使用jdk8 stream简化集合操作

　　使用stream的前提是对lambda表达式和函数式接口有一定的了解，同时对方法引用和普通传参的区别有一定的认识。

　　stream的三大特性：1、不存储数据2、不改变源数据3、延时执行。

　　stream优点：1、简化代码2、使用并行流可以利用多核特性，提升效率。

　　stream上的所有操作分为两类：中间操作和结束操作，中间操作只是一种标记，只有结束操作才会触发实际计算。

　　常用api如下

　　中间操作

　　filter：过滤流，过滤流中的元素，返回一个符合条件的Stream

　　map：转换流，将一种类型的流转换为另外一种流。（mapToInt、mapToLong、mapToDouble 返回int、long、double基本类型对应的Stream）

　　flatMap：简单的说，就是一个或多个流合并成一个新流。（flatMapToInt、flatMapToLong、flatMapToDouble 返回对应的IntStream、LongStream、DoubleStream流。）

　　distinct：返回去重的Stream。

　　sorted：返回一个排序的Stream。

　　peek：主要用来查看流中元素的数据状态。

　　limit：返回前n个元素数据组成的Stream。属于短路操作

　　skip：返回第n个元素后面数据组成的Stream。

　　结束操作

　　forEach: 循环操作Stream中数据。

　　toArray: 返回流中元素对应的数组对象。

　　reduce: 聚合操作，用来做统计。

　　collect: 聚合操作，封装目标数据。

　　min、max、count: 聚合操作，最小值，最大值，总数量。

　　anyMatch: 短路操作，有一个符合条件返回true。

　　allMatch: 所有数据都符合条件返回true。

　　noneMatch: 所有数据都不符合条件返回true。

　　findFirst: 短路操作，获取第一个元素。

　　findAny: 短路操作，获取任一元素。

　　forEachOrdered: 暗元素顺序执行循环操作。

　　流的创建demo

package com.example.test.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Random;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class StreamCreate {

    public static void main(String[] args) {

        unLimitStream2();
    }

    public static void testArrayStream(){
        int[] arr = new int[]{1,2,3,4,5};
        IntStream intStream = Arrays.stream(arr);
        Student[] students = new Student[]{Student.builder().name("zhangsan").age(12).score(12).build(),Student.builder().name("zhangsan").age(12).score(12).build()};
        Stream<Student> studentStream = Arrays.stream(students);
        Stream<Integer> integerStream =Stream.of(1,2,5,4,6);
        Stream<int[]> intArrayStrean = Stream.of(arr,arr);
        intArrayStrean.forEach(System.out::println);
    }

    public static void testCollectionStream(){
        List<String> strings = Arrays.asList("sdf","sdfdsf","ertert","sdfdsf");
        //创建普通流
        Stream<String> stringStream = strings.stream();
        //创建并行流
        Stream<String> parallelStream = strings.parallelStream();
    }

    public static void emptyStream(){
        Stream<Integer> stream = Stream.empty();
    }

    public static void unLimitStream(){
        Stream.generate((() -> "number" + new Random().nextInt())).limit(20).forEach(System.out::println);

    }

    public static void unLimitStream2(){
        Stream.iterate(0,x ->x+1).limit(10).forEach(System.out::println);
        Stream.iterate(0,x ->x).limit(10).forEach(System.out::println);
    }
}

　　流的操作demo

package com.example.test.stream;

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class OpepateStream {

    private static String[] arr = new String[]{"b","ab","abc","abcd","abcde"};

    public static void main(String[] args) {

        testFlatMap();
    }

    public static void testMap(){
        String[] str = {"sdf","sdfdsFFF","dfsGdsf"};
        Stream.of(str).map(t ->t.toUpperCase()).forEach(System.out::println);
    }
    public static void filter(){
        String[] str = {"sdf","sdfdsFFF","dfsGdsf"};
        Stream.of(str).filter(t -> ! t.equals("sdf")).forEach(System.out::println);
    }

    public static void testFlatMap(){
        List<String> list = Arrays.asList("beijing changcheng", "beijing gugong", "beijing tiantan",

                "gugong tiananmen");

        list.stream().map(item -> Arrays.stream(item.split(" "))).forEach(System.out::println);
        list.stream().flatMap(item -> Arrays.stream(item.split(" "))).forEach(System.out::println);
        List<Integer> a=new ArrayList<>();
        a.add(1);
        a.add(2);
        List<Integer> b=new ArrayList<>();
        b.add(3);
        b.add(4);
        List<Integer> figures=Stream.of(a,b).flatMap(u->u.stream()).collect(Collectors.toList());
        figures.forEach(f->System.out.println(f));

    }

    public static void sort(){
        String[] arr1 = {"abc","a","bc","abcd"};
        Arrays.stream(arr1).sorted(Comparator.comparing(String::length).reversed()).forEach(System.out::println);
    }

    public static void sort2(){
        String[] arr1 = {"abc","a","bc","abcd"};
        Arrays.stream(arr1).sorted(Comparator.comparing(String::length).reversed()).forEach(System.out::println);
    }

    public static void sort3(){
        String[] arr1 = {"abc","a","bc","abcd"};
        Arrays.stream(arr1).sorted(Comparator.comparing(OpepateStream::com1).thenComparing(String::length)).forEach(System.out::println);
    }

    public static char com1(String x){
        return x.charAt(0);
    }
    public static void testMaxAndMin(){
        Optional<String> option = Stream.of(arr).max(Comparator.comparing(String::length));
        option.ifPresent(System.out::println);
        Stream.of(arr).min(Comparator.comparing(String::length)).ifPresent(System.out::println);
    }

