Efficient way to find if map contains any of the keys from a list/iterable

Question

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?

Answer 1

for(String candidate : candidates) {
 if(fields.containsKey(candidate)) {
  return fields.get(candidate)
 }
}

the best way if null values are possibly in map, and if only the first detected key is required.

Answer 2

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.

Answer 3

In Java 8 you can have this:

boolean exists = Arrays.stream(candidates).anyMatch(fields::containsKey);

If you just want to know if any of candidates is key to the map.

If you want to know the first or any you can use:

Arrays.stream(candidates).filter(fields::containsKey).findAny();

or

Arrays.stream(candidates).filter(fields::containsKey).findFirst();

As per @Klapsa2503 answer above

Answer 4

My take:

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 candidate : candidates) {
    if (fields.containsKey(candidate)) {
        return fields.get(candidate);
    }
}

Answer 5

In Java 8 you can use this:

return candidates.stream()
            .filter(fields::containsKey)
            .findFirst()
            .map(fields::get)
            .orElse(null);