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This page shows operators that perform mathematical or other operations over an entire sequence of items emitted by an `Observable` or `Flowable`. Because these operations must wait for the source `Observable`/`Flowable` to complete emitting items before they can construct their own emissions (and must usually buffer these items), these operators are dangerous to use on `Observable`s and `Flowable`s that may have very long or infinite sequences.
# Outline
- [Mathematical Operators](#mathematical-operators)
- [`averageDouble`](#averagedouble)
- [`averageFloat`](#averagefloat)
- [`max`](#max)
- [`min`](#min)
- [`sumDouble`](#sumdouble)
- [`sumFloat`](#sumfloat)
- [`sumInt`](#sumint)
- [`sumLong`](#sumlong)
- [Standard Aggregate Operators](#standard-aggregate-operators)
- [`count`](#count)
- [`reduce`](#reduce)
- [`reduceWith`](#reducewith)
- [`collect`](#collect)
- [`collectInto`](#collectinto)
- [`toList`](#tolist)
- [`toSortedList`](#tosortedlist)
- [`toMap`](#tomap)
- [`toMultimap`](#tomultimap)
## Mathematical Operators
> The operators in this section are part of the [`RxJava2Extensions`](https://github.com/akarnokd/RxJava2Extensions) project. You have to add the `rxjava2-extensions` module as a dependency to your project. It can be found at [http://search.maven.org](http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.github.akarnokd%22).
> Note that unlike the standard RxJava aggregator operators, these mathematical operators return `Observable` and `Flowable` instead of the `Single` or `Maybe`.
*The examples below assume that the `MathObservable` and `MathFlowable` classes are imported from the `rxjava2-extensions` module:*
```java
import hu.akarnokd.rxjava2.math.MathObservable;
import hu.akarnokd.rxjava2.math.MathFlowable;
```
### averageDouble
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/average.html](http://reactivex.io/documentation/operators/average.html)
Calculates the average of `Number`s emitted by an `Observable` and emits this average as a `Double`.
#### averageDouble example
```java
Observable<Integer> numbers = Observable.just(1, 2, 3);
MathObservable.averageDouble(numbers).subscribe((Double avg) -> System.out.println(avg));
// prints 2.0
```
### averageFloat
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/average.html](http://reactivex.io/documentation/operators/average.html)
Calculates the average of `Number`s emitted by an `Observable` and emits this average as a `Float`.
#### averageFloat example
```java
Observable<Integer> numbers = Observable.just(1, 2, 3);
MathObservable.averageFloat(numbers).subscribe((Float avg) -> System.out.println(avg));
// prints 2.0
```
### max
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/max.html](http://reactivex.io/documentation/operators/max.html)
Emits the maximum value emitted by a source `Observable`. A `Comparator` can be specified that will be used to compare the elements emitted by the `Observable`.
#### max example
```java
Observable<Integer> numbers = Observable.just(4, 9, 5);
MathObservable.max(numbers).subscribe(System.out::println);
// prints 9
```
The following example specifies a `Comparator` to find the longest `String` in the source `Observable`:
```java
final Observable<String> names = Observable.just("Kirk", "Spock", "Chekov", "Sulu");
MathObservable.max(names, Comparator.comparingInt(String::length))
.subscribe(System.out::println);
// prints Chekov
```
### min
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/min.html](http://reactivex.io/documentation/operators/min.html)
Emits the minimum value emitted by a source `Observable`. A `Comparator` can be specified that will be used to compare the elements emitted by the `Observable`.
#### min example
```java
Observable<Integer> numbers = Observable.just(4, 9, 5);
MathObservable.min(numbers).subscribe(System.out::println);
// prints 4
```
### sumDouble
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/sum.html](http://reactivex.io/documentation/operators/sum.html)
Adds the `Double`s emitted by an `Observable` and emits this sum.
#### sumDouble example
```java
Observable<Double> numbers = Observable.just(1.0, 2.0, 3.0);
MathObservable.sumDouble(numbers).subscribe((Double sum) -> System.out.println(sum));
// prints 6.0
```
### sumFloat
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/sum.html](http://reactivex.io/documentation/operators/sum.html)
Adds the `Float`s emitted by an `Observable` and emits this sum.
#### sumFloat example
```java
Observable<Float> numbers = Observable.just(1.0F, 2.0F, 3.0F);
MathObservable.sumFloat(numbers).subscribe((Float sum) -> System.out.println(sum));
// prints 6.0
```
### sumInt
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/sum.html](http://reactivex.io/documentation/operators/sum.html)
Adds the `Integer`s emitted by an `Observable` and emits this sum.
