54:54.321  INFO 7356 --- [           main] c.j.design.java8.Stream.ErgodicString    : Starting ErgodicString on DESKTOP-87RMBG4 with PID 7356 (started by 46250 in E:\IdeaProjects\design)
    2019-02-20 15:54:54.323  INFO 7356 --- [           main] c.j.design.java8.Stream.ErgodicString    : No active profile set, falling back to default profiles: default
    2019-02-20 15:54:54.817  INFO 7356 --- [           main] c.j.design.java8.Stream.ErgodicString    : Started ErgodicString in 0.705 seconds (JVM running for 1.528)
    --------------------串行流stream---------------------
    KIRITO
    ASUNA
    ILLYASVIEL
    SAKURA
    单线程10590
    -----------------并行流parallelStream------------------
    KIRITO
    ASUNA
    ILLYASVIEL
    SAKURA
    多线程3313