Java 8 stream 经典示例
import com.google.common.collect.Lists; import lombok.extern.slf4j.Slf4j; import org.junit.Test; import java.util.*; import java.util.stream.Collectors; import java.util.stream.IntStream; import java.util.stream.Stream; /** * Java 8 – Streams * url:http://www.studytrails.com/java/java8/java8_streams/ * * @author gongxr */ @Slf4j public class StreamDemo { /** * 过滤：filter * 匹配操作：noneMatch、anyMatch */ @Test public void testMatch() { List<String> genre = new ArrayList<>(Arrays.asList("rock", "pop", "jazz", "reggae")); // a:2 long a = genre.stream().filter(s -> s.startsWith("r")).count(); log.info("a:{}", a); // 一个都匹配不上；noneMatch:true boolean noneMatch = genre.stream().peek(System.out::println).noneMatch(String::isEmpty); log.info("noneMatch:{}", noneMatch); // anyMatch:true boolean anyMatch = genre.stream().anyMatch(s -> s.indexOf("r") == 0); log.info("anyMatch:{}", anyMatch); } /** * forEach:如果想对流经的每个元素应用一个函数，从而改变某些状态，那么请用 forEach();最终操作。 * peek:如果想打印流经的每个元素的状态（日志或 debug），这时应该用 peek()；中间操作。 */ @Test public void testForEachPeek() { List<String> genre = new ArrayList<>(Arrays.asList("rock", "pop", "jazz", "reggae")); genre.stream().filter(s -> s.startsWith("r")).forEach(System.out::println); log.info("------------"); genre.stream().filter(s -> s.startsWith("r")).peek(System.out::println).collect(Collectors.toList()); log.info("------------"); // 一个都匹配不上；b:true boolean b = genre.stream().peek(System.out::println).noneMatch(String::isEmpty); log.info("b:{}", b); log.info("------------"); long count = genre.stream().peek(System.out::println).count(); log.info("count:{}", count); } /** * 数据去重（distinct） */ @Test public void listDistinctTest() { List<Integer> listNum = new ArrayList<>(Arrays.asList(1, 1, 1, 2, 2, 2, 3, 3)); log.info("listNum:{}", listNum); List<Integer> listNumNew = listNum.stream().distinct().collect(Collectors.toList()); log.info("listNumNew:{}", listNumNew); } /** * map和flatMap的对比 * map：分别对单个值进行单独处理 * flatMap：扁平化操作，将所有数据拉平一次性处理 */ @Test public void testMap() { List<String> list = new ArrayList<>(Arrays.asList("rock", "pop", "jazz", "reggae")); // 分别对单个值进行单独处理；mapList:[[RO, K], [POP], [JAZZ], [REGGAE]] List<List<String>> mapList = list.stream().map(x -> Arrays.stream(x.toUpperCase().split("C")).collect(Collectors.toList())).collect(Collectors.toList()); log.info("mapList:{}", mapList); // 将所有数据拉平一次性处理；flatMapList:[RO, K, POP, JAZZ, REGGAE] List<String> flatMapList = list.stream().flatMap(x -> Arrays.stream(x.toUpperCase().split("C"))).collect(Collectors.toList()); log.info("flatMapList:{}", flatMapList); // mapToInt、mapToLong、mapToDouble； integers:[1, 2] List<Integer> integers = Stream.of("1", "2").mapToInt(Integer::parseInt).boxed().collect(Collectors.toList()); log.info("integers:{}", integers); } /** * Collectors.toMap */ @Test public void testToMap() { List<User> list = getUserList(); Map<String, Integer> map = list.stream().collect(Collectors.toMap(User::getName, User::getAge, (k, v) -> k)); log.info("Map:{}", map); Map<String, User> map2 = list.stream().collect(Collectors.toMap(User::getName, t -> t)); log.info("Map2:{}", map2); Map<String, String> map3 = getUserList().stream().collect(Collectors.toMap(User::getName, User::getGender)); log.info("map3:{}", map3); } /** * 分组 * Collectors.groupingBy */ @Test public void testGroupingBy() { List<User> list = getUserList(); Map<String, List<User>> collect = list.stream().collect(Collectors.groupingBy(User::getGender)); log.info("根据性别分组:{}", collect.toString()); Map<String, Set<String>> collect2 = list.stream().collect(Collectors.groupingBy(User::getGender, Collectors.mapping(User::getName, Collectors.toSet()))); log.info("根据性别对名字进行分组：{}", collect2.toString()); } /** * 数据转List */ @Test public void