Java反转图

问题描述

我需要创建逆映射 - 选择唯一值并为它们找到键.似乎唯一的方法是迭代所有键/​​值对，因为 entrySet 返回一组不唯一的值?谢谢.

I need create inverse map - select unique values and for them find keys.Seems that only way is to iterate all key/value pairs, because entrySet returns set of so value not unique?Thanks.

推荐答案

地图中的值可能不是唯一的.但是，如果它们是(在您的情况下)，您可以按照您在问题中所写的那样做，并创建一个通用方法来转换它:

The values in a map may not be unique. But if they are (in your case) you can do as you wrote in your question and create a generic method to convert it:

private static <V, K> Map<V, K> invert(Map<K, V> map) {

    Map<V, K> inv = new HashMap<V, K>();

    for (Entry<K, V> entry : map.entrySet())
        inv.put(entry.getValue(), entry.getKey());

    return inv;
}

Java 8:

public static <V, K> Map<V, K> invert(Map<K, V> map) {
    return map.entrySet()
              .stream()
              .collect(Collectors.toMap(Entry::getValue, Entry::getKey));
}

用法示例:

public static void main(String[] args) {

    Map<String, Integer> map = new HashMap<String, Integer>();

    map.put("Hello", 0);
    map.put("World!", 1);

    Map<Integer, String> inv = invert(map);

    System.out.println(inv); // outputs something like "{0=Hello, 1=World!}"
}

旁注:put(.., ..) 方法将返回键的旧"值.如果它不为空，您可能会抛出一个 new IllegalArgumentException("Map values must be unique") 或类似的东西.

---

Side note: the put(.., ..) method will return the the "old" value for a key. If it is not null you may throw a new IllegalArgumentException("Map values must be unique") or something like that.

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