1. Overview
In our previous tutorial, A Guide to Java HashMap, we showed how to use HashMap in Java.
In this short tutorial, we’ll learn how to get a submap from a HashMap based on a list of keys.
2. Use Java 8 Stream
For example, suppose we have a HashMap and a list of keys:
Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "B");
map.put(3, "C");
map.put(4, "D");
map.put(5, "E");
List<Integer> keyList = Arrays.asList(1, 2, 3);
We can use Java 8 streams to get a submap based on keyList:
Map<Integer, String> subMap = map.entrySet().stream()
.filter(x -> keyList.contains(x.getKey()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
System.out.println(subMap);
The output will look like this:
{1=A, 2=B, 3=C}
3. Use retainAll() Method
We can get the map’s keySet and use the retainAll() method to remove all entries whose key is not in keyList:
map.keySet().retainAll(keyList);
Note that this method will edit the original map. If we don’t want to affect the original map, we can create a new map first using a copy constructor of HashMap:
Map<Integer, String> newMap = new HashMap<>(map);
newMap.keySet().retainAll(keyList);
System.out.println(newMap);
The output is the same as above.
4. Conclusion
In summary, we’ve learned two methods to get a submap from a HashMap based on a list of keys.
