functional style validation algorithm

Question

Imagine hierarchical path stored in ordered List and its validation

List<String> path = Lists.newArrayList("path","to","end");

Iterator<String> iterator = path.iterator();

while (iterator.hasNext()) {
    if (iterator.next() == null) {
        while (iterator.hasNext()) {
            if (iterator.next() != null) {
                throw new Exception("Invalid path");
            }
        }
    }
}

The algorithm is pretty straightforward.

Only leafs or nodes with all its descendant must be null.

Valid paths:

"path", "to", "end"
"path", "to", null
"path", null, null
null, null, null

Invalid paths:

"path", null, "end"
null, null, "end"
null, "to", "end"
null, "to", null

I am looking for way to rewrite it in a functional style.

I think you've got some typos in your "valid paths" which are making your question confusing. Did you mean to put quotes around null is those places? — castletheperson
– castletheperson, Commented Feb 13, 2017 at 23:53
What's wrong with non-functional solution? You said it yourself - your imperative algorithm is straightforward and relatively simple (I would only remove a second nested while, since you don't need it). It will gain no benefir from functional rewrite, and most likely will gain only complexity (I'm almost sure that functional implementation would require a Tuple class, for example). — M. Prokhorov
– M. Prokhorov, Commented Feb 14, 2017 at 15:32

Holger · Accepted Answer · 2017-02-14 16:01:33Z

3

In Java 9, you could use

if(path.stream().dropWhile(Objects::nonNull).anyMatch(Objects::nonNull))
    throw new Exception("Invalid path");

The logic is simple, first, drop all non-null values until the encounter of a null value (if any), then, fail if any non-null value follows.

Since Java 8 lacks the dropWhile operation, we have to prepend an alternative operation, e.g. using the Collection API rather than Stream API:

int firstNull = path.indexOf(null);
if(firstNull>=0 && path.stream().skip(firstNull).anyMatch(Objects::nonNull))
    throw new Exception("Invalid path");

edited Feb 14, 2017 at 16:01

answered Feb 14, 2017 at 15:55

Holger

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2 Comments

Jude Niroshan Over a year ago

as you have been among the top users in Java-8, would you please put some links about that Java-9 feature? :)

Holger Over a year ago

@Jude Niroshan: Here you go

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