Question

In Java, how can I create an array where each element is a list of objects containing a pair of keys.

Then given an index of this array, I should be able to search the list at the index provided a single key value and retrieve the corresponding value.

Problem Statement

I have around 2 million webpage name and corresponding to each name are categories (one or more) .

I can hash them down to say 1,00,000 using some hash function but obviously their will be collisions.

I want to build a data structure in Java that can store all this information, and then given a webpage and then, after hashing, given the index, I can find the categories of the web page.

Answer 1

It sounds like what you want is an array of maps:

List<Map<KeyType, ValueType>> myList = new ArrayList<Map<KeyType, ValueType>>();

This is how you add a map:

myList.add(new HashMap<KeyType, ValueType>());

This is how you add value to the map at index i with key as key:

myList.get(i).put(key, value);

To search for key in the map at index i use:

ValueType value = myList.get(i).get(key);

Edit: After your edit I think that the solution to your problem is to simply use HashMap<String, List<String>>. You’d use the name of the webpage as key and the list of category-names as values. There is no need to implement your own hash-map (it sounds like that was what you were doing).