How to optimize the updating of values in an ArrayList<Integer>

I want to store all values of a certain variable in a dataset and the frequency for each of these values. To do so, I use an ArrayList<String> to store the values and an ArrayList<Integer> to store the frequencies (since I can't use int). The number of different values is unknown, that's why I use ArrayList and not Array.

Example (simplified) dataset:

a,b,c,d,b,d,a,c,b

The ArrayList<String> with values looks like: {a,b,c,d} and the ArrayList<Integer> with frequencies looks like: {2,3,2,2}.

To fill these ArrayLists I iterate over each record in the dataset, using the following code.

public void addObservation(String obs){
    if(values.size() == 0){// first value
        values.add(obs);
        frequencies.add(new Integer(1));
        return;//added
    }else{
        for(int i = 0; i<values.size();i++)开发者_开发百科{
            if(values.get(i).equals(obs)){
                frequencies.set(i, new Integer((int)frequencies.get(i)+1));
                return;//added
            }
        }
        // only gets here if value of obs is not found
        values.add(obs);
        frequencies.add(new Integer(1));
    }
}

However, since the datasets I will use this for can be very big, I want to optimize my code, and using frequencies.set(i, new Integer((int)frequencies.get(i)+1)); does not seem very efficient.

That brings me to my question; how can I optimize the updating of the Integer values in the ArrayList?

Use a HashMap<String,Integer>

Create the HashMap like so

HashMap<String,Integer> hm = new HashMap<String,Integer>();

Then your addObservation method will look like

public void addObservation(String obs) {
    if( hm.contains(obs) )
        hm.put( obs, hm.get(obs)+1 );
    else
        hm.put( obs, 1 );
}

I would use a HashMap or a Hashtable as tskzzy suggested. Depending on your needs I would also create an object that has the name, count as well as other metadata that you might need.

So the code would be something like:

Hashtable<String, FrequencyStatistics> statHash = new Hashtable<String, FrequencyStatistics>();
for (String value : values) {
    if (statHash.get(value) == null) {
        FrequencyStatistics newStat = new FrequencyStatistics(value);
        statHash.set(value, newStat);
    } else {
        statHash.get(value).incrementCount();
    }
}

Now, your FrequencyStatistics objects constructor would automatically set its inital count to 1, while the incrementCound() method would increment the count, and perform any other statistical calculations that you might require. This should also be more extensible in the future than storing a hash of the String with only its corresponding Integer.