Generic Conversion Between List<T> and String

Here's a reusable generic "converter" interface for converting between comma-separated strings and Lists of a given type:

/**
 * Converts between comma-separated string and lists of type T.
 */
public abstract class AbstractConverter<T> {

    protected abstract T getInstance(String s);

    /**
     * Converts a list of items into a comma-separated string
     */
    public String toString(List<T> items)
    {
        String s= "";
        String comma = "";
        for (T item : items) {
            s += comma + item;
            comma = ",";
        }
        return s;
    }

    /**
     * Converts a comma-separated string into a list of items.
     */
    public List<T> toList(String csv)
    {
        if (csv == null)
        {
            return new ArrayList<T>();
        }

        String[] items = csv.split(",");
        List<T> theList = new ArrayList<T>();
        for (String item : items) {
            T thisItem = getInstance(item); 
            theList.add(thisItem);
        }
        return theList;
    }

    /**
     * Converts a comma-separated string into a list of items, excluding any items in the given
     * exclude list.
     */
    public List<T> toList(String csv, List<T> excludes)
    {
        if (csv == null)
        {
            return new ArrayList<T>();
        }

        String[] items = csv.split(",");
        List<T> theList = new ArrayList();
        for (String item : items) {
            T thisItem = getInstance(item);
            if (!excludes.contains(thisItem))
            {
                theList.add(thisItem);
            }
        }
        return theList;
    }
}

I can now provide concrete converter classes of types as needed, for example:

/**
 * Converts between comma-separated string and lists of MyType objects.
 */
public class MyConverter extends AbstractConverter<MyType> {

    // provide an instance of the custom type
    protected MyType getInstance(String s)
    {
        return new MyType(s);
    }

    // optionally enforce a singleton, if needed
    private MyConverter() {
        // EMPTY
    }

    // provide a convenience for static utility classes
    private static AbstractConverter<MyType> cvt = new MyConverter();
    public static AbstractConverter<MyType> inst()
    {
        return cvt;
    }
}

Since I want to make things convenient for clients of this, I now provide a static utility class that precludes the need to instantiate anything:

public class MyHelper {

    private static AbstractConverter cvt = MyConverter.inst();

    public static String toString(List<MyType> items)
    {
        return cvt.toString(items);
    }

    public static List<MyType> toList(String csv)
    {
        return cvt.toList(csv);
    }

    public static List<MyType> toList(String csv, List<MyType> excludes)
    {
        return cvt.toList(csv, excludes);
    }
}

Now my clients can call e.g. MyHelper.toList("x, y, z") instead of MyConverter.inst().toList("x, y, z"). A small advantage, but it does help with readability and reinforces the notion that this is a collection of utility functions.