How to check if a Java 8 Stream is empty

Question

How can I check if a Stream is empty and throw an exception if it s not  as a non-terminal operation   Basically  I m looking for something equivalent to the code below  but without materializing the stream in-between  In particular  the check should not occur before the stream is actually consumed by a terminal operation   public Stream lt Thing gt  getFilteredThings         Stream lt Thing gt  stream   getThings   stream                    filter Thing  isFoo                   filter Thing  isBar       return nonEmptyStream stream     - gt            throw new RuntimeException  No foo bar things available                 private static  lt T gt  Stream lt T gt  nonEmptyStream Stream lt T gt  stream  Supplier lt T gt  defaultValue        List lt T gt  list   stream collect Collectors toList         if  list isEmpty    list add defaultValue get         return list stream

User · Answer

I think should be enough to map a boolean

In code this is:

boolean isEmpty = anyCollection.stream()
    .filter(p -> someFilter(p)) // Add my filter
    .map(p -> Boolean.TRUE) // For each element after filter, map to a TRUE
    .findAny() // Get any TRUE
    .orElse(Boolean.FALSE); // If there is no match return false

User · Answer

You must perform a terminal operation on the Stream in order for any of the filters to be applied  Therefore you can t know if it will be empty until you consume it   Best you can do is terminate the Stream with a findAny   terminal operation  which will stop when it finds any element  but if there are none  it will have to iterate over all the input list to find that out   This would only help you if the input list has many elements  and one of the first few passes the filters  since only a small subset of the list would have to be consumed before you know the Stream is not empty   Of course you ll still have to create a new Stream in order to produce the output list

User · Answer

I would simply use  stream count   gt 0

User · Answer

The other answers and comments are correct in that to examine the contents of a stream  one must add a terminal operation  thereby  consuming  the stream  However  one can do this and turn the result back into a stream  without buffering up the entire contents of the stream  Here are a couple examples   static  lt T gt  Stream lt T gt  throwIfEmpty Stream lt T gt  stream        Iterator lt T gt  iterator   stream iterator        if  iterator hasNext              return StreamSupport stream Spliterators spliteratorUnknownSize iterator  0   false         else           throw new NoSuchElementException  empty stream             static  lt T gt  Stream lt T gt  defaultIfEmpty Stream lt T gt  stream  Supplier lt T gt  supplier        Iterator lt T gt  iterator   stream iterator        if  iterator hasNext              return StreamSupport stream Spliterators spliteratorUnknownSize iterator  0   false         else           return Stream of supplier get               Basically turn the stream into an Iterator in order to call hasNext   on it  and if true  turn the Iterator back into a Stream  This is inefficient in that all subsequent operations on the stream will go through the Iterator s hasNext   and next   methods  which also implies that the stream is effectively processed sequentially  even if it s later turned parallel   However  this does allow you to test the stream without buffering up all of its elements   There is probably a way to do this using a Spliterator instead of an Iterator  This potentially allows the returned stream to have the same characteristics as the input stream  including running in parallel

User · Answer

This may be sufficient in many cases  stream findAny   isPresent

User · Answer

Following Stuart s idea  this could be done with a Spliterator like this   static  lt T gt  Stream lt T gt  defaultIfEmpty Stream lt T gt  stream  Stream lt T gt  defaultStream        final Spliterator lt T gt  spliterator   stream spliterator        final AtomicReference lt T gt  reference   new AtomicReference lt  gt         if  spliterator tryAdvance reference  set             return Stream concat Stream of reference get     StreamSupport stream spliterator  stream isParallel            else           return defaultStream            I think this works with parallel Streams as the stream spliterator   operation will terminate the stream  and then rebuild it as required  In my use-case I needed a default Stream rather than a default value  that s quite easy to change if this is not what you need

User · Answer

If you can live with limited parallel capablilities  the following solution will work   private static  lt T gt  Stream lt T gt  nonEmptyStream      Stream lt T gt  stream  Supplier lt RuntimeException gt  e         Spliterator lt T gt  it stream spliterator        return StreamSupport stream new Spliterator lt T gt              boolean seen          public boolean tryAdvance Consumer lt   super T gt  action                boolean r it tryAdvance action               if  seen  amp  amp   r  throw e get                seen true              return r                    public Spliterator lt T gt  trySplit     return null            public long estimateSize     return it estimateSize              public int characteristics     return it characteristics             false       Here is some example code using it   List lt String gt  l Arrays asList  hello    world    nonEmptyStream l stream      - gt new RuntimeException  No strings available       forEach System out  println   nonEmptyStream l stream   filter s- gt s startsWith  x                      - gt new RuntimeException  No strings available       forEach System out  println     The problem with  efficient  parallel execution is that supporting splitting of the Spliterator requires a thread-safe way to notice whether either of the fragments has seen any value in a thread-safe manner  Then the last of the fragments executing tryAdvance has to realize that it is the last one  and it also couldn   t advance  to throw the appropriate exception  So I didn   t add support for splitting here

[java] How to check if a Java 8 Stream is empty?

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