I am given HashMap<K, V>.
How to get list of all keys from it, where corresponding value is assignable from class I?
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Stefan Zobel
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J. Doe
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1Possible duplicate of [How to return a list of keys from a Hash Map?](http://stackoverflow.com/questions/6589744/how-to-return-a-list-of-keys-from-a-hash-map) – Todd Sewell Feb 14 '17 at 21:43
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3Seriously, you need to at least try something. HashMap is well documented, and you should know about loops. – JB Nizet Feb 14 '17 at 21:44
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I know, but aren't there more efficient ways to do this instead of iterating through HashMap? – J. Doe Feb 14 '17 at 21:46
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Sure there are more concise ways, if you use Java-8 streams – Thomas Fritsch Feb 14 '17 at 21:49
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Yes, I can use Java-8 streams. Can you tell me how to do it this way? – J. Doe Feb 14 '17 at 21:53
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3There is a big difference between "efficient" and "concise". Using a stream won't prevent iterating through the entries of the map. – JB Nizet Feb 14 '17 at 21:57
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@JBNizet ok, you are right – Thomas Fritsch Feb 14 '17 at 22:15
3 Answers
3
Using Java 8 you can do
Map<K, V> map = new HashMap<>();
List<K> list = map.entrySet().stream()
.filter(e -> e.getValue() instanceof I)
.map(e -> e.getKey())
.collect(Collectors.toList());
Thomas Fritsch
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where corresponding value is assignable from class I
Do you mean value.getClass().isAssignableFrom(I.class) ? If it's this the case:
Map<K, V> map = ...
List<K> keys = new ArrayList<K>();
for(K key : map.keySet()) {
V value = map.get(key);
if(value.getClass().isAssignableFrom(I.class)) {
keys.add(key);
}
}
//Now you have a list of keys associated to values that are assignable from I.class
Flood2d
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You can filter the values of the Map using Streams.
Map<K, V> map = new HashMap<>();
List<K> collect = map.entrySet()
.stream()
.filter(entry -> I.class.isInstance(entry.getValue()))
.map(Map.Entry::getKey)
.collect(toList());
alayor
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