CODEWITHSUNDEEP
hashmapjava-8
Senthil Kumaran
Senthil Kumaran

Reputation: 56931

Java 8. Idiomatic way create an indexed map from keyset?

I want to create a new map from the keyset() of an existing HashMap that maps a key to its index in the map.

Here's my attempt, which works:

Map<String, Integer> keysToIndex = new HashMap<>();
Integer index = 0;
for (String obj: hashMap.keySet()) {
    keysToIndex.put(obj, index);
    index += 1;
}

Does Java 8 provide features to create a map of each key's index, or is there a more idiomatic way to write this snippet?

Upvotes: 3

Views: 586

Answers (1)

Bohemian
Bohemian

Reputation: 425198

Sets aren't ordered, so the "index" of an element in a Set is arbitrary (albeit unique). The "index" is simply the current size of the map, so you can use the result map's state to provide the index:

Map<String, Integer> keysToIndex = new HashMap<>();
hashMap.keySet().forEach(k -> keysToIndex.put(k, keysToIndex.size()));

This is more or less what your code is doing, but more compact since no extra variables or instructions are needed.

Incidentally, you can drop one more method call by using forEach() directly on the source map (and ignoring the value argument of the BiConsumer):

Map<String, Integer> keysToIndex = new HashMap<>();
hashMap.forEach((k, v) -> keysToIndex.put(k, keysToIndex.size()));

Upvotes: 3

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