ArrayList as key in HashMap

by

Yes you can have ArrayLists as a keys in a hash map, but it is a very bad idea since they are mutable.

If you change the ArrayList in any way (or any of its elements), the mapping will basically be lost, since the key won’t have the same hashCode as it had when it was inserted.

The rule of thumb is to use only immutable data types as keys in a hash map. As suggested by Alex Stybaev, you probably want to create a Bigram class like this:

final class Bigram {

    private final String word1, word2;

    public Bigram(String word1, String word2) {
        this.word1 = word1;
        this.word2 = word2;
    }

    public String getWord1() {
        return word1;
    }

    public String getWord2() {
        return word2;
    }

    @Override
    public int hashCode() {
        return word1.hashCode() ^ word2.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        return (obj instanceof Bigram) && ((Bigram) obj).word1.equals(word1)
                                       && ((Bigram) obj).word2.equals(word2);
    }
}

More Related Contents:

  1. How to put HashMap into different elements with same key
  2. Get ArrayList entry of a HashMap Entry [closed]
  3. What are the differences between a HashMap and a Hashtable in Java?
  4. How to loop through a HashMap in JSP?
  5. How can I convert JSON to a HashMap using Gson?
  6. How to sort a HashMap in Java [duplicate]
  7. Fuzzy string search library in Java [closed]
  8. What happens when a duplicate key is put into a HashMap?
  9. Is a HashMap thread-safe for different keys?
  10. How to update a value, given a key in a hashmap?
  11. What is the significance of load factor in HashMap?
  12. Why there is no ConcurrentHashSet against ConcurrentHashMap
  13. Can an array be used as a HashMap key?
  14. Sorting hashmap based on keys
  15. What is the optimal capacity and load factor for a fixed-size HashMap?
  16. HashMap implementation in Java. How does the bucket index calculation work?
  17. HashMap to return default value for non-found keys?
  18. ConcurrentHashMap and Hashtable in Java [duplicate]
  19. HashMap and int as key
  20. Why does HashMap require that the initial capacity be a power of two?
  21. Java: how to convert HashMap to array
  22. Android – Get value from HashMap
  23. get string value from HashMap depending on key name
  24. How to train the Stanford NLP Sentiment Analysis tool
  25. JDK 8 – “The type java.util.Map$Entry cannot be resolved” [duplicate]
  26. How do I create some variable type alias in Java
  27. Theoretical limit for number of keys (objects) that can be stored in a HashMap?
  28. what is the best way to get a sub HashMap based on a list of Keys?
  29. What is the Java equivalent of Objective-C’s NSDictionary?
  30. Is Java HashMap.clear() and remove() memory effective?

Leave a Comment