How to search between two Streams in Java 8

Question

If I have 2 Streams like coming in a method as shown below

public Stream<Transaction> getPendingTransaction(Stream<PendingTransaction> pendingTransactionStream,Stream<ProcessedTransaction> processedTransactionStream){ }

and I want to find all objects which are present in pendingTransactionStream which are also present in processedTransactionStream based upon some criteria like

if transaction.getId() is same for an Transaction object present in pendingTransactionStream and processedTransactionStreamthen that object is same and we can collect them in a list.

I tried doing like this but its giving error

processedTransactionStream
        .filter( (processedTransaction)->
        {
            pendingTransactionStream.anyMatch(s->s.getTransactionId().equals(processedTransaction.getTransactionId()) );
        } 
        ).collect(Collectors.toList());

Answer 1

Well, you can't consume the pendingTransactionStream Stream multiple times. You can transform it to a List (or even better, a Set) of transaction IDs to use in the filter method.

Set<String> pending = pendingTransactionStream.map(PendingTransaction::getTransactionId)
                                              .collect(Collectors.toSet());
List<ProcessedTransaction> processed = 
    processedTransactionStream.filter(pt -> pending.contains(pt.getTransactionId()))
                              .collect(Collectors.toList());

Answer 2

You can't iterate Streams more than once. So your current code doesn't work (you get an exception like IllegalStateException: Stream already closed. From the java doc:

A stream should be operated on (invoking an intermediate or terminal stream operation) only once.

A possible solution would be to convert the pendingTransactionStream into a map where the key is the type of the id (I use string, because I don't know the keyType):

Actually a Set would be better as you don't need the PendingTransaction for anything else, for an example have a look at @Eran's answer

Map<String, PendingTransaction> pendingTransactionMap = pendingTransactionStream
    .collect(PendingTransaction::getId, Function.identity());

And then filter your processedTransactionStream, by checking if the id is in the map:

List<ProcessedTransaction> processedTransactionList = processedTransactionStream
    .filter(p -> pendingTransactionMap.containsKey(p.getId()))
    .collect(Collectors.toList());