检查列表是否为HashMap中的值

Question

我想检查一个值是否存在。

我的代码：

public static final Map<String, List<String>> m = new HashMap<>();

if(m.containsValue("gummi")) {...}

我的问题是值是一个列表。如何检查清单？

Answer 1

So I am going to interpret the question, as wanting to tell whether any value of the Map contains the target String.

There's probably a more appropriate third-party bi-direction multimap implementation. If you are reading this in the future such a type may be in Java SE - please edit this answer.

Given the data structures, this is going to require searching through the entire data. It may be better to have Map mapping the other way. Let's ignore that.

The streams one-liner expression solution:

m.values().stream().anyMatch(list -> list.contains("gummi"))

Edit: For Horse Voice, a non-stream version.

flatContains(m.values(), "gummi")

where

public static boolean flatContains(
    Iterable<? extends Collection<?>> collections,
    Object value
) {
    for (Collection<?> collection : collections) {
        if (collection.contains(value)) {
            return true;
        }
    }
    return false;
}

Often avoiding streams makes for more readable code (and faster). However, in this case, I would go straight for the streams. If flatMap was involved, the choice would be much closer.

Answer 2

引用List.equalsJavadoc：

换句话说，如果两个列表包含相同顺序的相同元素，则它们被定义为相等。

因此，要检查某个值是否存在，您可以创建一个列表并调用containsValue该列表List，因为该方法将调用equals以确定相等性。如果相同的元素具有相同的顺序，则会找到匹配项。

List您可以使用以下命令轻松地从值数组创建Arrays.asLista ：

if(m.containsValue(Arrays.asList("gummi"))) {...}

示例代码：

Map<String, List<String>> m = new HashMap<>();
List<String> list = new ArrayList<>();

list.add("1");
list.add("2");
m.put("key", list);

if (m.containsValue(Arrays.asList("1", "2"))) {
    System.out.println("contains value");
}