CollectorEx

This library provides new collectors for using with java 8 streams. Some collectors are provide similar functionality like SQL window functions. ###Install Add to project via Maven:

<dependency>
  <groupId>com.github.agzamovr</groupId>
  <artifactId>collectorex</artifactId>
  <version>0.1</version>
</dependency>

or Gradle:

compile 'com.github.agzamovr:collectorex:0.1'

##Getting started

1. Rank collector
2. NTile collector
3. Distinct collector
4. Summing collectors
5. Mode collector
6. Multi value map collector

###Rank collector Rank collector calculates the rank for stream of objects using given comparator. If objects are implements Comparable interface then comparator may be omitted. Equal objects receive the same rank. Number of tied rows added to the next rank. Therefore, the ranks may not be consecutive numbers. To produce consecutive numbers use dense rank collector. Here is example of rank and dense rank comparators which returns sorted map with ranks as a key and list of objects as a value for corresponding key:

List<Integer> list = Arrays.asList(1, 2, 3, 4, 4, 3, 2, 1);

SortedMap<Integer, List<Integer>> rankedMap = list.stream().collect(CollectorEx.rank());
SortedMap<Integer, List<Integer>> denseRankedMap = list.stream().collect(CollectorEx.denseRank());

System.out.println(rankedMap);
System.out.println(denseRankedMap);
// {1=[1, 1], 3=[2, 2], 5=[3, 3], 7=[4, 4]}
// {1=[1, 1], 2=[2, 2], 3=[3, 3], 4=[4, 4]}

Alternatively objects can be mapped to ranks:

List<Integer> list = Arrays.asList(-1, -2, -3, -4, -4, -3, -2, -1);

Map<Integer, Integer> rankedMap = list.stream().collect(CollectorEx.mapObjToRank());
Map<Integer, Integer> denseRankedMap = list.stream().collect(CollectorEx.mapObjToDenseRank());

System.out.println(rankedMap);
System.out.println(denseRankedMap);
// {-1=7, -2=5, -3=3, -4=1}
// {-1=4, -2=3, -3=2, -4=1}

If required custom downstream collector can be provided:

List<Integer> list = Arrays.asList(1, 2, 3, 4, 4, 3, 2, 1);

SortedMap<Integer, Set<Integer>> rankedMap = list.stream().collect(CollectorEx.rank(Collectors.toSet()));

System.out.println(rankedMap);
// {1=[1], 3=[2], 5=[3], 7=[4]}

And finally the full version with custom comparator, rank comparator, dense rank flag and custom downstream collector:

List<Integer> list = Arrays.asList(1, 2, 3, 4, 4, 3, 2, 1);
Comparator<Integer> intComparator = Integer::compare;

SortedMap<Integer, Set<Integer>> rankedMap = list.stream()
              .collect(CollectorEx.rank(intComparator, intComparator.reversed(), false, Collectors.toSet()));

System.out.println(rankedMap);
// {7=[4], 5=[3], 3=[2], 1=[1]}

###NTile collector NTile collector divides stream of objects into a number of buckets using given comparator. If objects are implements Comparable interface then comparator may be omitted. Default collector returns list which indexes corresponds to tile number with collection of objects as values for each tile.

List<Integer> list = Arrays.asList(null, 1, 1, 2, 3, null);

List<List<Integer>> result = list.stream().collect(CollectorEx.ntile(2));

System.out.println(result);
// [[1, 1, 2], [3, null, null]]

Custom comparator may be provided for sorting. Here is example of custom comparator which puts nulls before non null values:

List<Integer> list = Arrays.asList(null, 1, 1, 2, 3, null);
Comparator<Integer> integerComparator = Comparator.nullsFirst(Integer::compareTo);

List<List<Integer>> result = list.stream().collect(CollectorEx.ntile(2, integerComparator));

System.out.println(result);
// [[null, null, 1], [1, 2, 3]]

Custom collector can be passed as downstream collector:

List<Integer> list = Arrays.asList(null, 1, 1, 2, 3, null);

List<Set<Integer>> result = list.stream().collect(CollectorEx.ntile(2, Collectors.toSet()));

System.out.println(result);
// [[1, 2], [null, 3]]

###Distinct collector Distinct collector return distinct elements of stream using given mapper function.

