How to Get All Keys From Map With a Value in Java
Suppose we want to get all keys from a hashmap that match a certain value in Java.
Let’s start with this hashmap.
Map<String, Integer> map = new HashMap<>();
map.put("corgi", 1);
map.put("pug", 2);
map.put("shih tzu", 3);
map.put("husky", 1);
1. Filtering with the Stream API (Java 8+)#
If we’re using JDK8+, we can use the Stream API to obtain all keys matching a certain value.
List<String> getKeysWithValue(Map<String, Integer> map, Integer value) {
return map
.entrySet()
.stream()
.filter(e -> Objects.equals(e.getValue(), value))
.map(Map.Entry::getKey)
.collect(Collectors.toList());
}
We want to use Objects.equals() for comparisons here because hashmaps can contain null values.
2. Filtering with standard loops#
If we’re using JDK7, we can obtain the same functionality using standard for loops.
List<String> getKeysWithValue(Map<String, Integer> map, Integer value) {
List<String> keys = new ArrayList<String>();
for(String key : map.keySet()) {
if(Objects.equals(map.get(key), value))
keys.add(key);
}
return keys;
}