#### sumInt example
```java
Observable<Integer> numbers = Observable.range(1, 100);
MathObservable.sumInt(numbers).subscribe((Integer sum) -> System.out.println(sum));
// prints 5050
```
### sumLong
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/sum.html](http://reactivex.io/documentation/operators/sum.html)
Adds the `Long`s emitted by an `Observable` and emits this sum.
#### sumLong example
```java
Observable<Long> numbers = Observable.rangeLong(1L, 100L);
MathObservable.sumLong(numbers).subscribe((Long sum) -> System.out.println(sum));
// prints 5050
```
## Standard Aggregate Operators
> Note that these standard aggregate operators return a `Single` or `Maybe` because the number of output items is always know to be at most one.
### count
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/count.html](http://reactivex.io/documentation/operators/count.html)
Counts the number of items emitted by an `Observable` and emits this count as a `Long`.
#### count example
```java
Observable.just(1, 2, 3).count().subscribe(System.out::println);
// prints 3
```
### reduce
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/reduce.html](http://reactivex.io/documentation/operators/reduce.html)
Apply a function to each emitted item, sequentially, and emit only the final accumulated value.
#### reduce example
```java
Observable.range(1, 5)
.reduce((product, x) -> product * x)
.subscribe(System.out::println);
// prints 120
```
### reduceWith
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/reduce.html](http://reactivex.io/documentation/operators/reduce.html)
Apply a function to each emitted item, sequentially, and emit only the final accumulated value.
#### reduceWith example
```java
Observable.just(1, 2, 2, 3, 4, 4, 4, 5)
.reduceWith(TreeSet::new, (set, x) -> {
set.add(x);
return set;
})
.subscribe(System.out::println);
// prints [1, 2, 3, 4, 5]
```
### collect
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/reduce.html](http://reactivex.io/documentation/operators/reduce.html)
Collect items emitted by the source `Observable` into a single mutable data structure and return an `Observable` that emits this structure.
#### collect example
```java
Observable.just("Kirk", "Spock", "Chekov", "Sulu")
.collect(() -> new StringJoiner(" \uD83D\uDD96 "), StringJoiner::add)
.map(StringJoiner::toString)
.subscribe(System.out::println);
// prints Kirk 🖖 Spock 🖖 Chekov 🖖 Sulu
```
### collectInto
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/reduce.html](http://reactivex.io/documentation/operators/reduce.html)
Collect items emitted by the source `Observable` into a single mutable data structure and return an `Observable` that emits this structure.
#### collectInto example
*Note: the mutable value that will collect the items (here the `StringBuilder`) will be shared between multiple subscribers.*
```java
Observable.just('R', 'x', 'J', 'a', 'v', 'a')
.collectInto(new StringBuilder(), StringBuilder::append)
.map(StringBuilder::toString)
.subscribe(System.out::println);
// prints RxJava
```
### toList
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/to.html](http://reactivex.io/documentation/operators/to.html)
Collect all items from an `Observable` and emit them as a single `List`.
#### toList example
```java
Observable.just(2, 1, 3)
.toList()
.subscribe(System.out::println);
// prints [2, 1, 3]
```
### toSortedList
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/to.html](http://reactivex.io/documentation/operators/to.html)
Collect all items from an `Observable` and emit them as a single, sorted `List`.
#### toSortedList example
```java
Observable.just(2, 1, 3)
.toSortedList(Comparator.reverseOrder())
.subscribe(System.out::println);
// prints [3, 2, 1]
```
### toMap
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/to.html](http://reactivex.io/documentation/operators/to.html)
Convert the sequence of items emitted by an `Observable` into a `Map` keyed by a specified key function.
#### toMap example
```java
Observable.just(1, 2, 3, 4)
.toMap((x) -> {
// defines the key in the Map
return x;
}, (x) -> {
// defines the value that is mapped to the key
return (x % 2 == 0) ? "even" : "odd";
})
.subscribe(System.out::println);
// prints {1=odd, 2=even, 3=odd, 4=even}
```
### toMultimap
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/to.html](http://reactivex.io/documentation/operators/to.html)
Convert the sequence of items emitted by an `Observable` into a `Collection` that is also a `Map` keyed by a specified key function.
#### toMultimap example
```java
Observable.just(1, 2, 3, 4)
.toMultimap((x) -> {
// defines the key in the Map
return (x % 2 == 0) ? "even" : "odd";
}, (x) -> {
// defines the value that is mapped to the key
return x;
})
.subscribe(System.out::println);
// prints {even=[2, 4], odd=[1, 3]}
```
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