testArrayToList() { int[] arr = {1, 2, 3, 4, 5}; List<Integer> list = IntStream.of(arr).boxed().collect(Collectors.toList()); log.info(list.toString()); String[] arr2 = {"a", "b", "c"}; List<String> list2 = Stream.of(arr2).collect(Collectors.toList()); log.info(list2.toString()); } /** * sorted:排序 */ @Test public void testSorted() { int[] arr = {1, 2, 3, 4, 5}; int[] arrReverse = IntStream.of(arr).boxed().sorted(Comparator.reverseOrder()).mapToInt(Integer::intValue).toArray(); Arrays.stream(arrReverse).boxed().forEach(x -> log.info(x.toString())); } /** * 统计个数(List to Map) */ @Test public void testStat() { List<String> list = Arrays.asList("a", "a", "b", "c", "b", "b"); // toMap(key取值，value取值，key已存在的处理逻辑，实例对象类型) LinkedHashMap<String, Integer> map = list.stream().collect(Collectors.toMap(k -> k, k -> 1, Integer::sum, LinkedHashMap::new)); map.forEach((k, v) -> System.out.println(k + ":" + v)); System.out.println("-----"); map = list.stream().collect(Collectors.toMap(k -> k, k -> 1, Integer::sum, LinkedHashMap::new)); map.forEach((k, v) -> System.out.println(k + ":" + v)); } /** * 多个元素合并为一个元素(reduce) * reduce：按照特定规则处理每个元素，然后返回处理后的结果 */ @Test public void testReduce() { List<Integer> list = Arrays.asList(1, 2, 3, 4, 5); int n = list.stream().reduce(0, Integer::sum); log.info("n:【{}】", n); List<String> list2 = Arrays.asList("a", "a", "b", "c", "b", "b"); StringBuffer sb = list2.stream().map(StringBuffer::new).reduce(new StringBuffer(), StringBuffer::append); log.info("StringBuffer:【{}】", sb); } /** * 数学运算 * 最大值，最小值，平均值等数学操作 */ @Test public void testCompute() { int max = IntStream.rangeClosed(10, 20).max().getAsInt(); int min = IntStream.rangeClosed(10, 20).min().getAsInt(); double average = IntStream.rangeClosed(10, 20).average().getAsDouble(); long count = IntStream.rangeClosed(10, 20).count(); int sum = IntStream.rangeClosed(10, 20).sum(); log.info("max:{}", max); log.info("min:{}", min); log.info("aDouble:{}", average); log.info("count:{}", count); log.info("sum:{}", sum); } /** * IntStream:数字流 * boxed：用于将流中的元素转换为对应的对象类型 */ @Test public void testIntStream() { // range是左闭右开，还有rangeClose，左闭右闭；integersList:[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] List<Integer> integersList = IntStream.range(0, 10).boxed().collect(Collectors.toList()); log.info("integersList:{}", integersList); // 随机生成一个数字列表，integers2:[681, 309, 672] List<Integer> integers2 = IntStream.generate(() -> new Random().nextInt(1000)).limit(3).boxed().collect(Collectors.toList()); log.info("integers2:{}", integers2); // 迭代函数，生成一个有序无限的Int数据流 integers3:[1, 2, 4] List<Integer> integers3 = IntStream.iterate(1, x -> x * 2).limit(3).boxed().collect(Collectors.toList()); log.info("integers3:{}", integers3); // 和并流，使用concat方法将连个不同流合并为一个流输出 integers4:[1, 2, 3, 4] IntStream intStream = IntStream.of(1, 2); IntStream intStream2 = IntStream.builder().add(3).add(4).build(); List<Integer> integers4 = IntStream.concat(intStream, intStream2).boxed().collect(Collectors.toList()); log.info("integers4:{}", integers4); } public List<User> getUserList() { User user1 = new User("Mike", 11, "男", "Street #1"); User user2 = new User("Jill", 22, "女", "Street #2"); User user3 = new User("Tom", 33, "男", "Street #3"); User user4 = new User("Lily", 22, "女", "Street #4"); List<User> list = Lists.newArrayList(user1, user2, user3, user4); return list; } }
User.Class
1 package org.study2.java8.stream; 2 3 4 public class User { 5 6 private int id; 7 private String name; 8 9 public User(int id, String name) { 10 this.id = id; 11 this.name = name; 12 } 13 14 public int getId() { 15 return id; 16 } 17 18 public String toString() { 19 return "User [id=" + id + ", name=" + name + "]"; 20 } 21 22 }
详细讲解参考：
https://www.cnblogs.com/andywithu/p/7404101.html
