Java8的stream API与 C#的 LINQ 拓展方法对比

为方便初学 Java8/C# 集合操作的人,特意写下这篇文章.

前期准备

C#版

java版

单集合

分类筛选

• 计数(Count)

 Date time1 = convertLocalDateToTimeZone(LocalDate.of(1990, 1, 1)); //0 Long count1 = list1.stream().filter(o -> o.getBirthday().equals(time1)).count();

 int count1 = list1.Where(o => o.Birthday.Equals(new DateTime(1990, 1, 1)) && o.Sex == Sex.Male).Count(); long count2 = list1.Where(o => o.Birthday.Equals(new DateTime(1990, 1, 1)) && o.Sex == Sex.Male).LongCount(); /* 0 0 */

• 分组(GroupBy)

 Map<Sex, List<Person>> group1 = list1.stream().collect(Collectors.groupingBy(Person::getSex)); Iterator it = group1.entrySet().iterator(); while (it.hasNext()) { Map.Entry<Sex, List<Person>> groupByItem = (Map.Entry) it.next(); Sex sex = groupByItem.getKey(); out.println(sex); groupByItem.getValue().forEach(person -> { out.println(new Gson().toJson(person)); }); } /* 输出结果: Male {"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"Male"} Female {"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"Female"} X {"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"} */

var group1 = list1.GroupBy(o => o.Sex); //当我们使用 GroupBy（） 扩展方法时，使用了延迟执行。 这意味着，当你遍历集合的时候,下一个要出现的项目可能会或者可能不会被加载。 这是一个很大的性能改进，但它会引起有趣的副作用。 list1.RemoveAll(o => o.Sex == Sex.X);//定义 groupby 集合后对原集合进行修改,会发现group1里面已经没了 Sex=X的分组 foreach (var groupByItem in group1) { Sex sex = groupByItem.Key; System.Console.WriteLine(sex); foreach (Person person in groupByItem) { System.Console.WriteLine(JsonConvert.SerializeObject(person)); } } /* 输出结果: {"Height":165,"Weight":50,"Birthday":"1981-01-01T00:00:00","Hobbies":["吃飯","逛街"],"Identifier":"1","Address":"北京","Sex":2} Male {"Height":170,"Weight":50,"Birthday":"1982-02-01T00:00:00","Hobbies":["吃飯","看電影"],"Identifier":"2","Address":"北京","Sex":1} Female {"Height":165,"Weight":50,"Birthday":"1981-01-01T00:00:00","Hobbies":["吃飯","逛街"],"Identifier":"1","Address":"北京","Sex":2} Male {"Height":170,"Weight":50,"Birthday":"1982-02-01T00:00:00","Hobbies":["吃飯","看電影"],"Identifier":"2","Address":"北京","Sex":1} */ //该 ToLookup（） 方法创建一个类似 字典（Dictionary ） 的列表List, 但是它是一个新的 .NET Collection 叫做 lookup。 Lookup，不像Dictionary, 是不可改变的。 这意味着一旦你创建一个lookup, 你不能添加或删除元素。 var group2 = list1.ToLookup(o => o.Sex); foreach (var groupByItem in group2) { Sex sex = groupByItem.Key; foreach (Person person in groupByItem) { System.Console.WriteLine(sex); System.Console.WriteLine(JsonConvert.SerializeObject(person)); } } /* 输出结果: {"Height":165,"Weight":50,"Birthday":"1981-01-01T00:00:00","Hobbies":["吃飯","逛街"],"Identifier":"1","Address":"北京","Sex":3} {"Height":170,"Weight":50,"Birthday":"1982-02-01T00:00:00","Hobbies":["吃飯","看電影"],"Identifier":"2","Address":"北京","Sex":3} */

与此对比,stream没有RemoveAll的操作

• 匹配的第一项(findFirst/First,FirstOrDefault)

 Person after90 = list1.stream() .filter(o -> o.getBirthday().after(convertLocalDateToTimeZone(LocalDate.of(1990, 1, 1)))) .findFirst() .orElse(null); // null

 var after90 = list1.Where(o => o.Birthday >= new DateTime(1990, 1, 1)).First();//如果结果为空,将会导致异常,所以一般极少使用该方法 //An unhandled exception of type 'System.InvalidOperationException' occurred in System.Linq.dll: 'Sequence contains no elements' after90 = list1.Where(o => o.Birthday >= new DateTime(1990, 1, 1)).FirstOrDefault(); var after00 = list1.Where(o => o.Birthday >= new DateTime(2000, 1, 1)).FirstOrDefault();

