Java 8:使用流检查两个列表中的公共元素 [英] Java 8: check for common elements in two lists using streams

问题描述

根据用户名，我正在寻找一条语句来检查两个用户列表中是否有匹配项.

I'm looking for a statment to check if there is any match in two lists of Users, according to Username.

List<User> a;
List<User> b;
for (User user : a) {
    for (User newUser : b) {
        if (user.getName().equals(newUser.getName())) {
        }
    }
}

如何在Java 8中编写此代码?这样的事情:

How can I write this in java 8? Somthing like this:

List<User> intersect = a.stream()
                    .filter(User::getName)
                    .collect(Collectors.toList());

推荐答案

使用hashCode和equals正确定义了User(否则，您可以尝试使用TreeSet而不是HashSet)，请执行设置操作:

When User is correctly defined with a hashCode and equals (otherwise you might try TreeSet instead of HashSet), do set-operations:

Set<User> common = new HashSet<>(a);
common.retainAll(b);

如果User.getName不用于相等:

Set<User> common = new TreeSet<>(Comparator.comparing(User::getName));
common.addAll(a);
common.retainAll(b);

列表(也作为流)上的两个嵌套的for循环的复杂度为O(N²)， 而这是O(N.log N).

Two nested for loops on lists (also as streams) would have complexity O(N²), whereas this is O(N.log N).

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