    public static void findFirst(){
        String str = Stream.of(arr).parallel().filter(t -> t.length()>3).findFirst().orElse("noting");
        System.out.println(str);
    }

    public static void testFindAny(){
        Optional<String> optional = Stream.of(arr).parallel().filter(x->x.length()>3).findAny();
        optional.ifPresent(System.out::println);
    }

    public static void reduceTest(){
        Optional<Integer> optional = Stream.of(1,2,3).reduce((x,y) -> x+y);
        optional.ifPresent(System.out::println);
    }

    public static Optional<Double> inverse(Double x) {
        return x == 0 ? Optional.empty() : Optional.of(1 / x);
    }

    public static Optional<Double> squareRoot(Double x) {
        return x < 0 ? Optional.empty() : Optional.of(Math.sqrt(x));
    }

    public static void testStream2() {
        double x = 4d;
        Optional<Double> result1 = inverse(x).flatMap(OpepateStream::squareRoot);
        result1.ifPresent(System.out::println);
        Optional<Double> result2 = Optional.of(4.0).flatMap(OpepateStream::inverse).flatMap(OpepateStream::squareRoot);
        result2.ifPresent(System.out::println);
    }

    public static void testToMap(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 10; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        studentList.add(Student.builder().name("student08").age(12).score(1000).build());
        studentList.add(Student.builder().name("student08").age(12).score(234).build());
        studentList.add(Student.builder().name("student08").age(12).score(123).build());
        studentList.add(Student.builder().name("student08").age(12).score(3000).build());
        Map<String,Integer> map = studentList.stream().collect(Collectors.toMap(Student::getName,Student::getScore,(a,b) -> b));
        map.forEach((x,y) -> System.out.println(x + "->" + y));

    }

    public static void toAssign(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
            HashSet<Student> s = studentList.stream().collect(Collectors.toCollection(HashSet::new));
            s.forEach(System.out::println);
    }

    public static void aggregation(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        IntSummaryStatistics summaryStatistics = studentList.stream().collect(Collectors.summarizingInt(Student::getScore));
        System.out.println("getAverage->"+summaryStatistics.getAverage());
        System.out.println("getMax->"+summaryStatistics.getMax());
        System.out.println("getMin->"+summaryStatistics.getMin());
        System.out.println("getCount->"+summaryStatistics.getCount());
        System.out.println("getSum->"+summaryStatistics.getSum());
    }

    public static void group(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        Map<Boolean,Integer> map  = studentList.stream().collect(Collectors.groupingBy((x -> x.getScore()>50) ,Collectors.summingInt(Student::getScore)));
        map.forEach((x,y) -> {
            System.out.println(x);
            System.out.println(y);
        });
        Map<String,Set<Integer>> map4 = studentList.stream().collect(Collectors.groupingBy(Student::getName,Collectors.mapping(Student::getScore,Collectors.toSet())));
        map4.forEach((x,y)-> System.out.println(x+"->"+y));
    }

    public static void group1(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        Map<Boolean,Integer> map  = studentList.stream().collect(Collectors.groupingBy((x -> x.getScore()>50) ,Collectors.summingInt(Student::getScore)));
        map.forEach((x,y) -> {
            System.out.println(x);
            System.out.println(y);
        });
        Map<String,List<Student>> map4 = studentList.stream().collect(Collectors.groupingBy(Student::getName));
        map4.forEach((x,y)-> System.out.println(x+"->"+y));
    }

}

　　并行流

package com.example.test.stream;

import java.util.stream.Stream;

public class ParalellTest {

    public void peek1(int x) {
        System.out.println(Thread.currentThread().getName() + ":->peek1->" + x);
    }

    public void peek2(int x) {
        System.out.println(Thread.currentThread().getName() + ":->peek2->" + x);
    }

    public void peek3(int x) {
        System.out.println(Thread.currentThread().getName() + ":->final result->" + x);
    }

    public static void main(String[] args) {
        ParalellTest paralellTest = new ParalellTest();
        Stream<Integer> stream = Stream.iterate(1,x -> x+1).limit(10);
        stream.peek(paralellTest::peek1).filter(x -> x>5)
                .peek(paralellTest::peek2).filter(x -> x<9)
                .peek(paralellTest::peek3).forEach(System.out::println);

        System.out.println("-------------------------------------");
        Stream<Integer> stream2 = Stream.iterate(1,x -> x+1).limit(10).parallel();
        stream2.peek(paralellTest::peek1).filter(x -> x>5)
                .peek(paralellTest::peek2).filter(x -> x<9)
                .peek(paralellTest::peek3).forEach(System.out::println);
    }

}

　　综合应用

package com.example.test.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamTest {

    public static void main(String[] args) {
        test2();
    }

    public static void test1(){
        Random random = new Random();
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        List<String> names =  studentList.stream().filter(t -> t.getScore()>50)
                .sorted(Comparator.comparingDouble(Student::getScore).reversed())
                .map(Student::getName).collect(Collectors.toList());
        System.out.println(names);
    }

    public static void test2(){
        List<Student> studentList = new ArrayList<Student>(){
            {
                for (int i = 0; i < 100; i++) {
                    add(Student.builder().name("student" + i).age(12).score(i).build());
                }
            }
        };
        Stream<Student> stream = studentList.stream().filter(StreamTest::filter)
                .sorted(Comparator.comparingDouble(Student::getScore).reversed());
        System.out.println("--------------");
        System.out.println(stream.map(Student::getName).collect(Collectors.toList()));

    }

    private static boolean filter(Student student){
        System.out.println("123123123");
        return student.getScore() >50;
    }
}