List<Integer> list = Arrays.asList(1, 2, 2, 1, -1, null);

Collection<Integer> result = list.stream().collect(CollectorEx.distinct(Function.identity()));

System.out.println(result);
//[-1, null, 1, 2]

Custom collector can be passed as downstream collector:

List<Integer> list = Arrays.asList(-1, -2, 2, 1, 1);

List<Integer> result = list.stream().collect(CollectorEx.distinct(Math::abs, Collectors.toList()));

System.out.println(result);
// [1, 2]

If you need to distinguish elements by more than one property you may use rankCollector. In the following example there is list of maps. Each map contains two entries with keys email and name. We use comparator to distinguish maps by this two keys:

List<Map<String, String>> mapList = new ArrayList<>();
Map<String, String> map = new HashMap<>();
map.put("email", "some@email.com");
map.put("name", "John");
mapList.add(map);
map = new HashMap<>();
map.put("email", "some@email.com");
map.put("name", "Rob");
mapList.add(map);
map = new HashMap<>();
map.put("email", "some@email.com");
map.put("name", "Rob");
mapList.add(map);
Comparator<Map<String, String>> distinctComparator =
        comparing((Map<String, String> m) -> m.get("email"))
                .thenComparing(m -> m.get("name"));
List<Map<String, String>> bidAuthorsInfo = mapList.stream()
        .collect(CollectorEx.rankDistinct(distinctComparator));
bidAuthorsInfo.forEach(System.out::println);
//{name=John, email=some@email.com}
//{name=Rob, email=some@email.com}

Rank distinct collector uses rank collector internally and selects distinct elements by their calculated ranks. ###Summing collectors Summing collectors are returns cumulative sum for each stream element in a given order. There are four types of this collector for int, long, double and BigDecimal types. Example:

List<Integer> list = Arrays.asList(1, 2, 3);

List<Integer> result = list.stream().collect(CollectorEx.summingInt(i -> i));

System.out.println(result);
//[1, 3, 6]

Custom comparator and downstream collector can be passed as:

List<Integer> list = asList(1, 2, 3);
Comparator<Integer> integerComparator = Integer::compareTo;

Set<Integer> result = list.stream().collect(CollectorEx.summingInt(i -> i,
        integerComparator.reversed(),
        Collectors.toSet()));

System.out.println(result);
//[3, 5, 6]

###Mode collector Mode collector returns collection of elements which appears most often in a stream.

List<Integer> list = Arrays.asList(1, 3, 6, 6, 6, 6, 7, 7, 12, 12, 17);

Set<Integer> result = list.stream().collect(CollectorEx.mode());

System.out.println(result);
//[6]

Custom mapping function and downstream collector can be passed as:

List<Integer> list = Arrays.asList(1, -1, 2, -2, 3, 4);

List<Integer> result = list.stream().collect(CollectorEx.mode(Math::abs,
        Collectors.toList()));

System.out.println(result);
//[1, 2]

###Multi value map collector Multi value map collector converts stream of map or map entries to multi value map. Example:

List<Map<Integer, Integer>> list = Arrays.asList(
        singletonMap(null, null), singletonMap(0, 0),
        singletonMap(1, 1), singletonMap(1, -1),
        singletonMap(2, 2), singletonMap(2, -2));

Map<Integer, List<Integer>> result = list.stream().collect(CollectorEx.mapStreamToMultiValueMap());

System.out.println(result);
//{0=[0], null=[null], 1=[1, -1], 2=[2, -2]}

Custom collector can be passed as downstream collector:

List<Map<Integer, Integer>> list = Arrays.asList(
                singletonMap(null, null), singletonMap(0, 0),
                singletonMap(1, 1), singletonMap(1, -1),
                singletonMap(2, 2), singletonMap(2, -2));

Map<Integer, Long> result = list.stream()
        .collect(CollectorEx.mapStreamToMultiValueMap(Collectors.counting()));

System.out.println(result);
//{0=1, null=1, 1=2, 2=2}

======================

License

The MIT License (MIT)

Copyright (c) 2016 Rustam Agzamov

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.