Stream常用方式最全汇总

• 各种数据结构转流
• 高级收集器，返回各种数据类型
• 流的拼接
• 排序
• 分区与分组
• 归约
• 其他

  1 package stream;
  2 
  3 import java.util.ArrayList;
  4 import java.util.Arrays;
  5 import java.util.Collection;
  6 import java.util.Comparator;
  7 import java.util.HashMap;
  8 import java.util.HashSet;
  9 import java.util.IdentityHashMap;
 10 import java.util.Iterator;
 11 import java.util.LinkedHashMap;
 12 import java.util.List;
 13 import java.util.Map;
 14 import java.util.Map.Entry;
 15 import java.util.Random;
 16 import java.util.Set;
 17 import java.util.StringJoiner;
 18 import java.util.function.Function;
 19 import java.util.stream.Collectors;
 20 import java.util.stream.IntStream;
 21 import java.util.stream.Stream;
 22 import java.util.stream.StreamSupport;
 23 
 24 public class Test10 {
 25     
 26     static List numbers = Arrays.asList(1, 5, 3, 2, 4);
 27     
 28     static List strs = Arrays.asList("b2", "b1", "a2", "c", "b", "a");
 29     
 30     static List> strsList = Arrays.asList(strs);
 31     
 32     static Person p1 = new Person(GenderEnum.MALE, 20, "Jack");
 33     static Person p2 = new Person(GenderEnum.FEMALE, 20, "Lily");
 34     static Person p3 = new Person(GenderEnum.MALE, 10, "John");
 35     static Person p4 = new Person(GenderEnum.FEMALE, 10, "Lucy");
 36     static Person p5 = new Person(GenderEnum.MALE, 20, "Eason");
 37 
 38     static List persons = Arrays.asList(p1, p2, p3, p4, p5);
 39 
 40     static void method1() {
 41         // 常用数据结构转流的方式
 42 
 43         // Collection接口 提供了stream()方法
 44 
 45         // 1. List转流
 46         List list = new ArrayList<>();
 47         Stream streamA = list.stream();
 48 
 49         // ------------------------------------------------------------
 50         
 51         // 2. Set转流
 52         Set set = new HashSet<>();
 53         Stream streamB = set.stream();
 54 
 55         // ------------------------------------------------------------
 56         
 57         // 3. Map转流
 58         Map map = new HashMap<>();
 59         Stream> streamC = map.entrySet().stream();
 60         Stream streamD = map.values().stream();
 61 
 62         // ------------------------------------------------------------
 63         
 64         // 4. 数组转流
 65         String[] arr = { "A", "B", "C" };
 66         Arrays.stream(arr);
 67 
 68         // ------------------------------------------------------------
 69         
 70         // 5. Iterable转流
 71         Iterable iterable = new ArrayList<>();
 72         StreamSupport.stream(iterable.spliterator(), false);
 73     }
 74     
 75     static void method2() {
 76         // 流的拼接
 77         
 78         List listA = Arrays.asList("s1", "s2", "s3");
 79         List listB = Arrays.asList("t1", "t2", "t3");
 80 
 81         // Stream.concat()实现多个流拼接
 82         Stream streamA = Stream.concat(listA.stream(), listB.stream());
 83 
 84         // Stream.of()实现拼接
 85         // 多个元素
 86         Stream streamB = Stream.of("A", "B", "C");
 87         // 多个List
 88         Stream streamC = Stream.of(listA, listB).flatMap(Collection::stream);
 89         // 多个Stream
 90         Stream streamD = Stream.of(listA.stream(), listB.stream()).flatMap(Function.identity());
 91 
 92         // flatMap实现子元素拼接
 93         List items = new ArrayList<>();
 94         Stream streamE = items.stream().flatMap(i -> i.getSubItems().stream());
 95     }
 96 
 97     static void method3() {
 98         // 高级收集器
 99     
100         // 1. 字符串拼接
101     
102         List strs = Arrays.asList("s1", "s2", "s3");
103     
104         // 无分隔符
105         String str1 = strs.stream().collect(Collectors.joining());
106         // 单个分隔符
107         String str2 = strs.stream().collect(Collectors.joining(","));
108         // 前后缀及分隔符
109         String str3 = strs.stream().collect(Collectors.joining(",", "[", "]"));
110     
111         // ----------------------------------------------------------------------------------------------------
112     
113         // 返回集合
114     
115         // 2. 返回数组
116     
117         // Object数据（无参默认）
118         Object[] arr1 = strs.stream().toArray();
119         // 特定类型数据
120         String[] arr2 = strs.stream().toArray(String[]::new);
121         // 数组指定长度
122         String ids = "1, 2, 3, 4";
123         String[] arr3 = Arrays.asList(ids.split(",")).stream().map(String::trim).toArray(size -> new String[size]);
124     
125         // ----------------------------------------------------------------------------------------------------
126         
127         // 3. 返回List
128         // 默认List
129         List list1 = strs.stream().filter(str -> str.length() > 5).map(String::toUpperCase).collect(Collectors.toList());
130         // 返回特定类型如ArrayList
131         List list2 = strs.stream().filter(str -> str.length() > 5).map(String::toUpperCase).collect(Collectors.toCollection(ArrayList::new));
132     
133         // ----------------------------------------------------------------------------------------------------
134 
135         // 4. 返回Map
136         
137         // List转Map
138         List items = Arrays.asList(new Item("0001", "name"), new Item("0002", "name2"));
139         // 成员作为value
