How to get the key from a value in a HashMap in Java?
The answer is to find key-value pairs by looping through the entrySet or using a Stream. First obtain the entrySet of the HashMap, then iterate each entry and compare whether the value matches the target value. If it matches, return the corresponding key; if there are multiple matching values, all matching keys can be stored in a list; you can also use the Java 8 Stream API to implement a concise search, but the time complexity is O(n). For frequent reverse queries, it is recommended to use a bidirectional mapping such as Guava's BiMap.
There's no direct method in Java's HashMap to get a key from a value, because HashMap is designed for fast lookups of values by keys, not the reverse. But you can find the key associated with a specific value by iterating through the map entries.
Iterate Through Entry Set
To retrieve a key based on its value, loop over the map's entry set and check each value:
- Use entrySet() to access key-value pairs.
- Check if the value matches your target.
- Return the corresponding key when found.
Example:
HashMap<string integer> map = new HashMap();
map.put("apple", 1);
map.put("banana", 2);
map.put("cherry", 3);
Integer targetValue = 2;
String key = null;
for (Map.Entry<string integer> entry : map.entrySet()) {
if (targetValue.equals(entry.getValue())) {
key = entry.getKey();
break;
}
}
System.out.println(key); // Output: banana
</string></string>Handle Multiple Matching Values
If multiple keys could have the same value and you want all matching keys, collect them in a list:
List<string> keys = new ArrayList();
for (Map.Entry<string integer> entry : map.entrySet()) {
if (targetValue.equals(entry.getValue())) {
keys.add(entry.getKey());
}
}
</string></string>Using Java 8 Streams (Single Match)
You can use streams for a more concise approach:
String key = map.entrySet().stream()
.filter(entry -> targetValue.equals(entry.getValue()))
.map(Map.Entry::getKey)
.findFirst()
.orElse(null);
Note: This returns only the first match. Use .collect(Collectors.toList()) if you need all keys.
Finding a key from a value isn't efficient in large maps (O(n) time), so consider using a bidirectional map (like BiMap from Google Guava) if you frequently need reverse lookups.
Basically just loop through or stream the entries until you find the value you're looking for.
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