How to check if exists any duplicate in Java 8 Streams?

Question

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?

Answer 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);
}