How to extract only one allowed element from a stream? - java

I have a list of elements, and want to extract the value of the fields' propery.
Problem: all elements should have the same property value.
Can I do better or more elegant than the following?
Set<String> matches = fields.stream().map(f -> f.getField()).collect(Collectors.toSet());
if (matches.size() != 1) throw new IllegalArgumentException("could not match one exact element");
String distrinctVal = matches.iterator().next(); //continue to use the value
Is this possible directly using the stream methods, eg using reduce?

Your current solution is good. You can try this way also to avoid collecting.
Use distinct() then count()
if (fields.stream().map(f -> f.getField()).distinct().count() != 1)
throw new IllegalArgumentException("could not match one exact element");
To get the value
String distrinctVal = fields.get(0).getField();

Well you could certainly do this in several ways but as to which is more elegant can vary from person to person.
Anyway if you were to attempt this via streams this is how i would have done it:
With a slight modification to my answer here you could do:
boolean result = fields.stream()
.map(f -> f.getField())
.distinct()
.limit(2) // ENABLE SHORT CIRCUITING
.count() != 1;
if (result) throw new IllegalArgumentException("could not match one exact element");
String distinctVal = fields.get(0).getField();
The benefit of this approach is basically utilising limit(2) to enable optimisation where possible.
Conclusion : your current approach is quite good actually so I wouldn't be surprised if you were to stick to that but you also have the choice of this approach where you can short-circuit the pipeline.

That would be the reduce solution.
Optional<String> distinctVal = fields.stream()
.map(f -> f.getField())
.reduce((a, b) -> {
if(a != b) throw new IllegalArgumentException("could not match one exact element");
return a;
});

Depending on the frequency of invocation and the size of your set, the iterative code can be significantly faster.
public boolean allEqual(Collection<Fields> fields) {
if (fields.size() > 1) {
String last;
boolean first = true;
for (Field field : fields) {
String thisString = field.getField();
if (first) {
last = thisString;
} else {
if (!StringUtils.equals(last, thisString)) {
return false;
}
}
}
}
return true;
}
While this is not a streaming solution, it aborts when the first mismatch is found, which - depending on the input - can be significantly faster.

Similar to this:
String distrinctVal = fields.stream()
.map(f -> f.getField())
.reduce((a, b)
-> { throw new IllegalArgumentException("could not match one exact element");}
).get();

As others have said, it’s largely a matter of taste. Here’s mine.
String distinctVal = fields.iterator().next().getField();
if (fields.stream().map(Field::getField).anyMatch(e -> ! e.equals(distinctVal)) {
throw new IllegalArgumentException("could not match one exact element");
}
//continue to use the value
(Code is not tested; forgive typos.)
I didn’t particularly code with performance efficiency in mind, but the code will only search until the first non-matching string, so should be efficient.

Related

see if all elements in a Java collection end with one of the members of a List

I have a List of File in Java 11.
I want to check in a one-liner solution if all the files end with one of the extensions listed in a collection.
So I have
List<File> filesInOutput
and
List<String> wantedExtensions
with elements ".html" and ".png".
I want to check whether all the files in filesInOutput end with either ".html" or ".png", if the filesInOutput contains a file ending with ".pdf", for example, I want to return false.
I have done this code:
boolean allMatch = true;
for(File fileInOutput : filesInOutput) {
boolean matches = false;
for(String wantedExtension : wantedExtensions) {
matches = fileInOutput.getPath().endsWith(wantedExtension);
if (matches) {
break;
}
}
if (!matches) {
allMatch = false;
break;
}
}
return allMatch;
Ideally I would like to do this with filesInOutput.stream().filter()... in a one-line solution, but the fact that the extensions we admit are in a collection makes this solution harder.
Still a double loop, but a lambda :)
Set<String> extensions = new HashSet<>(wantedExtensions);
filesInOutput.stream()
.map(file -> file.getPath())
.allMatch(filePath ->
extensions.stream()
.anyMatch(filePath::endsWith));
Of course you want something like this:
Set<String> extensions = new HashSet<>(wantedExtensions);
filesInOutput.stream()
.map(file -> getExtension(file.getPath()))
.allMatch(extensions::contains);
You just need to come up with a method to get the extension. If you search you can find some options on SO using regex or the approach in the other answer.
I haven't run this myself, and maybe this can be improved, but I think this should work, shouldn't it?
Boolean allMatch = filesInOutput.stream().map(file -> file.getName().substring(file.getName().lastIndexOf("."))).allMatch(name -> wantedExtensions.contains(name));
Streams conveniently give us an allMatch operator

