1.介绍

熟悉C#的lambda 表达式，想要使用 LINQ 在 Java，是个问题，虽然 Java8用函数式接口已经实现了部分lambda 表达式，但是还是不够灵活。

2.Linqer简介

Linqer 是在 Java 中的可以使用 LINQ 语言集成查询 (LINQ)。可以弥补目前Java8在 LINQ方面的一些不足，实现了在 c# 中的(selectMany，Union, Intersect, Except等)。

3.使用方法

如果你使用 Maven，请在以下 pom.xml 应用以下包。

<dependency>

    <groupId>com.github.jlinqer</groupId>

    <artifactId>jlinqer</artifactId>

    <version>1.0.0</version>

</dependency>

如果没有使用Maven 时，直接使用 jLinqer jar，下载地址：

https://oss.sonatype.org/content/groups/public/com/github/jlinqer/jlinqer/1.0.0/jlinqer-1.0.0.jar

4.LINQ jLinqer 对比

功能	LINQ(C#)	jLinqer(Java)	Stream(Java)
【基本】
筛选	Where	where	filter
投影	Select	select	map
排序（升序）	OrderBy	orderBy	sorted
排序（降序）	OrderByDescending	orderByDescending	n/a
按升序对序列中的元素执行后续排序	ThenBy	thenBy	n/a
按降序对序列中的元素执行后续排序。	ThenByDescending	thenByDescending	n/a
将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。	SelectMany	selectMany	flatMap
【提取】
跳过序列中指定数量的元素，然后返回剩余的元素。	Skip	skip	skip
只要满足指定的条件，就跳过序列中的元素，然后返回剩余元素。	SkipWhile	skipWhile	n/a
从序列的开头返回指定数量的连续元素。	Take	take	limit
只要满足指定的条件，就会返回序列的元素，然后跳过剩余的元素。	TakeWhile	takeWhile	n/a
【合成】
连接两个序列	Concat	concat	concat
生成两个序列的交集	Intersect	intersect	n/a
生成两个序列的并集	Union	union	n/a
生成两个序列的差集	Except	except	n/a
基于匹配键对两个序列的元素进行关联	Join	join	n/a
基于键相等对两个序列的元素进行关联并对结果进行分组	GroupJoin	groupJoin	n/a
反转序列中元素的顺序	Reverse	reverse	n/a
将指定函数应用于两个序列的对应元素，以生成结果序列	Zip	zip	n/a
【分组和总计】
返回序列中的非重复元素	Distinct	distinct	distinct
累加器函数	Aggregate	aggregate	reduce
对序列中的元素进行分组	GroupBy	groupBy	Collectors.groupingBy
计算数值序列的平均值	Average	averageXXX	Collectors.summarizingXXX
返回序列中的元素数量	Count	count	n/a
返回一个 Int64，表示序列中的元素的数量	LongCount	longCount	count
返回值序列中的最大值	Max	max	max
返回值序列中的最小值	Min	min	min
计算数值序列之和	Sum	sumXXX	Collectors.summarizingXXX
返回序列中的第一个元素	First	first	findFirst
返回序列中的第一个元素；如果未找到元素，则返回默认值	FirstOrDefault	firstOrDefault	n/a
返回序列的最后一个元素	Last	last	n/a
返回序列中的最后一个元素；如果未找到元素，则返回默认值	LastOrDefault	lastOrDefault	n/a
返回值序列的单个特定元素	Single	single	n/a
返回序列中单个特定元素，如果未找到这样的元素，则返回默认值	SingleOrDefault	singleOrDefault	n/a
返回指定序列的元素；如果序列为空，则返回集合中的类型参数的默认值。	DefaultIfEmpty	defaultIfEmpty	n/a
返回序列中指定索引处的元素	ElementAt	elementAt	n/a
返回序列中指定索引处的元素；如果索引超出范围，则返回默认值	ElementAtOrDefault	elementAtOrDefault	n/a
确定序列中的所有元素是否满足条件	All	all	allMatch
确定序列中的任何元素是否存在或满足条件	Any	any	anyMatch
【区间】
返回一个具有指定的类型参数的空	Empty	empty	n/a
生成指定范围内的整数的序列	Range	range	n/a
生成包含一个重复值的序列	Repeat	repeat	n/a
【其他】
根据相等比较器确定两个序列是否相等	SequenceEqual	sequenceEqual	n/a
强制转换为指定的类型	Cast	cast	n/a
根据指定类型筛选 IEnumerable 的元素	OfType	ofType	n/a

5.使用方法

Where

 List<Integer> list = new List<>(1, 2, 3);