140         Map map1 = items.stream().collect(Collectors.toMap(Item::getItemId, Item::getName));
141         // 元素作为value
142         Map map2 = items.stream().collect(Collectors.toMap(Item::getItemId, Function.identity()));
143         // value重复取第一个
144         Map map3 = items.stream().collect(Collectors.toMap(Item::getItemId, Function.identity(), (n1, n2) -> n1));
145         // value重复自定义异常，指定返回类型
146         Map map4 = items.stream()
147                 .collect(Collectors.toMap(Item::getItemId, Function.identity(), (a, b) -> {
148                     throw new IllegalStateException("cannot add duplicate key: " + a);
149                 }, IdentityHashMap::new));
150         
151         // Map转Map
152         Map map = new HashMap<>();
153         Map map_ = map.entrySet().stream().filter(e -> e.getValue() > 10).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
154     }
155 
156     static void method4() {
157         // 排序 sorted
158         
159         // 数字排序
160     
161         List numbers = Arrays.asList(1, 5, 3, 2, 4);
162     
163         // 1. 升序(默认）
164         numbers.stream().sorted();
165         // 1, 2, 3, 4, 5
166     
167         // 2. 降序
168         numbers.stream().sorted(Comparator.reverseOrder());
169         // 5, 4, 3, 2, 1
170     
171         // --------------------------------------------------------------------------------
172     
173         // 字符排序
174     
175         List strs = Arrays.asList("b2", "b1", "a2", "c", "b", "a");
176     
177         // 1. 默认
178         List strSorted1 = strs.stream().sorted().collect(Collectors.toList());
179         // a, a2, b, b1, b2, c
180     
181         // 2. 自定义：先比较长度
182         strs.stream().sorted((s1, s2) -> s1.length() - s2.length());
183         List strSorted2 = strs.stream()
184                 .sorted(Comparator.comparing(String::length).thenComparing(Comparator.naturalOrder()))
185                 .collect(Collectors.toList());
186         // a, b, c, a2, b1, b2
187     
188         // --------------------------------------------------------------------------------
189     
190         // 自定义类型排序
191     
192 
193         // 1. 可实现Comparable接口
194         List pVal = persons.stream().sorted().collect(Collectors.toList());
195         /*
196          * Person [gender=MALE, age=10, name=John] 
197          * Person [gender=MALE, age=20, name=Eason] 
198          * Person [gender=MALE, age=20, name=Jack] 
199          * Person [gender=FEMALE, age=10, name=Lucy]
200          * Person [gender=FEMALE, age=20, name=Lily]
201          * 
202          */
203         
204         // List可直接排序，传入compareTo方法引用
205         List personsNew = new ArrayList<>(persons);
206         personsNew.sort(Person::compareTo);
207     
208         // 2. 使用比较器链，排序方式更灵活
209         persons.sort(Comparator.comparing(Person::getAge).thenComparing(Person::getGender).thenComparing(Person::getName));
210          /*
211          * Person [gender=MALE, age=10, name=John]
212          * Person [gender=FEMALE, age=10, name=Lucy]
213          * Person [gender=MALE, age=20, name=Eason]
214          * Person [gender=MALE, age=20, name=Jack]
215          * Person [gender=FEMALE, age=20, name=Lily]
216          * 
217          */
218     }
219 
220     static void method5() {
221         // 分组与分区
222         
223         // 分组
224         Map> grouped1 = persons.stream().collect(Collectors.groupingBy(Person::getGender));
225         // 子分组，多个分组
226         Map>> grouped2 = persons.stream()
227                 .collect(Collectors.groupingBy(Person::getGender, LinkedHashMap::new,
228                         Collectors.groupingBy(Person::getAge, LinkedHashMap::new, Collectors.toList())));
229     
230         // --------------------------------------------------------------------------------
231         
232         // 分区
233         Map> partitioned1 = persons.stream().collect(Collectors.partitioningBy(person -> person.getAge() <= 6));
234         // 分区并计数
235         Map partitioned2 = persons.stream().collect(Collectors.partitioningBy(person -> person.getAge() <= 6, Collectors.counting()));
236     }
237     
238     static  void method6() {
239         // 归约 reduce
240         
241         // 数字
242         Integer reduced1 = numbers.stream().reduce((x, y) -> x + y).get();
243         // 数字，指定初始值
244         Integer reduced2 = numbers.stream().reduce(0, (x, y) -> x + y);
245         
246         // 字符串拼接
247         String reduced3 = strs.stream().reduce((s1, s2) -> s1 + s2).get();
248         // 字符串拼接，指定前缀
249         String reduced4 = strs.stream().reduce("prefix", (s1, s2) -> s1 + s2);
250 
251         // --------------------------------------------------------------------------------
252 
253         // 数字，求最小值
254         Integer min = numbers.stream().reduce(Math::min).get();
255         // 数字，求最大值
256         Integer max = numbers.stream().reduce(Math::max).get();
257         // 数字，求和
258         Integer sum = numbers.stream().reduce(Integer::sum).get();
259 
260         // --------------------------------------------------------------------------------
261         
262         // 实现流的拼接
263         Stream reduced5 = strsList.stream().map(list -> list.stream()).reduce(Stream::concat).get();
264         
265         Iterable[] inputs = null;