How to convert the following code to Java 8 streams and lambdas

I have a complicated requirement where a list records has comments in it. We have a functionality of reporting where each and every change should be logged and reported. Hence as per our design, we create a whole new record even if a single field has been updated.
Now we wanted to get history of comments(reversed sorted by timestamp) stored in our db. After running query I got the list of comments but it contains duplicate entries because some other field was changed. It also contains null entries.
I wrote the following code to remove duplicate and null entries.
List<Comment> toRet = new ArrayList<>();
dbCommentHistory.forEach(ele -> {
//Directly copy if toRet is empty.
if (!toRet.isEmpty()) {
int lastIndex = toRet.size() - 1;
Comment lastAppended = toRet.get(lastIndex);
// If comment is null don't proceed
if (ele.getComment() == null) {
return;
}
// remove if we have same comment as last time
if (StringUtils.compare(ele.getComment(), lastAppended.getComment()) == 0) {
toRet.remove(lastIndex);
}
}
//add element to new list
toRet.add(ele);
});
This logic works fine and have been tested now, But I want to convert this code to use lambda, streams and other java 8's feature.
You can use the following snippet:
Collection<Comment> result = dbCommentHistory.stream()
.filter(c -> c.getComment() != null)
.collect(Collectors.toMap(Comment::getComment, Function.identity(), (first, second) -> second, LinkedHashMap::new))
.values();
If you need a List instead of a Collection you can use new ArrayList<>(result).
If you have implemented the equals() method in your Comment class like the following
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
return Objects.equals(comment, ((Comment) o).comment);
}
you can just use this snippet:
List<Comment> result = dbCommentHistory.stream()
.filter(c -> c.getComment() != null)
.distinct()
.collect(Collectors.toList());
But this would keep the first comment, not the last.
If I'm understanding the logic in the question code you want to remove consecutive repeated comments but keep duplicates if there is some different comment in between in the input list.
In this case a simply using .distinct() (and once equals and hashCode) has been properly defined, won't work as intended as non-consecutive duplicates will be eliminated as well.
The more "streamy" solution here is to use a custom Collector that when folding elements into the accumulator removes the consecutive duplicates only.
static final Collector<Comment, List<Comment>, List<Comment>> COMMENT_COLLECTOR = Collector.of(
ArrayDeque::new, //// supplier.
(list, comment) -> { /// folder
if (list.isEmpty() || !Objects.equals(list.getLast().getComment(), comment.getComment()) {
list.addLast(comment);
}
}),
(list1, list2) -> { /// the combiner. we discard list2 first element if identical to last on list1.
if (list1.isEmpty()) {
return list2;
} else {
if (!list2.isEmpty()) {
if (!Objects.equals(list1.getLast().getComment(),
list2.getFirst().getComment()) {
list1.addAll(list2);
} else {
list1.addAll(list2.subList(1, list2.size());
}
}
return list1;
}
});
Notice that Deque (in java.util.*) is an extended type of List that have convenient operations to access the first and last element of the list. ArrayDeque is the nacked array based implementation (equivalent to ArrayList to List).
By default the collector will always receive the elements in the input stream order so this must work. I know it is not much less code but it is as good as it gets. If you define a Comment comparator static method that can handle null elements or comment with grace you can make it a bit more compact:
static boolean sameComment(final Comment a, final Comment b) {
if (a == b) {
return true;
} else if (a == null || b == null) {
return false;
} else {
Objects.equals(a.getComment(), b.getComment());
}
}
static final Collector<Comment, List<Comment>, List<Comment>> COMMENT_COLLECTOR = Collector.of(
ArrayDeque::new, //// supplier.
(list, comment) -> { /// folder
if (!sameComment(list.peekLast(), comment) {
list.addLast(comment);
}
}),
(list1, list2) -> { /// the combiner. we discard list2 first element if identical to last on list1.
if (list1.isEmpty()) {
return list2;
} else {
if (!sameComment(list1.peekLast(), list2.peekFirst()) {
list1.addAll(list2);
} else {
list1.addAll(list2.subList(1, list2.size());
}
return list1;
}
});
----------
Perhaps you would prefer to declare a proper (named) class that implements the Collector to make it more clear and avoid the definition of lambdas for each Collector action. or at least implement the lambdas passed to Collector.of by static methods to improve readability.
Now the code to do the actual work is rather trivial:
List<Comment> unique = dbCommentHistory.stream()
.collect(COMMENT_COLLECTOR);
That is it. However if it may become a bit more involved if you want to handle null comments (element) instances. The code above already handles the comment's string being null by considering it equals to another null string:
List<Comment> unique = dbCommentHistory.stream()
.filter(Objects::nonNull)
.collect(COMMENT_COLLECTOR);
Your code can be simplified a bit. Notice that this solution does not use stream/lambdas but it seems to be the most succinct option:
List<Comment> toRet = new ArrayList<>(dbCommentHistory.size());
Comment last = null;
for (final Comment ele : dbCommentHistory) {
if (ele != null && (last == null || !Objects.equals(last.getComment(), ele.getComment()))) {
toRet.add(last = ele);
}
}
The outcome is not exactly the same as the question code as in the latter null elements might be added to the toRet but it seems to me that you actually may want to remove the completely instead. Is easy to modify the code (make it a bit longer) to get the same output though.
If you insist in using a .forEach that would not be that difficult, in that case last whould need to be calculated at the beggining of the lambda. In this case you may want to use a ArrayDeque so that you can coveniently use peekLast:
Deque<Comment> toRet = new ArrayDeque<>(dbCommentHistory.size());
dbCommentHistory.forEach( ele -> {
if (ele != null) {
final Comment last = toRet.peekLast();
if (last == null || !Objects.equals(last.getComment(), ele.getComment())) {
toRet.addLast(ele);
}
}
});

