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

Question

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++){
            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?

Answer 1

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 );
}