reusing stream inside a stream java

Question

Hi I just wanted to know what is wrong with my code and how can I improve it as it throws an error "stream has already been operated upon or closed" thank you here is my code:

Stream<String> stream = Files.lines(path)

stream.filter(item -> !item.contains("String")).forEach(item -> {
            Pattern pattern = Pattern.compile(REGEX);
            Matcher matcher = pattern.matcher(item);
            if (!stream.filter(items -> items.length() > 12).collect(Collectors.toList()).isEmpty() == true) {
                log.error("Item must not exceed 12 numbers {}", item);
                streamMessage.set("Items must not exceed 12 numbers");
            }
            else if(matcher.matches() == true){
                log.error("Invalid String format {}", item);
                streamMessage.set("Invalid String format");
            }
        });

I wanted to loop inside the file and check per line if the characters there exceeded 12 characters or not

What exactly are you trying to achieve? Can you share a sample file and the result you'd like to get for it? — Mureinik
– Mureinik, Commented Jan 19, 2022 at 9:06
What do you want to achieve here? Besides the log messages, nothing comes out of your stream processing? — GhostCat
– GhostCat, Commented Jan 19, 2022 at 9:07
@CedrixCedrix even with the edit I'm not sure I understand. Do you mean you want to read the file and omit lines longer than 12 characters? — Mureinik
– Mureinik, Commented Jan 19, 2022 at 9:10
You're immediately closing the stream in the first iteration: stream.close(). On the next iteration, the stream access will throw the exception. — gru
– gru, Commented Jan 19, 2022 at 9:11
@CedrixCedrix stream.close() is unnecessary, but it's not the cause of the error. — Eran
– Eran, Commented Jan 19, 2022 at 9:16

Eran · Accepted Answer · 2022-01-19 10:04:56Z

You cannot reuse the Stream. You can create two Streams using Files.lines(path), but given your code, I don't think it's necessary.

You can find all the elements that are too long using a single Stream:

List<String> tooLong = 
    stream.filter(item -> !item.contains("String"))
          .filter(items -> items.length() > 12)
          .collect(Collectors.toList());

if (tooLong.size () > 0) {
    log.error("Items must not exceed 12 numbers {}", tooLong.toString ());
    streamMessage.set("Items must not exceed 12 numbers");
}

EDIT:

After your question edit, it appears you want to do two checks for each element of the Stream. You can still do it with a single Stream:

stream.filter(item -> !item.contains("String"))
      .forEach(item -> {
        Pattern pattern = Pattern.compile(REGEX);
        Matcher matcher = pattern.matcher(item);
        if (item.length() > 12) {
            log.error("Item must not exceed 12 numbers {}", item);
            streamMessage.set("Items must not exceed 12 numbers");
        } else if(matcher.matches()) {
            log.error("Invalid String format {}", item);
            streamMessage.set("Invalid String format");
        }
    });

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