查找映射是否包含列表/可迭代项中的任何键的有效方法 [英] Efficient way to find if map contains any of the keys from a list/iterable

问题描述

I need to check if a map contains any of the keys from a list, and if it does then return the first matching value. The naive approach that comes to mind is to do it in two nested loops:

Map<String, String> fields = new HashMap<String, String>();
fields.put("a", "value a");
fields.put("z", "value z");
String[] candidates = "a|b|c|d".split("|");
for (String key : fields.keySet()){
    for (String candidate : candidates) {
        if (key.equals(candidate)){
            return fields.get(key);
        }
    }
}

Is there a nicer and more efficient way, possibly one relying on the Java standard library?

解决方案

Surely something like:

for (String candidate : candidates) {
     String result = fields.get(key);
     if (result != null) {
         return result;
     }
}

The above only performs one map lookup per candidate key. It avoids the separate test for presence plus extraction, since extracting a non-existant key will simply give you a null. Note (thanks Slanec) that a null value for a valid key is indistinguishable from a non-existant key for this solution.

I don't quite understand why you're performing the case conversion, btw.

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