如何改善HashMap数组中的搜索

Question

A search in any HashMap (or any hash based data structure) requires a single hash operation ie O(1). However, when we have to search through an array of HashMaps then a single search will requires O(n) hash operations, where n is the size of array. I was wondering since array is a collections of consecutive memory locations, therefore there could be an efficient method to reduce the search from O(n) to O(1) in array of HashMap. Or we can design some object that in fact have the advantage of array as well as require single hash operation for search. Any suggestion ???

Consider the scenario, You are processing elements from different users and you want to keep them based on user profiles (separate from each other). The most memory efficient way is to keep them separate is using array of HashMap.

Answer 1

The answer for this is in your question. When you're using an ordinary Array to store these elements, there's no way to get O(1) complexity when searching it.

You stated that the complexity of searching a Hash Map is O(1), so why not store those Hash Maps... in a Hash Map? I'll leave the implementation of that up to you, haha.

Answer 2

I've edited your question to include the hypothetical scenario of various user accounts, each with various properties. As mentioned in a different answer, the solution here is a map of maps. There are two separate searches occurring:

1. Find the right user
2. Find the right property of this user

Each of these searches can be done with a separate map.

The user map:

class Application {

    ...

    Map<String, User> userMap = new HashMap<>();

    public User getUser(String userName) {
        return userMap.get(userName);
    }

    ...
}

The property map:

class User {

    ...

    Map<String, Property> propertyMap = new HashMap<>();

    public Property getProperty(String propertyName) {
        return propertyMap .get(propertyName);
    }

    ...
}

Now to find the property named favoriteTowel of the user named Arthur Dent :

myApplication.getUser("Arthur Dent").getProperty("favoriteTowel");

Answer 3

Go Go For java.util packages, Go Go for Java Collections

look at set ( Hashset , enumset ) and hash ( HashMap , linkedhash ..., idnetityhash ..) based implementations. they have O(1) for contains()

Below are stats for reference.