 List<Integer> actual = list.where(x -> x == 1 || x == 3).toList();

 assertEquals(true , actual.contains(1));

 assertEquals(false, actual.contains(2));

 assertEquals(true , actual.contains(3));

Select

 List<Person> list = new List<>(

 new Person("React" , 1),

 new Person("Angular" , 3),

 new Person("Backbone", 5)

 );

 List<String> actual = list.select(x -> x.name).toList();

 assertEquals("React" , actual.get(0));

 assertEquals("Angular" , actual.get(1));

 assertEquals("Backbone", actual.get(2));

OrderBy

 List<String> list = new List<>("Backbone", "Angular", "React");

 List<String> actual = list.orderBy(x -> x).toList();

 assertEquals("Angular" , actual.get(0));

 assertEquals("Backbone", actual.get(1));

 assertEquals("React" , actual.get(2));

OrderByDescending

 List<String> list = new List<>("Backbone", "Angular", "React");

 List<String> actual = list.orderByDescending(x -> x).toList();

 assertEquals("React" , actual.get(0));

 assertEquals("Backbone", actual.get(1));

 assertEquals("Angular" , actual.get(2));

ThenBy

 List<Person> list = new List<>(

 new Person("Angular2", 2),

 new Person("Angular1", 2),

 new Person("React" , 1)

 );

 List<String> actual = list.orderBy(x -> x.age).thenBy(x -> x.name).toList();

 assertEquals("React" , actual.get(0).name);

 assertEquals("Angular1", actual.get(1).name);

 assertEquals("Angular2" , actual.get(2).name);

ThenByDescending

 List<Person> list = new List<>(

 new Person("Angular2", 2),

 new Person("Angular1", 2),

 new Person("React" , 1)

 );

 List<String> actual = list.orderBy(x -> x.age).thenByDescending(x -> x.name).toList();

 assertEquals("React" , actual.get(0).name);

 assertEquals("Angular2", actual.get(1).name);

 assertEquals("Angular1" , actual.get(2).name);

SelectMany

 List<Person> list = new List<>(

 new Person("Angular", 3, new List("1.0.1", "1.0.2")),

 new Person("React" , 1, new List("2.0.1", "2.0.2"))

 );

 List<String> actual = list.selectMany(x -> x.versionHistory).toList();

 assertEquals("1.0.1", actual.get(0));

 assertEquals("1.0.2", actual.get(1));

 assertEquals("2.0.1", actual.get(2));

 assertEquals("2.0.2", actual.get(3));

Skip

 List<Integer> list = new List<>(1, 2, 3);

 List<Integer> actual = list.skip(2).toList();

 assertEquals(3, actual.get(0).intValue());

SkipWhile

 List<Integer> list = new List<>(1, 2, 3, 4, 5);

 List<Integer> actual = list.skipWhile(x -> x <= 3).toList();

 assertEquals(4, actual.get(0).intValue());

 assertEquals(5, actual.get(1).intValue());

Take

 List<String> list = new List<>("Backbone", "Angular", "React");

 List<String> actual = list.take(2).toList();

 assertEquals(2, actual.size());

 assertEquals("Backbone", actual.get(0));

 assertEquals("Angular" , actual.get(1));

TakeWhile

 List<String> list = new List<>("Backbone", "Angular", "React");

 List<String> actual = list.takeWhile(x -> x.equals("Backbone") || x.equals("Angular")).toList();

 assertEquals(2, actual.size());

 assertEquals("Backbone", actual.get(0));

 assertEquals("Angular" , actual.get(1));

Concat

 List<Integer> first = new List<>(1, 2);

 List<Integer> second = new List<>(2, 3);

 List<Integer> actual = first.concat(second).toList();

 assertEquals(1, actual.get(0).intValue());

 assertEquals(2, actual.get(1).intValue());

 assertEquals(2, actual.get(2).intValue());

 assertEquals(3, actual.get(3).intValue());

Intersect

 List<Integer> first = new List<>(1, 2, 3);

 List<Integer> second = new List<>(1, 3);

 List<Integer> actual = first.intersect(second).toList();

 assertEquals(1, actual.get(0).intValue());

 assertEquals(3, actual.get(1).intValue());

Union

 List<Integer> first = new List<>(1, 2, 3);

 List<Integer> second = new List<>(0, 1, 3, 4);

 List<Integer> actual = first.union(second).toList();

 assertEquals(5, actual.size());

 assertEquals(1, actual.get(0).intValue());

 assertEquals(2, actual.get(1).intValue());

 assertEquals(3, actual.get(2).intValue());

 assertEquals(0, actual.get(3).intValue());

 assertEquals(4, actual.get(4).intValue());

Except

 List<Integer> first = new List<>(1, 2, 3);

 List<Integer> second = new List<>(1, 3);

