Creating an unordered map of <char, int> in java

Question

So I need to have a some sort of multiset of characters, where adding a duplicate character increases the cardinality by 1, and the multiplicity of characters should not drastically increase the memory that the object takes up.

This will be implemented with some sort of map where characters are keys, that hold a value representing the number of that character is represented in the set.

However, I'm struggling to figure out which collection would be best for this (I was looking at hashmap) and how to declare this data type. I was doing something like this

Map m = new HashMap(char, int);

But the above is an incorrect declaration, and I'm not sure how to exactly approach this.

Answer 1

Try this declaration:

Map<Character, Integer> m = new HashMap<Character, Integer>();

You can then add characters as such:

char c = //...;
if (m.containsKey(c))
    m.put(c, m.get(c) + 1);
else
    m.put(c, 1);

Answer 2

I would implement it using int[] (for ASCII) or int[][] for unicode. Given the memory footprint of storing boxed integers in a hash map with all the hashing conflicts introduced by using numbers, and characters are nothing but numbers, as keys.

public class CharacterBag {

    private int[][] data = new int[255][];

    public void add(char ch) {
        int[] bin = data[ch >> 8];
        if (bin == null) bin = data[ch >> 8] = new int[255];
        bin[ch & 0xFF]++;
    }

    public int frequency(char ch) {
        int[] bin = data[ch >> 8];
        if (bin == null) return 0;
        return bin[ch & 0xFF];
    }

}

This starts with a memory footprint of zero, and adds 2K for each unicode page. Typically text uses characters from one or two unicode pages.

In comparison using HashMap will have the whole overhead of storing boxed primitives in a list of linked lists. For each character there will an object of the Entry class with two pointers pointing to a boxed key and a linked list object with all its fields and which holds an object of class Node with a forward and a backward pointer and which has a pointer to the boxed integer … shall I go on? ;)

Answer 3

Map<Character, Integer> charCount = new HashMap<Character, Integer>();

public void addCharacter(char c) {
    Integer value = charCount.get(c);
    if (value == null) {
        charCount.put(c, 1);
    } else {
        charCount.put(c, value + 1);
    }
}