what is the best way to get a sub HashMap based on a list of Keys?

by

With Java8 streams, there is a functional (elegant) solution. If keys is the list of keys to keep and map is the source Map.

keys.stream()
    .filter(map::containsKey)
    .collect(Collectors.toMap(Function.identity(), map::get));

Complete example:

    List<Integer> keys = new ArrayList<>();
    keys.add(2);
    keys.add(3);
    keys.add(42); // this key is not in the map

    Map<Integer, String> map = new HashMap<>();
    map.put(1, "foo");
    map.put(2, "bar");
    map.put(3, "fizz");
    map.put(4, "buz");

    Map<Integer, String> res = keys.stream()
        .filter(map::containsKey)
        .collect(Collectors.toMap(Function.identity(), map::get));

    System.out.println(res.toString());

Prints: {2=bar, 3=fizz}

EDIT add a filter for keys that are absent from the map

More Related Contents:

  1. How to print all the values for a key in HashMap in java
  2. what is the method to delete hashmap entry
  3. How to check if an integer is in a set of given ranges? [closed]
  4. Java 8 Streams hashmap
  5. How to get percentile in java
  6. How to loop through a HashMap in JSP?
  7. Is the order of values retrieved from a HashMap the insertion order
  8. Java Compare Two Lists
  9. How to preserve insertion order in HashMap? [duplicate]
  10. java.lang.OutOfMemoryError: GC overhead limit exceeded [duplicate]
  11. HashMap Java 8 implementation
  12. Java associative-array
  13. Performance ConcurrentHashmap vs HashMap
  14. Java HashMap performance optimization / alternative
  15. Reverse HashMap keys and values in Java
  16. HashMap to return default value for non-found keys?
  17. ArrayList as key in HashMap
  18. Copying a HashMap in Java
  19. Java HashMap nested generics with wildcards
  20. How to prove that HashMap in java is not thread-safe
  21. Is there a way to get the value of a HashMap randomly in Java?
  22. What exactly is bucket in hashmap?
  23. Changing HashMap keys during iteration
  24. Java HashMap – deep copy
  25. Why Hashtable does not allow null keys or values?
  26. Thread safe Hash Map?
  27. Comparing two hashmaps for equal values and same key sets?
  28. Sorting the Map in descending order based on the value [duplicate]
  29. How do I access nested HashMaps in Java?
  30. Explanation of HashMap#hash(int) method

Leave a Comment