Stream流與Lambda表達式（一） 雜談

時間  2019-12-04
標籤 stream lambda 表達式 雜談 欄目 Apache 简体版
原文   原文鏈接

1、流 轉換爲數組、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 陳楊
 */

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

    private Stream<String> stream = Stream.of("Kirito", "Asuna", "Illyasviel", "Sakura");

    @Test
    public void testListChange() {


        //   將流轉換爲數組
        //   System.out.println("-------------將流轉換爲數組---------------");
        //   String[] array = stream.toArray(len -> new String[len]);
        //   String[] array = stream.toArray(String[]::new);
        //   Arrays.asList(array).stream().forEach(System.out::println);


        //    將流轉換爲集合
        //    System.out.println("-------------將流轉換爲集合---------------");
        //    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));


    }

}
.   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: 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:\IdeaProjects\design)
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

2、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 陳楊
 */

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

    private List<String> names;

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


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

    @Test
    public void testComparatorDetail() {


        //  對名字進行升序排序
        Collections.sort(names);
        this.println();

        //  對名字進行降序排序
        names.sort(Collections.reverseOrder());
        this.println();


        //  按照姓名的字符串長度升序排序 相同長度-->比較前兩個字符-->按照字符的ASCII碼大小升序排序
        names.sort(Comparator.comparingInt(String::length)
                .thenComparing(str -> str.charAt(0))
                .thenComparing(str -> str.charAt(1))
        );
        this.println();


        //  按照姓名的字符串長度降序排序 相同長度-->比較前兩個字符-->按照字符的ASCII碼大小降序排序
        names.sort(Comparator.comparingInt(String::length)
                .thenComparing(str -> str.charAt(0))
                .thenComparing(str -> str.charAt(1))
                .reversed());
        this.println();


        //  按照姓名的字符串長度降序排序 相同長度-->按照字符的ASCII碼大小排序(不區分大小寫)
        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:\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]
-----------------------------------------

3、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 陳楊
 */

@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 每一個字符串  按照排列前後順序拼接 造成一個長字符串
        //    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 對每一個字符串元素  將全部字母變成大寫字母
        System.out.println("-----------------------------------------\n");
        List<String> upperCase = list.stream().map(String::toUpperCase).collect(Collectors.toList());
        upperCase.forEach(System.out::println);


        //    集合中 對每一個字符串元素  將全部字母變成小寫字母
        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與Map的區別:\n");

        //    map:  對多個list 分別map Fuction 映射  造成多個list
        System.out.println("-----------------------------------------\n");
        System.out.println("進行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： 對多個list 進行flat扁平化 後再進行map Fuction 映射  造成一個list
        System.out.println("-----------------------------------------\n");
        System.out.println("FlatMap扁平化進行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與Map的區別:

-----------------------------------------

進行map映射:
[KIRITO]
[ASUNA]
[ILLYASVIEL]
[SAKURA]
-----------------------------------------

FlatMap扁平化進行map映射:
KIRITO
ASUNA
ILLYASVIEL
SAKURA

4、內部迭代與外部迭代

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 陳楊
 */

@SpringBootTest
@RunWith(SpringRunner.class)
//迭代本質
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());
        }


        //    內部迭代
        System.out.println("-----------------------------------------\n");
        System.out.println("內部迭代");
        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()));

        //    備註：
        //    內部迭代與SQL語句屬於描述性語言
        //    集合關注的是數據與數據存儲
        //    流關注的是數據的計算
        //    流中間操做返回的都是Stream對象 泛型取決於中間操做的類型
        //    流終止操做： 無返回值（forEach） 返回值是其餘類型
    }


}
.   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: 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
-----------------------------------------

內部迭代
Kirito
Asuna

5、串行流與並行流 簡單性能測試

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 陳楊
 */

@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("單線程" + 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("多線程" + millis);
    }


    @Test
    public void testErgodicString() {

        List<String> list = Arrays.asList("Kirito", "Asuna", "Illyasviel", "Sakura");


        //  需求： 將數組中每一個元素各個字母大寫 並放入集合

        System.out.println("--------------------串行流stream---------------------");
        // spliterator 分割迭代器
        //  串行流stream() 單線程處理
        /*
         *  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() 沒有參數做爲輸入
        //  但把調用toUpperCase的對象做爲T做爲輸入 返回的R是return的對象結果
        //  List<String> collect = list.stream().map(String::toUpperCase).collect(Collectors.toList());

        Function<String, String> function = String::toUpperCase;


        //等價於 (String str)  -> str.toUpperCase()
        //方法引用 類的類型::實例方法 對應的lambda表達式 第一個輸入參數 是調用此方法的對象
        List<String> collect = list.stream().map(function).collect(Collectors.toList());

        collect.forEach(System.out::println);

        this.testNormal();

        System.out.println("-----------------並行流parallelStream------------------");
        //  並行流parallelStream() 多線程處理
        /*
         *  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
單線程10590
-----------------並行流parallelStream------------------
KIRITO
ASUNA
ILLYASVIEL
SAKURA
多線程3313
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