如何检查 Java 8 Streams 中是否存在任何重复项? [英] How to check if exists any duplicate in Java 8 Streams?

问题描述

在 java 8 中，检查 List 是否包含任何重复项的最佳方法是什么?

In java 8, what's the best way to check if a List contains any duplicate?

我的想法是这样的:

list.size() != list.stream().distinct().count()

这是最好的方法吗?

推荐答案

您的代码需要遍历所有元素.如果你想确保没有重复的简单方法，比如

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;
}

会更有效率，因为它可以在找到第一个非唯一元素时立即给你 false.

would be more efficient since it can give you false immediately when first non-unique element would be found.

此方法也可以使用 Stream#allMatch 也是短路的(对于第一个元素立即返回 false不满足提供的条件)

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));
}

或作为评论中提到的@Holger

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

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