使用jdk8 stream简化集合操作
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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;
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