Java流参数限制没有限制（MongoDB不一致）

Question

I have a method which return list of items and takes a limit (used by Stream#limit ) as parameter:

public List<Integer> getItems(Long limit) {
    return IntStream.range(1, 10)
            .limit(limit)
            .boxed()
            .collect(Collectors.toList());
}  

How to set the parameter to take all items (with no limit)?

My attempts:

    Long limit5 = 5L;
    System.out.println("With limit 5:" + getItems(limit5));
    // works fine: 5 items

    Long noLimitZero = 0L;
    System.out.println("Without limit (zero): " + getItems(noLimitZero));
    // why 0 mean "no items" instead of "all items"

   Long noLimitNegative = -1L;
    System.out.println("Without limit (negative number): " + getItems(noLimitNegative));
    // IllegalArgumentException

    Long noLimitNull = null;
    System.out.println("Without limit (null): " + getItems(noLimitNull));
    // NullPointerException

Passing Long.MAX_VALUE is not a solution.

MongoDB inconsistency

For example MongoDB's FindIterable#limit can take 0 or null as no limit.

public List<Integer> getItems(Long limit) {
    MongoDatabase mongo = new MongoClient().getDatabase("example");
    MongoCollection<Document> documents = mongo.getCollection("items");
    FindIterable<Document> founded = documents.find();
    List<Integer> items = new ArrayList<>();
    for (Document doc : founded.limit(limit.intValue())) {
        items.add(doc.getInteger("number"));
    }
    return items;
}

This inconsistency between methods causes incompatibility, for example one interface with method List<Integer> getItems(Long limit) and two implementations: in memory and MongoDB.

Consistency in methods Stream#skip and FindIterable#skip is preserved.

          --------------------------
          | Java       | Mongo     |
------------------------------------
limit = 0 | none items | all items |
------------------------------------
skip = 0  | none skip  | none skip |
------------------------------------

Refactor method with Stream#limit

I guess there is no way to pass "no limit" parameter to Stream#limit , so I must refactor this method to takes "limit" and 0 or null or -1 as "no limit".

public static List<Integer> getItems(Long limit) {
    if (limit == null || limit == 0 || limit == -1) {
        return IntStream.range(1, 10)
                .boxed()
                .collect(Collectors.toList());
    } else {
        return IntStream.range(1, 10)
                .limit(limit)
                .boxed()
                .collect(Collectors.toList());
    }
}

Or:

public static List<Integer> getItems(Long limit) {
    IntStream items = IntStream.range(1, 10);
    if (limit != null && limit != 0 && limit != -1) {
        items = items.limit(limit);
    }
    return items.boxed()
            .collect(Collectors.toList());
}

There is a better way to achieve consistency between methods limit ?

Answer 1

So there's several layers of problems with what you're trying to do.

You say "practicality isn't an argument" and that's fine, but let me just point out that Long.MAX_VALUE does exceed the amount of atoms on earth, so the probability that you're getting more entries than that from a database is really small. Not to mention that you go on collecting that data into a list so you might run into memory issues in your own application as well.

So the second thing is that the semantics of limit() is that it imposes a fixed limit on the number of entries and "infinity" is not a fixed limit; hence limit() just isn't what you're looking for.

Third, you seem to be looking for a way around that so there us a pattern you can use, and that's maintaining your own counter. What you want is something like an AtomicBigInteger which doesn't exist in the JDK but is shown here .

So what you'd do is create a Predicate like this

class BelowValue<T> implements Predicate<T> {
    BigInteger limit = BigInteger.ZERO;
    AtomicBigInteger counter = new AtomicBigInteger();

    public BelowValue(BigInteger limit) {
        this.limit = limit;
    }        
    public BelowValue() {}

    public boolean test(T ignored) {
        // short circuit on zero
        if (BigInteger.ZERO.compareTo(limit) == 0) { return true; }

        // check actual condition
        return  counter.incrementAndGet().compareTo(limit) > 0;
    }
}

and then you can use it in your stream with (Java 8)

Predicate<T> filter = new BelowValue<>(limit);
return stream
    .filter(filter)
    .boxed()
    .collect(Collectors.toList());

Note however that filter is not a short-circuiting operation, so if you have an infinite stream, this will not terminate (and be very inefficient if your stream is much longer than the limit size).

Java 9's takeWhile is short-circuiting, so you can substitute that for filter in the above example.