Question

I’m trying to create a simple class to read a csv file and store the contents in an

ArrayList<ArrayList<T>>.  

I’m creating a generic class CsvReader so that I can handle data of different types: int, double, String. If I had, say, a csv file of doubles, I was imagining I would use my class like this:

//possible method 1
CsvReader<Double> reader = new CsvReader<Double>();
ArrayList<ArrayList<Double>> contents = reader.getContents();

//possible method 2
CsvReader reader = new CsvReader(Double.class);
ArrayList<ArrayList<Double>> contents = reader.getContents();

But method 1 won’t work since type erasure prevents you from writing code like

rowArrayList.add(new T(columnStringValue)); 

But I can’t even make the passing in a Double.class solution work. The problem is that what’s really going on is that I need my class “parameterized” (in the general sense of that word, not the technical java generics sense) on a type with the following property: it has a ctor accepting a single String argument. That is, to create the row ArrayLists on, say, a Double csv file, I’d need to write:

StringTokenizer st = new StringTokenizer(line,",");
ArrayList<Double> curRow = new ArrayList<Double>();
while (st.hasMoreTokens()) {
 curRow.add(new Double(st.nextToken());
}

Having passed in Double.class, I could get its String ctor using

  Constructor ctor = c.getConstructor(new Class[] {String.class});

but this has two problems. Most importantly, this is a general constructor that will return a type Object, which I cannot then downcast into a Double. Second, I would be missing “type” checking on the fact that I am requiring my passed in class to have a String arg constructor.

My question is: How can I properly implement this general purpose CsvReader?

Thanks,
Jonah

Answer 1

I’m not sure a generic CSV reader would be this simple to use (and to create, by the way).

The first question that comes to my mind is: What if the CSV contains three columns: first an integer, then a string and finally a date? How would you use your generic CSV reader?

Anyway, lets suppose you want to create a CSV reader where all columns are of the same type. As you said, you can’t parametrize a class on a type “that accepts a String as constructor”. Java just doesn’t allow that. The solution using reflection is a good start. But what if your class doesn’t take a String as parameter in one of its constructors?

Here you can come with an alternative: a parser that would take your String and return an object of the correct type. Create a generic interface, and make some implementations for the type you want to crawl:

public interface Parser<T> {

    T parse(String value);

}

And then, implement:

public class StringParser implements Parser<String> {

    public String parse(String value) {
        return value;
    }

}

Then, you CSV reader can take a Parser as one of its parameters. Then, it can use this parser to convert each String into a Java object.

With this solution, you get rid of the not-so-pretty reflection your where using. And you can convert to any type, you just have to implement a Parser.

Your reader will look like this:

public CSVReader<T> {

    Parser<T> parser;

    List<T> getValues() {
        // ...
    }

}

Now, back at the problem where a CSV file can have multiple types, just improve your reader a little. All you need is a list of parsers (one per column) instead of one that parse all columns.

Hope that helps 🙂