• 遍历(ForEach)

 list1.stream().forEach(o -> { //在ForEach當中可對集合進行操作 o.setSex(Sex.X); }); list1.forEach(o -> { out.println(new Gson().toJson(o)); }); /* {"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"} {"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"} {"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"} */

 list1.ForEach(item => { //在ForEach當中可對集合進行操作 item.Sex = Sex.X; }); list1.ForEach(item => { System.Console.WriteLine(JsonConvert.SerializeObject(item)); });

• 极值Max/Min

 //IntStream的max方法返回的是OptionalInt,要先判断有没有值再读取值.isPresent=false 时直接getAsInt会报错.mapToLong,mapToDouble同理 OptionalInt maxHeightOption = list1.stream().mapToInt(Person::getHeight).max(); //字符串拼接、数值的 sum、min、max、average 都是特殊的 reduce。 //当集合为长度0的集合时会返回起始值Integer.MIN_VALUE,起始值也不能乱传,个中缘由我暂不清楚 int maxHeight = list1.stream().mapToInt(Person::getHeight).reduce(Integer.MIN_VALUE, Integer::max); out.println(maxHeight); if (maxHeightOption.isPresent()) { maxHeight = maxHeightOption.getAsInt(); out.println(maxHeight); } //mapToInt参数的2种写法都一样,我比较喜欢以下写法,但是 idea 会报 warning OptionalInt minWeightOption = list1.stream().mapToInt(o -> o.getHeight()).min(); int minWeight = list1.stream().mapToInt(o -> o.getHeight()).reduce(Integer.MAX_VALUE, Integer::min);

 int maxHeight = list1.Select(o => o.Height).Max(); //同 list1.Max(o => o.Height); int minWeight = list1.Min(o => o.Weight); 

• 跳过(skip/Skip),截取(limit/Take)

 //skip和 limit参数都是long, 这个要注意 list1.stream().skip(1L).limit(2L);

排序

• 去重复(Distinct)

list1.stream().map(Person::getIdentifier).distinct();

list1.Select(o=>o.Identifier).Distinct();

 list1.Skip(1).Take(2);

• 升序(sort/OrderBy)

 out.println("------------------------------------|升序|------------------------------------"); list1 = list1.stream().sorted(Comparator.comparing(Person::getBirthday)).collect(Collectors.toList()); out.println(new Gson().toJson(list1)); list1 = list1.stream().sorted((left, right) -> left.getBirthday().compareTo(right.getBirthday())).collect(Collectors.toList()); out.println(new Gson().toJson(list1));

 //升序 list1 = list1.OrderBy(o => o.Birthday).ToList();

• 降序(sort/OrderByDescending)

 out.println("------------------------------------|降序|------------------------------------"); list1 = list1.stream().sorted(Comparator.comparing(Person::getBirthday).reversed()).collect(Collectors.toList()); out.println(new Gson().toJson(list1)); list1 = list1.stream().sorted((left, right) -> right.getBirthday().compareTo(left.getBirthday())).collect(Collectors.toList()); out.println(new Gson().toJson(list1)); 

 //降序 list1 = list1.OrderByDescending(o => o.Birthday).ToList();

多集合

• 交集 list1 ∩ list2

 out.println("------------------------------------|交集 list1 ∩ list2|------------------------------------"); list1.stream().filter(o -> list2.contains(o)).collect(Collectors.toList());

//连接,下面表示把 list1和 list2当中相同身份证号的取出来,生成一个新的集合 //实际上, join 有另外的用法,类似 sqlserver 里面的多表连接,将不同数据源结合到一起,生成新的数据结构 var intersect = list1.Join(list2, o => o.Identifier, o => o.Identifier, (a, b) => a).ToList(); //交集 list1 ∩ list2 intersect = list1.Intersect(list2).ToList();

• 并集list1 ∪ list2

 out.println("------------------------------------|并集list1 ∪ list2 |------------------------------------"); list1.addAll(list2);

 //并集list1 ∪ list2 var union = list1.Union(list2).ToList();

• 差集list1 - list2

 out.println("------------------------------------|差集list1 - list2|------------------------------------"); list1.stream().filter(item1 -> !list2.contains(item1)).collect(Collectors.toList());

 //差集list1 - list2 var except = list1.Except(list2).ToList();

数据结构转换

 out.println("------------------------------------|数据结构转换|------------------------------------"); List<Person> list3 = list1.stream().filter(o -> true).collect(Collectors.toList()); ArrayList<Person> list4 = list1.stream().filter(o -> true).collect(Collectors.toCollection(ArrayList::new)); Set<Person> list5 = list1.stream().filter(o -> true).collect(Collectors.toSet()); Object[] list6 = list1.stream().toArray(); Person[] list7 = list1.stream().toArray(Person[]::new);

 //数据结构转换 list1.ToArray(); //注意如果 key 重复,ToDictionary会导致出错 list1.ToDictionary(o => o.Identifier, o => o); list1.ToHashSet();