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

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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 using Stream#allMatch which also is short-circuit (returns false immediately for first element which doesn’t fulfill provided condition)
(assuming non-parallel streams and thread-safe environment)

public static <T> boolean areAllUnique(List<T> list){
    Set<T> set = new HashSet<>();
    return list.stream().allMatch(t -> set.add(t));
}

which can be farther shortened as @Holger pointed out in comment

public static <T> boolean areAllUnique(List<T> list){
    return list.stream().allMatch(new HashSet<>()::add);
}

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