'How to check if exists any duplicate in Java 8 Streams?
In java 8, what's the best way to check if a List contains any duplicate?
My idea was something like:
list.size() != list.stream().distinct().count()
Is it the best way?
Solution 1:[1]
Your code would need to iterate over all elements. If you want to make sure that there are no duplicates simple method like
public static <T> boolean areAllUnique(List<T> list){
Set<T> set = new HashSet<>();
for (T t: list){
if (!set.add(t))
return false;
}
return true;
}
would be more efficient since it can give you false
immediately when first non-unique element would be found.
This method could also be rewritten as (assuming non-parallel streams and thread-safe environment) using Stream#allMatch
which also is short-circuit (returns false immediately for first element which doesn't fulfill provided condition)
public static <T> boolean areAllUnique(List<T> list){
Set<T> set = new HashSet<>();
return list.stream().allMatch(t -> set.add(t));
}
or as @Holger mentioned in comment
public static <T> boolean areAllUnique(List<T> list){
return list.stream().allMatch(new HashSet<>()::add);
}
Solution 2:[2]
I used the following:
1. return list.size() == new HashSet<>(list).size();
.
I'm not sure how it compares to:
2. return list.size() == list.stream().distinct().count();
and
3. return list.stream().sequential().allMatch(new HashSet<>()::add);
in terms of performance.
The last one (#3) has possibility to handle not only collections (e.g. lists), but also streams (without explicitly collecting them).
Upd.: The last one (#3) seems to be the best not only because it can handle pure streams, but also because it stops on the first duplicate (while #1 and #2 always iterate till the end) — as @Pshemo said in comment.
Solution 3:[3]
You can use the counting collector.
Stream.of(1, 3, 4, 6, 7, 5, 6)
.collect(Collectors.groupingBy(
Function.identity(), Collectors.counting()))
.entrySet().stream().anyMatch(e -> e.getValue() > 1)
Solution 4:[4]
Started this class as a StreamTool, but I think there must be an even better way with reduce or similar:
public class StreamTool {
/**
* Whether stream records are unique in that stream.
* @param <T> Type of records
* @param records
* @return true if there are no duplicates, false otherwise
*/
public static <T> boolean isUnique(Stream<T> records) {
return records.allMatch(new HashSet<>()::add);
}
}
Solution 5:[5]
Given array arr,
arr.length != Arrays.stream(arr).distinct().count()
will help check for duplicates
Solution 6:[6]
Use set.add()
it is faster.
Set<T> items = new HashSet<>();
list.stream().filter(n -> !items.add(n))
.collect(Collectors.toSet());
Sources
This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.
Source: Stack Overflow
Solution | Source |
---|---|
Solution 1 | |
Solution 2 | |
Solution 3 | Will Humphreys |
Solution 4 | geekdenz |
Solution 5 | Pshemo |
Solution 6 | itro |