Kotlin Destructuring when/if statement

So I have a String i would want to check if I should split into two, or return some default value. Like this:
val myString = "firstPart-secondPart"
val (first, second) = when (myString.contains("-")) {
true -> myString.split('-', limit = 2)
else -> ?? <-- How would i return ("Default1", "Default2") so the destructuring still works?
}
So my question is, how do i return two default strings, so that the deconstructing works? I've used String.split() before in order to deconstruct and it's really nice.
How to return 2 values for destructuring
You need to return a type matching the above type, split returns a list, so you could use this:
listOf("Default1", "Default2")
Full code
val myString = "firstPart-secondPart"
val (first, second) = when (myString.contains("-")) {
true -> myString.split('-', limit = 2)
else -> listOf("Default1", "Default2")
}
Why this works
As both branches return List<String> you can treat the whole when block as a List<String>, so it can be destructured to get the values from it.
Possible cleanup
val myString = "firstPart-secondPart"
val (first, second) = when {
myString.contains("-") -> myString.split('-', limit = 2)
else -> listOf("Default1", "Default2")
}
This may make more sense, assuming you are going to add more conditions, otherwise an if may make more sense.
As an alternative to the good and correct answer of jrtapsell, you could use destructured Pairs:
val (first, second) = when (myString.contains("-")) {
true -> myString.split('-', limit = 2).let { it[0] to it[1] }
else -> "Default1" to "Default2"
}
Note 1: The resulting list with two elements is transformed to a Pair with the help of let.
Note 2: The infix function to is used to create Pairs here.

