CODEWITHSUNDEEP
javamemorymemory-managementcollections
Deca
Deca

Reputation: 1225

Java HashMap to store in different buckets internally

I have couple of scenarios related to storing of HashMap, which I am not aware how to accomplish.

Case 1: As there are buckets on which objects are saved, and hashcode will be taken into consideration while saving it. Now say, there are 5 buckets and I want to have my own control on which bucket to save it. Is there a way to achieve it? Say, By internal mechanism, it was going to be saved into bucket 4, but I wanted to save that particular object into bucket 1.

Case 2: Similarly, If I see that out of 5 buckets, 1 bucket was getting much more load than other, and I want to do a load balancing kind of job by moving it to different buckets. How can that be accomplished?

Upvotes: 0

Views: 866

Answers (4)

Corey Berigan
Corey Berigan

Reputation: 634

The underlying mechanism for bucket creation and placement are abstracted away. For case 1, you can simply use objects as your keys for the bucket placement. For case 2, you cannot see the actual placement of objects directly.

Although, what you can do is use a Multimap which you can treat the keys as if they were buckets. It's basically a map from keys to collections. Here you can check any given key(bucket) and see how many items you have placed in there. Here you can satisfy requirements from both cases. This is probably as close as you're going to get without actually tampering with the internal bucketing mechanism.

From the link, here is a snippet:

public class MutliMapTest {
    public static void main(String... args) {
  Multimap<String, String> myMultimap = ArrayListMultimap.create();

  // Adding some key/value
  myMultimap.put("Fruits", "Bannana");
  myMultimap.put("Fruits", "Apple");
  myMultimap.put("Fruits", "Pear");
  myMultimap.put("Vegetables", "Carrot");

  // Getting the size
  int size = myMultimap.size();
  System.out.println(size);  // 4

  // Getting values
  Collection<string> fruits = myMultimap.get("Fruits");
  System.out.println(fruits); // [Bannana, Apple, Pear]

  Collection<string> vegetables = myMultimap.get("Vegetables");
  System.out.println(vegetables); // [Carrot]

  // Iterating over entire Mutlimap
  for(String value : myMultimap.values()) {
   System.out.println(value);
  }

  // Removing a single value
  myMultimap.remove("Fruits","Pear");
  System.out.println(myMultimap.get("Fruits")); // [Bannana, Pear]

  // Remove all values for a key
  myMultimap.removeAll("Fruits");
  System.out.println(myMultimap.get("Fruits")); // [] (Empty Collection!)
 }

Upvotes: 1

YamYamm
YamYamm

Reputation: 401

With HashMap and with all Collections whose names start with Hash the more important part is the hasCode generated by the domain object that you are trying to store. That's why every object has a hashCode implementation(implicity with object.hashCode() or explicitely).

First of all HashMap tries to accomplish what you stated in case 2(sort of). If your hashCode implementation is good, meaning can produce evenly dispersed hashCode values for variety of objects than load of the buckets of HashMap is more or less evenly distributed, and you don't have to anything(other than writing a good hashCode function.). Also you can somehow manupulate the balance by implementing your hascode accordingly by producing same hashcode for objects that you want them to be in the same bucket.

If you want to have complete control on the internals of the hashMap than you should implement your own HashMap by implementing Map interface.

Upvotes: 1

Marko Topolnik
Marko Topolnik

Reputation: 200148

There is fundamentally no way to achieve load balancing in a hashtable. The quintessential property of this structure is direct access to exactly the bucket which must hold the requested key. Any balancing scheme would involve reshuffling the objects among buckets and destroy this property. This is the reason why good-quality hashcodes are vital to the proper operation of a hashtable.

Additionally note that you can't even control bucket selection by manipulating the hashCode() method of your objects, because hashcodes of any two equal objects must match, and because any self-respecting hashtable implementation will additionally shuffle the bits of the value retrieved from hashCode() to ensure better dispersion.

Upvotes: 2

sdgfsdh
sdgfsdh

Reputation: 37045

The implementations are designed so that you shouldn't have to worry about these details.

If you wanted to control these more carefully, then you can create your own class implementing Map.

Upvotes: 1

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