Stream流与Lambda表达式（一）杂谈

Harpsichord1207 发布于2019-08-16 15:13 / 3697人阅读

摘要：一流转换为数组集合陈杨将流转换为数组将流转换为数组将流转换为集合将流转换为集合解析

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 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 
         *               Collector> toList() {
         *          return new CollectorImpl<>((Supplier>) ArrayList::new, List::add,
         *                 (left, right) -> { left.addAll(right); return left; },
         *                 CH_ID);
        }*/

        //    List list = stream.collect(Collectors.toList());
        //    List linkedList = stream.collect(LinkedList::new,LinkedList::add,LinkedList::addAll);
        List 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 >
         *        Collector toCollection(Supplier collectionFactory) {
         *              return new CollectorImpl<>(collectionFactory, Collection::add,
         *             (r1, r2) -> { r1.addAll(r2); return r1; },
         *              CH_ID);
        }*/

        //    List list =stream.collect(Collectors.toCollection(ArrayList::new));
        //    List linkedList =stream.collect(Collectors.toCollection(ArrayList::new));
        //    Set treeSet =stream.collect(Collectors.toCollection(TreeSet::new));
        //    Set hashSet =stream.collect(Collectors.toCollection(HashSet::new));


    }

}

  .   ____          _            __ _ _
 / / ___"_ __ _ _(_)_ __  __ _    
( ( )\___ | "_ | "_| | "_ / _` |    
 /  ___)| |_)| | | | | || (_| |  ) ) ) )
  "  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.1.2.RELEASE)

2019-02-20 15:47:22.310  INFO 1348 --- [           main] com.java.design.java8.Stream.ListChange  : Starting ListChange on DESKTOP-87RMBG4 with PID 1348 (started by 46250 in E:IdeaProjectsdesign)
2019-02-20 15:47:22.311  INFO 1348 --- [           main] com.java.design.java8.Stream.ListChange  : No active profile set, falling back to default profiles: default
2019-02-20 15:47:22.947  INFO 1348 --- [           main] com.java.design.java8.Stream.ListChange  : Started ListChange in 0.914 seconds (JVM running for 1.774)
Kirito
Asuna
Illyasviel
Sakura
class java.util.ArrayList

二、集合排序

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.*;

/**
 * @author 陈杨
 */

@SpringBootTest
@RunWith(SpringRunner.class)
public class ComparatorDetail {

    private List names;

    @Before
    public void init() {
        names = Arrays.asList("Kirito", "Asuna", "Sinon", "Yuuki", "Alice");
    }


    public void println() {
        System.out.println(names);
        System.out.println("-----------------------------------------
");
    }

    @Test
    public void testComparatorDetail() {


        //  对名字进行升序排序
        Collections.sort(names);
        this.println();

        //  对名字进行降序排序
        names.sort(Collections.reverseOrder());
        this.println();


        //  按照姓名的字符串长度升序排序 相同长度-->比较前两个字符-->按照字符的ASCII码大小升序排序
        names.sort(Comparator.comparingInt(String::length)
                .thenComparing(str -> str.charAt(0))
                .thenComparing(str -> str.charAt(1))
        );
        this.println();


        //  按照姓名的字符串长度降序排序 相同长度-->比较前两个字符-->按照字符的ASCII码大小降序排序
        names.sort(Comparator.comparingInt(String::length)
                .thenComparing(str -> str.charAt(0))
                .thenComparing(str -> str.charAt(1))
                .reversed());
        this.println();


        //  按照姓名的字符串长度降序排序 相同长度-->按照字符的ASCII码大小排序(不区分大小写)
        names.sort(Comparator.comparingInt(String::length)
                .thenComparing(String.CASE_INSENSITIVE_ORDER));
        this.println();

    }
}

  .   ____          _            __ _ _
 / / ___"_ __ _ _(_)_ __  __ _    
( ( )\___ | "_ | "_| | "_ / _` |    
 /  ___)| |_)| | | | | || (_| |  ) ) ) )
  "  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: 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:IdeaProjectsdesign)
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 list = Arrays.asList("Kirito", "Asuna", "Illyasviel", "Sakura");

    private List> listMap =
            Arrays.asList(Collections.singletonList("Kirito"), Collections.singletonList("Asuna"),
                    Collections.singletonList("Illyasviel"), Collections.singletonList("Sakura"));

    private List> 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("-----------------------------------------
");
        List 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("-----------------------------------------
");
        List lowerCase = list.stream().map(String::toLowerCase).collect(Collectors.toList());
        lowerCase.forEach(System.out::println);


        System.out.println("-----------------------------------------
");
        System.out.println("FlatMap与Map的区别:
");

        //    map:  对多个list 分别map Fuction 映射  形成多个list
        System.out.println("-----------------------------------------
");
        System.out.println("进行map映射:");


        List> 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("-----------------------------------------
");
        System.out.println("FlatMap扁平化进行map映射:");


        List 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:IdeaProjectsdesign)
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 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("-----------------------------------------
");
        System.out.println("外部迭代");
        List 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("-----------------------------------------
");
        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:IdeaProjectsdesign)
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 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 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 stream() {
         *       return StreamSupport.stream(spliterator(), false);}
         */

        //  List 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 collect = list.stream().map(String::toUpperCase).collect(Collectors.toList());

        Function 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 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 parallelStream() {
         *       return StreamSupport.stream(spliterator(), true);}
         */

        List 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:IdeaProjectsdesign)
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

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