266         Iterator iterator = Stream.of(inputs).map(it -> StreamSupport.stream(it.spliterator(), false)).reduce(Stream::concat).orElseGet(Stream::empty).iterator();
267 
268     }
269     
270     static void method7() {
271         // findFirst 返回第一个符合条件
272         strs.stream().filter(str -> str.length() > 10).findFirst().get();
273         // findAny 返回任意一个符合条件，串行流和findFirst相同，并行流返回最先有值的线程
274         strs.stream().filter(str -> str.length() > 10).findAny().get();
275         // anyMatch 任意符合
276         boolean flag1 = strs.stream().anyMatch(str -> str.length() > 10);
277         // allMatch 全部符合
278         boolean flag2 = strs.stream().allMatch(str -> str.length() > 10);
279         // noneMatch 全不符合
280         boolean flag3 = strs.stream().noneMatch(str -> str.length() > 10);
281         
282         // --------------------------------------------------------------------------------
283         
284         // distinct 去重
285         strs.stream().distinct();
286         // count 计数
287         strs.stream().filter(str -> str.length() > 10).count();
288         // skip 跳过
289         strs.stream().filter(str -> str.length() > 10).skip(3);
290         // limit 限定数量
291         strs.stream().filter(str -> str.length() > 10).limit(3);
292         
293         // --------------------------------------------------------------------------------
294         
295         // 遍历
296         strs.stream().forEach(System.out::println);
297         
298     }
299     
300     static void method8() {
301         // IntStream
302         
303         // 使用IntStream生成50个连续字符
304         
305         IntStream.range(0, 50).mapToObj(s -> "*").collect(Collectors.joining());
306         IntStream.range(0, 50).mapToObj(s -> "*").collect(Collectors.joining(""));
307         IntStream.rangeClosed(1, 50).mapToObj(s -> "*").reduce((s1, s2) -> s1 + s2).get();
308         IntStream.rangeClosed(1, 50).mapToObj(s -> "*").reduce(new StringJoiner("", "[", "]"), StringJoiner::add, StringJoiner::merge);
309         
310         // 生成随机数
311         Random random = new Random();
312         IntStream.generate(random::nextInt);
313         
314         // 生成特定元素
315         IntStream.iterate(1, i -> i * 2);
316     }
317 
318     static class Person implements Comparable {
319         private GenderEnum gender;
320         private int age;
321         private String name;
322 
323         public Person(GenderEnum gender, int age, String name) {
324             this.gender = gender;
325             this.age = age;
326             this.name = name;
327         }
328 
329         public GenderEnum getGender() {
330             return gender;
331         }
332 
333         public void setGender(GenderEnum gender) {
334             this.gender = gender;
335         }
336 
337         public int getAge() {
338             return age;
339         }
340 
341         public void setAge(int age) {
342             this.age = age;
343         }
344 
345         public String getName() {
346             return name;
347         }
348 
349         public void setName(String name) {
350             this.name = name;
351         }
352 
353         @Override
354         public int compareTo(Person o) {
355             if (this.gender != o.gender) {
356                 return this.gender.compareTo(o.gender);
357             }
358             if (this.age != o.age) {
359                 return this.age - o.age;
360             }
361 
362             return this.name.compareTo(o.name);
363         }
364 
365         @Override
366         public String toString() {
367             return "Person [gender=" + gender + ", age=" + age + ", name=" + name + "]";
368         }
369     }
370 
371     enum GenderEnum {
372         MALE, FEMALE
373     }
374     
375     static class Item {
376         private String itemId;
377         private String name;
378         private String value;
379         private List subItems;
380 
381         public Item(String itemId, String name) {
382             this.itemId = itemId;
383             this.name = name;
384         }
385 
386         public String getItemId() {
387             return itemId;
388         }
389 
390         public void setItemId(String itemId) {
391             this.itemId = itemId;
392         }
393 
394         public String getName() {
395             return name;
396         }
397 
398         public void setName(String name) {
399             this.name = name;
400         }
401 
402         public String getValue() {
403             return value;
404         }
405 
406         public void setValue(String value) {
407             this.value = value;
408         }
409 
410         public List getSubItems() {
411             return subItems;
412         }
413 
414         public void setSubItems(List subItems) {
415             this.subItems = subItems;
416         }
417 
418         @Override
419         public String toString() {
420             return "Item [itemId=" + itemId + ", name=" + name + ", value=" + value + ", subItems=" + subItems + "]";
421         }
422     }
423 
424     public static void main(String[] args) {
425         method5();
426     }
427 }

 

转载于:https://www.cnblogs.com/blouson/p/Stream.html