This section explains operators with which you conditionally emit or transform Observables, or can do boolean evaluations of them:

### Conditional Operators

### Outline

- [`amb`](#all)
- [`defaultIfEmpty`](#defaultIfEmpty)
- [`skipUntil`](#skipUntil)
- [`skipWhile`](#skipWhile)
- [`takeUntil`](#takeUntil)
- [`takeWhile`](#takeUntil)

## amb

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Completable`
 
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/amb.html](http://reactivex.io/documentation/operators/amb.html)

given two or more source Observables, emits all of the items from the first of these Observables to emit an item

```java
    Observable source1 = Observable.range(1, 5);
    Observable source2 = Observable.range(6, 5);
    Observable.amb(new ArrayList(Arrays.asList(source1, source2)))
        .subscribe(next -> System.out.printf("next: %s\n", next), // onNext
            throwable -> System.out.printf("error: %s\n", throwable), //onError
            () -> System.out.println("Completed") //onComplete
        );
```
## defaultIfEmpty

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/defaultifempty.html](http://reactivex.io/documentation/operators/defaultifempty.html)

emit items from the source Observable, or emit a default item if the source Observable completes after emitting no items

```java
 Observable.empty().defaultIfEmpty(1).blockingSubscribe(next -> System.out.printf("next: %s\n", next), // onNext
                throwable -> System.out.printf("error: %s", throwable), //onError
                () -> System.out.println("Completed") //onComplete
        );
```

## skipUntil

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`
 
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/skipuntil.html](http://reactivex.io/documentation/operators/skipuntil.html)

discard items emitted by a source Observable until a second Observable emits an item, then emit the remainder of the source Observable's items

```java
Observable observable1 = Observable.range(1, 10).doOnNext(next -> Thread.sleep(1000));
        
observable1.skipUntil(Observable.timer(3, TimeUnit.SECONDS))
    .subscribe(next -> System.out.printf("next: %s\n", next), // onNext
        throwable -> System.out.printf("error: %s", throwable), //onError
        () -> System.out.println("Completed") //onComplete
    );
```
## skipWhile

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/skipwhile.html](http://reactivex.io/documentation/operators/skipwhile.html)

discard items emitted by an Observable until a specified condition is false, then emit the remainder

```java
Observable.range(1, 10).skipWhile(next -> next < 5)
    .subscribe(next -> System.out.printf("next: %s\n", next), // onNext
        throwable -> System.out.printf("error: %s", throwable), //onError
        () -> System.out.println("Completed") //onComplete
    );
```

## takeUntil

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/takeuntil.html](http://reactivex.io/documentation/operators/takeuntil.html)

emits the items from the source Observable until a second Observable emits an item or issues a notification

```java
Observable.range(1, 10).takeUntil(value -> value >= 5)
    .subscribe(next -> System.out.printf("next: %s\n", next), // onNext
        throwable -> System.out.printf("error: %s", throwable), //onError
        () -> System.out.println("Completed") //onComplete
    );
```

## takeWhile

**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/takewhile.html](http://reactivex.io/documentation/operators/takewhile.html)

emit items emitted by an Observable as long as a specified condition is true, then skip the remainder

```java
     Observable.range(1, 10).takeWhile(value -> value <= 5)
                .subscribe(next -> System.out.printf("next: %s\n", next), // onNext
                        throwable -> System.out.printf("error: %s", throwable), //onError
                        () -> System.out.println("Completed") //onComplete
                );
```

### Boolean Operators

### Outline

- [`all`](#all)
- [`contains`](#contains)
- [`isEmpty`](#isEmpty)
- [`sequenceEqual`](#sequenceEqual)

## all
**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/all.html](http://reactivex.io/documentation/operators/all.html)

determine whether all items emitted by an Observable meet some criteria

```java
Flowable.range(0,10).doOnNext(next -> System.out.println(next)).all(integer -> integer<10).
    blockingSubscribe(success->System.out.println("Success: "+success));
```

## contains
**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/contains.html](http://reactivex.io/documentation/operators/contains.html)

determine whether an Observable emits a particular item or not

```java
Flowable.range(1,10).doOnNext(next->System.out.println(next))
    .contains(4).blockingSubscribe(contains->System.out.println("contains: "+contains));
```

## isEmpty
**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_off.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/contains.html](http://reactivex.io/documentation/operators/contains.html)

determine whether the source Publisher is empty

```java
Flowable.empty().isEmpty().subscribe(isEmpty -> System.out.printf("isEmpty: %s", isEmpty));
```

## sequenceEqual
**Available in:** ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Flowable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Observable`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Maybe`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Single`, ![image](https://raw.github.com/wiki/ReactiveX/RxJava/images/checkmark_on.png) `Completable`

**ReactiveX documentation:** [http://reactivex.io/documentation/operators/sequenceequal.html](http://reactivex.io/documentation/operators/sequenceequal.html)

test the equality of the sequences emitted by two Observables

```java
Flowable<Integer> flowable1 = Flowable.range(1,3).doOnNext(next-> System.out.print("flowable1: "+next + " "));

Flowable<Integer> flowable2 = Flowable.range(1,3).doOnNext(next-> System.out.println("flowable2: "+next));

Flowable.sequenceEqual(Flowable.fromPublisher(flowable1),Flowable.fromPublisher(flowable2))
    .blockingSubscribe(sequenceEqual->System.out.println("sequenceEqual: "+sequenceEqual));
```