Limit a stream and find out if there are pending elements

I have the following code that I want to translate to Java 8 streams:
public ReleaseResult releaseReources() {
List<String> releasedNames = new ArrayList<>();
Stream<SomeResource> stream = this.someResources();
Iterator<SomeResource> it = stream.iterator();
while (it.hasNext() && releasedNames.size() < MAX_TO_RELEASE) {
SomeResource resource = it.next();
if (!resource.isTaken()) {
resource.release();
releasedNames.add(resource.getName());
}
}
return new ReleaseResult(releasedNames, it.hasNext(), MAX_TO_RELEASE);
}
Method someResources() returns a Stream<SomeResource> and ReleaseResult class is as follows:
public class ReleaseResult {
private int releasedCount;
private List<String> releasedNames;
private boolean hasMoreItems;
private int releaseLimit;
public ReleaseResult(List<String> releasedNames,
boolean hasMoreItems, int releaseLimit) {
this.releasedNames = releasedNames;
this.releasedCount = releasedNames.size();
this.hasMoreItems = hasMoreItems;
this.releaseLimit = releaseLimit;
}
// getters & setters
}
My attempt so far:
public ReleaseResult releaseReources() {
List<String> releasedNames = this.someResources()
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
return new ReleasedResult(releasedNames, ???, MAX_TO_RELEASE);
}
The problem is that I can't find a way to know if there are pending resources to process. I've thought of using releasedNames.size() == MAX_TO_RELEASE, but this doesn't take into account the case where the stream of resources has exactly MAX_TO_RELEASE elements.
Is there a way to do the same with Java 8 streams?
Note: I'm not looking for answers like "you don't have to do everything with streams" or "using loops and iterators is fine". I'm OK if using an iterator and a loop is the only way or just the best way. It's just that I'd like to know if there's a non-murky way to do the same.
Since you don’t wanna hear that you don’t need streams for everything and loops and iterators are fine, let’s demonstrate it by showing a clean solution, not relying on peek:
public ReleaseResult releaseReources() {
return this.someResources()
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE+1)
.collect(
() -> new ReleaseResult(new ArrayList<>(), false, MAX_TO_RELEASE),
(result, resource) -> {
List<String> names = result.getReleasedNames();
if(names.size() == MAX_TO_RELEASE) result.setHasMoreItems(true);
else {
resource.release();
names.add(resource.getName());
}
},
(r1, r2) -> {
List<String> names = r1.getReleasedNames();
names.addAll(r2.getReleasedNames());
if(names.size() > MAX_TO_RELEASE) {
r1.setHasMoreItems(true);
names.remove(MAX_TO_RELEASE);
}
}
);
}
This assumes that // getters & setters includes getters and setters for all non-final fields of your ReleaseResult. And that getReleasedNames() returns the list by reference. Otherwise you would have to rewrite it to provide a specialized Collector having special non-public access to ReleaseResult (implementing another builder type or temporary storage would be an unnecessary complication, it looks like ReleaseResult is already designed exactly for that use case).
We could conclude that for any nontrivial loop code that doesn’t fit into the stream’s intrinsic operations, you can find a collector solution that basically does the same as the loop in its accumulator function, but suffers from the requirement of always having to provide a combiner function. Ok, in this case we can prepend a filter(…).limit(…) so it’s not that bad…
I just noticed, if you ever dare to use that with a parallel stream, you need a way to reverse the effect of releasing the last element in the combiner in case the combined size exceeds MAX_TO_RELEASE. Generally, limits and parallel processing never play well.
I don't think there's a nice way to do this. I've found a hack that does it lazily. What you can do is convert the Stream to an Iterator, convert the Iterator back to another Stream, do the Stream operations, then finally test the Iterator for a next element!
Iterator<SomeResource> it = this.someResource().iterator();
List<String> list = StreamSupport.stream(Spliterators.spliteratorUnknownSize(it, Spliterator.ORDERED), false)
.filter(resource -> !resource.isTaken())
.limit(MAX_TO_RELEASE)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
return new ReleaseResult(list, it.hasNext(), MAX_TO_RELEASE);
The only thing I can think of is
List<SomeResource> list = someResources(); // A List, rather than a Stream, is required
List<Integer> indices = IntStream.range(0, list.size())
.filter(i -> !list.get(i).isTaken())
.limit(MAX_TO_RELEASE)
.collect(Collectors.toList());
List<String> names = indices.stream()
.map(list::get)
.peek(SomeResource::release)
.map(SomeResource::getName)
.collect(Collectors.toList());
Then (I think) there are unprocessed elements if
names.size() == MAX_TO_RELEASE
&& (indices.isEmpty() || indices.get(indices.size() - 1) < list.size() - 1)

How should I check whether a Stream<T> is sorted?