 List<Integer> actual = first.except(second).toList();

 assertEquals(2, actual.get(0).intValue());

Join

 List<Javascript> outer = new List<>(

 new Javascript("Angular", 1),

 new Javascript("React" , 4),

 new Javascript("ES2016" , 5)

 );

 List<Javascript> inner = new List<>(

 new Javascript("Angular", 2),

 new Javascript("Angular", 3),

 new Javascript("ES2016" , 6),

 new Javascript("ES7" , 7)

 );

 Function<Javascript, String> outerKey = (x) -> x.name;

 Function<Javascript, String> innerKey = (y) -> y.name;

 BiFunction<Javascript, Javascript, Javascript> selector = (x, y) -> new Javascript(x.name, y.age);

 List<Javascript> actual = outer.join(inner, outerKey, innerKey, selector).toList();

 assertEquals(3, actual.size());

 assertEquals("Angular", actual.get(0).name);

 assertEquals("Angular", actual.get(1).name);

 assertEquals("ES2016" , actual.get(2).name);

 assertEquals(2, actual.get(0).age);

 assertEquals(3, actual.get(1).age);

 assertEquals(6, actual.get(2).age);

GroupJoin

 List<Javascript> outer = new List<>(

 new Javascript("Angular", 1),

 new Javascript("React" , 4),

 new Javascript("ES2016" , 5)

 );

 List<Javascript> inner = new List<>(

 new Javascript("Angular", 2),

 new Javascript("Angular", 3),

 new Javascript("ES2016" , 6),

 new Javascript("ES7" , 7)

 );

 Function<Javascript, String> outerKey = (x) -> x.name;

 Function<Javascript, String> innerKey = (y) -> y.name;

 BiFunction<Javascript, IEnumerable<Javascript>, Javascript> selector = (x, y) -> new Javascript(x.name, y.select(z -> z.age));

 List<Javascript> actual = outer.groupJoin(inner, outerKey, innerKey, selector).toList();

 assertEquals(3, actual.size());

 assertEquals("Angular", actual.get(0).name);

 assertEquals("React" , actual.get(1).name);

 assertEquals("ES2016" , actual.get(2).name);

 assertEquals(2, actual.get(0).ages.elementAt(0));

 assertEquals(3, actual.get(0).ages.elementAt(1));

 assertEquals(0, actual.get(1).ages.count());

 assertEquals(6, actual.get(2).ages.elementAt(0));

Reverse

 List<Integer> list = new List<>(1, 2, 3);

 List<Integer> actual = list.reverse().toList();

 assertEquals(3, actual.get(0).intValue());

 assertEquals(2, actual.get(1).intValue());

 assertEquals(1, actual.get(2).intValue());

Zip

 List<Integer> first = new List<>(1, 2, 3);

 List<String> second = new List<>("Angular", "React", "Backbone");

 List<Integer> actual = first.zip(second, (x, y) -> String.format("%s %d", x, y)).toList();

 assertEquals("1 Angular" , actual.get(0));

 assertEquals("2 React" , actual.get(1));

 assertEquals("3 Backbone", actual.get(2));

Distinct

 List<Integer> list =new List<>(1, 2, 3,1, 2, 3, 4);

 List<Integer> actual = list.distinct().toList();

 assertEquals(1, actual.get(0).intValue());

 assertEquals(2, actual.get(1).intValue());

 assertEquals(3, actual.get(2).intValue());

 assertEquals(4, actual.get(3).intValue());

Aggregate

 List<Integer> list = new List<>(1, 2, 3);

 int actual = list.aggregate((sum, elem) -> sum + elem);

 assertEquals(6, actual);

GroupBy

 List<Person> list = new List<>(

 new Person("React" , 1),

 new Person("Angular" , 1),

 new Person("Backbone", 5)

 );

 Map<Integer, IEnumerable<Person>> actual = list.groupBy(x -> x.age);

 assertEquals(true, actual.get(1).any(x -> x.name.equals("React")));

 assertEquals(true, actual.get(1).any(x -> x.name.equals("Angular")));

 assertEquals(true, actual.get(5).any(x -> x.name.equals("Backbone")));

Average

 List<Long> listLong = new List<>(1l, 2l, 3l, 4l);

 double actualLong = listLong.averageLong(x -> x);

 assertEquals(2.5d, actualLong, 0);

Count

 List<String> list = new List<>("Backbone", "Angular", "React");

 long actual = list.longCount();

 int actualNone = list.count(x -> x.equals("jquery"));

 assertEquals(3, actual);

 assertEquals(0, actualNone);

Max

 List<Double> listDouble = new List<>(1d, 2d, 3d);

 double actualDouble = listDouble.max(x -> x);

 assertEquals(3d, actualDouble, 0);

