Question

I am very familiar with functional langauges such as Scheme, and Haskell. I’m trying to solve a problem in Java and struggling, possibly because I’m still in a functional mindset.

I want to write:

public void doQueryAndStoreData(String query, <? extends Collection> storeIn) {
    /* make a jdbc query, get ResultSet */
    ResultSet rset = ...;
    ProcessResultSet proc = new ProcessResultSet();
    proc.process(rset, storeIn);
    /* clean up */
}

with an interface like:

private interface IProcessResultSet<C> {
    public void process(ResultSet rset, C storeIn);
}

and a class implementing the interface like:

private class ProcessResultSet implements IProcessResultSet<? extends Collection> {
    public void process(ResultSet rset, Map storeIn) {
        /* do something */
    }

    public void process(ResultSet rset, List storeIn) {
        /* do something else */
    }
}

so that the first method can call the appropriate process based on what type of storeIn it’s given.

In Haskell I could write

class Storeable c a where
    store :: a -> c a -> c a

doQueryAndStoreData :: Storeable c a => ResultSet a -> c a -> c a
doQueryAndStoreData (ResultSet rs) coll = foldr store coll rs

and provide Storeable instances for whatever collection type I want to store my ResultSet in.

Is this the correct approach in Java? Because I feel like I’m somewhat fighting the langauge to accomplish this.

Answer 1

No, Java doesn’t do that.

You’d need to do something like:

public <T> void doQueryAndStoreData(
    String query,
    T storeIn,
    ResultSetProcessor<T> processor
) {

Or much more likely:

public void doQueryAndStoreData(
    String query,
    ResultSetHandler handler // may contain processor and storeIn
) {

I hope I don’t need to mention SQL injection vulnerabilities are a bad thing. (Also Map is not a Collection in Java (it is in C# but C# Collection isn’t very useful).)