With an Iterable<T>, it's easy:
T last = null;
for (T t : iterable) {
if (last != null && last.compareTo(t) > 0) {
return false;
}
last = t;
}
return true;
But I can't think of a clean way to do the same thing for a Stream<T> that avoids consuming all the elements when it doesn't have to.
There are several methods to iterate over the successive pairs of the stream. For example, you can check this question. Of course my favourite method is to use the library I wrote:
boolean unsorted = StreamEx.of(sourceStream)
.pairMap((a, b) -> a.compareTo(b) > 0)
.has(true);
It's short-circuit operation: it will finish as soon as it find the misorder. Also it works fine with parallel streams.
This is a sequential, state holding solution:
IntStream stream = IntStream.of(3, 3, 5, 6, 6, 9, 10);
final AtomicInteger max = new AtomicInteger(Integer.MIN_VALUE);
boolean sorted = stream.allMatch(n -> n >= max.getAndSet(n));
Parallelizing would need to introduce ranges. The state, max might be dealt with otherwise, but the above seems most simple.
You can grab the Stream's underlying spliterator and check it it has the SORTED characteristic. Since it's a terminal operation, you can't use the Stream after (but you can create another one from this spliterator, see also Convert Iterable to Stream using Java 8 JDK).
For example:
Stream<Integer> st = Stream.of(1, 2, 3);
//false
boolean isSorted = st.spliterator().hasCharacteristics(Spliterator.SORTED);
Stream<Integer> st = Stream.of(1, 2, 3).sorted();
//true
boolean isSorted = st.spliterator().hasCharacteristics(Spliterator.SORTED);
My example shows that the SORTED characteristic appears only if you get the Stream from a source's that reports the SORTED characteristic or you call sorted() at a point on the pipeline.
One could argue that Stream.iterate(0, x -> x + 1); creates a SORTED stream, but there is no knowledge about the semantic of the function applied iteratively. The same applies for Stream.of(...).
If the pipeline is infinite then it's the only way to know. If not, and that the spliterator does not report this characteristic, you'd need to go through the elements and see if it does not satisfy the sorted characteristic you are looking for.
This is what you already done with your iterator approach but then you need to consume some elements of the Stream (in the worst case, all elements). You can make the task parallelizable with some extra code, then it's up to you to see if it's worth it or not...
You could hijack a reduction operation to save the last value and compare it to the current value and throw an exception if it isn't sorted:
.stream().reduce((last, curr) -> {
if (((Comparable)curr).compareTo(last) < 0) {
throw new Exception();
}
return curr;
});
EDIT: I forked another answer's example and replaced it with my code to show it only does the requisite number of checks.
http://ideone.com/ZMGnVW
You could use allMatch with a multi-line lambda, checking the current value against the previous one. You'll have to wrap the last value into an array, though, so the lambda can modify it.
// infinite stream with one pair of unsorted numbers
IntStream s = IntStream.iterate(0, x -> x != 1000 ? x + 2 : x - 1);
// terminates as soon as the first unsorted pair is found
int[] last = {Integer.MIN_VALUE};
boolean sorted = s.allMatch(x -> {
boolean b = x >= last[0]; last[0] = x; return b;
});
Alternatively, just get the iterator from the stream and use a simple loop.
A naive solution uses the stream's Iterator:
public static <T extends Comparable<T>> boolean isSorted(Stream<T> stream) {
Iterator<T> i = stream.iterator();
if(!i.hasNext()) return true;
T current = i.next();
while(i.hasNext()) {
T next = i.next();
if(current == null || current.compareTo(next) > 0) return false;
current = next;
}
return true;
}
Edit: It would also be possible to use a spliterator to parallelize the task, but the gains would be questionable and the increase in complexity is probably not worth it.
I don't know how good it is , but i have just got an idea:
Make a list out of your Stream , Integer or Strings or anything.
i have written this for a List<String> listOfStream:
long countSorted = IntStream.range(1, listOfStream.size())
.map(
index -> {
if (listOfStream.get(index).compareTo(listOfStream.get(index-1)) > 0) {
return 0;
}
return index;
})
.sum();

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