Min

 List<BigDecimal> listBigDecimal = new List<>(

 new BigDecimal(1d),

 new BigDecimal(2d),

 new BigDecimal(3d)

 );

 BigDecimal actualBigDecimal = listBigDecimal.min(x -> x);

 assertEquals(1d, actualBigDecimal.doubleValue(), 0);

Sum

 List<Integer> listInt = new List<>(1, 2, 3);

 int actualInt = listInt.sumInt(x -> x);

 assertEquals(6, actualInt);

FirstOrDefault

 List<String> list = new List<>("Backbone", "Angular", "React");

 String actualFirst = list.firstOrDefault();

 String actualMatch = list.firstOrDefault(x -> x.equals("Angular"));

 String actualUnMatch = list.firstOrDefault(x -> x.equals("jquery"));

 assertEquals("Backbone", actualFirst);

 assertEquals("Angular" , actualMatch);

 assertEquals(null , actualUnMatch);

LastOrDefault

 List<Integer> list = new List<>(1, 2, 3);

 List<Integer> listEmpty = new List<>();

 int actual = list.lastOrDefault();

 Integer actualDefaultNone = listEmpty.lastOrDefault(x -> x == 0);

 assertEquals(3, actual);

 assertEquals(null, actualDefaultNone);

SingleOrDefault

 List<Integer> listMany = new List<>(1, 2, 3);

 List<Integer> listEmpty = new List<>();

 int actualFilter = listMany.singleOrDefault(x -> x == 3);

 Integer actualUnMatch = listEmpty.singleOrDefault(x -> x == 0);

 assertEquals(3, actualFilter);

 assertEquals(null, actualUnMatch);

DefaultIfEmpty

 List<String> listEmpty = new List<>();

 List<String> actualDefault = listEmpty.defaultIfEmpty("ES7").toList();

 assertEquals("ES7", actualDefault.get(0));

 ElementAtOrDefault

 List<Integer> list = new List<>(1, 2, 3);

 int actual = list.elementAtOrDefault(2);

 Integer actualDefault = list.elementAtOrDefault(3);

 assertEquals(3, actual);

 assertEquals(null, actualDefault);

All

 List<String> list = new List<>("Backbone", "Angular", "React");

 boolean actual = list.all(x -> x.equals("Angular") || x.equals("Backbone") || x.equals("React"));

 boolean actualNotFound = list.all(x -> x.equals("Angular") || x.equals("React"));

 assertEquals(true, actual);

 assertEquals(false, actualNotFound);

Any

 List<String> list = new List<>("Backbone", "Angular", "React");

 boolean actual = list.any(x -> x.equals("Angular"));

 boolean actualNotFound = list.any(x -> x.equals("jquery"));

 assertEquals(true, actual);

 assertEquals(false, actualNotFound);

Empty

 List<Double> actual = IEnumerable.empty(Double.class);

 assertEquals(0, actual.count());

Range

 List<Integer> actual = IEnumerable.range(-2, 3);

 assertEquals(-2, actual.get(0).intValue());

 assertEquals(-1, actual.get(1).intValue());

 assertEquals(0 , actual.get(2).intValue());

Repeat

 List<String> actual = IEnumerable.repeat(String.class, "Law of Cycles", 10);

 assertEquals(10, actual.count());

 assertEquals("Law of Cycles", actual.get(9));

 SequenceEqual

 List<Integer> first = new List<>(1, 2, 3);

 List<Integer> secondMatch = new List<>(1, 2, 3);

 List<Integer> secondUnMatchElem = new List<>(1, 2, 4);

 boolean actualMatch = first.sequenceEqual(secondMatch);

 boolean actualUnMatchElm = first.sequenceEqual(secondUnMatchElem);

 assertEquals(true, actualMatch);

 assertEquals(false, actualUnMatchElm);

Cast

 List<Object> list = new List<>(1, 2, 3);

 List<Integer> actual = list.cast(Integer.class).toList();

 assertEquals(1, actual.get(0).intValue());

 assertEquals(2, actual.get(1).intValue());

 assertEquals(3, actual.get(2).intValue());

OfType

 List<Object> list = new List<>(1, "2", 3, "4");

 List<String> actualStr = list.ofType(String.class).toList();

 List<Integer> actualInt = list.ofType(Integer.class).toList();

 assertEquals("2", actualStr.get(0));

 assertEquals("4", actualStr.get(1));

 assertEquals(1 , actualInt.get(0).intValue());

 assertEquals(3 , actualInt.get(1).intValue());

6.源代码

https://github.com/k--kato/jLinqer

Java的jLinqer包介绍