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
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();
