Java Iterator backed by a ResultSet

Question

I've got a class that implements Iterator with a ResultSet as a data member. Essentially the class looks like this:

public class A implements Iterator{
    private ResultSet entities;
    ...
    public Object next(){
        entities.next();
        return new Entity(entities.getString...etc....)
    }

    public boolean hasNext(){
        //what to do?
    }
    ...
}

How can I check if the ResultSet has another row so I can create a valid hasNext method since ResultSet has no hasNext defined itself? I was thinking doing SELECT COUNT(*) FROM... query to get the count and managing that number to see if there's another row but I'd like to avoid this.

Answer 1

It sounds like you are stuck between either providing an inefficient implementation of hasNext or throwing an exception stating that you do not support the operation.

Unfortunately there are times when you implement an interface and you don't need all of the members. In that case I would suggest that you throw an exception in that member that you will not or cannot support and document that member on your type as an unsupported operation.

Answer 2

You can get out of this pickle by performing a look-ahead in the hasNext() and remembering that you did a lookup to prevent consuming too many records, something like:

public class A implements Iterator{
    private ResultSet entities;
    private boolean didNext = false;
    private boolean hasNext = false;
    ...
    public Object next(){
        if (!didNext) {
            entities.next();
        }
        didNext = false;
        return new Entity(entities.getString...etc....)
    }

    public boolean hasNext(){
        if (!didNext) {
            hasNext = entities.next();
            didNext = true;
        }
        return hasNext;
    }
    ...
}

Answer 3

Its not a really bad idea in the cases where you need it, it's just that you often do not need it.

If you do need to do something like, say, stream your entire database.... you could pre-fetch the next row - if the fetch fails your hasNext is false.

Here is what I used:

/**
 * @author Ian Pojman <[e-mail]>
 */
public abstract class LookaheadIterator<T> implements Iterator<T> {
    /** The predetermined "next" object retrieved from the wrapped iterator, can be null. */
    protected T next;

    /**
     * Implement the hasNext policy of this iterator.
     * Returns true of the getNext() policy returns a new item.
     */
    public boolean hasNext()
    {
        if (next != null)
        {
            return true;
        }

        // we havent done it already, so go find the next thing...
        if (!doesHaveNext())
        {
            return false;
        }

        return getNext();
    }

    /** by default we can return true, since our logic does not rely on hasNext() - it prefetches the next */
    protected boolean doesHaveNext() {
        return true;
    }

    /**
     * Fetch the next item
     * @return false if the next item is null. 
     */
    protected boolean getNext()
    {
        next = loadNext();

        return next!=null;
    }

    /**
     * Subclasses implement the 'get next item' functionality by implementing this method. Implementations return null when they have no more.
     * @return Null if there is no next.
     */
    protected abstract T loadNext();

    /**
     * Return the next item from the wrapped iterator.
     */
    public T next()
    {
        if (!hasNext())
        {
            throw new NoSuchElementException();
        }

        T result = next;

        next = null;

        return result;
    }

    /**
     * Not implemented.
     * @throws UnsupportedOperationException
     */
    public void remove()
    {
        throw new UnsupportedOperationException();
    }
}

then:

    this.lookaheadIterator = new LookaheadIterator<T>() {
        @Override
        protected T loadNext() {
            try {
                if (!resultSet.next()) {
                    return null;
                }

                // process your result set - I use a Spring JDBC RowMapper
                return rowMapper.mapRow(resultSet, resultSet.getRow());
            } catch (SQLException e) {
                throw new IllegalStateException("Error reading from database", e);
            }
        }
    };
}

Answer 4

public class A implements Iterator<Entity>
{
    private final ResultSet entities;

    // Not required if ResultSet.isLast() is supported
    private boolean hasNextChecked, hasNext;

    . . .

    public boolean hasNext()
    {
        if (hasNextChecked)
           return hasNext;
        hasNext = entities.next();
        hasNextChecked = true;
        return hasNext;

        // You may also use !ResultSet.isLast()
        // but support for this method is optional 
    }

    public Entity next()
    {
        if (!hasNext())
           throw new NoSuchElementException();

        Entity entity = new Entity(entities.getString...etc....)

        // Not required if ResultSet.isLast() is supported
        hasNextChecked = false;

        return entity;
    }
}

Answer 5

entities.next returns false if there are no more rows, so you could just get that return value and set a member variable to keep track of the status for hasNext().

But to make that work you would also have to have some sort of init method that reads the first entity and caches it in the class. Then when calling next you would need to return the previously cached value and cache the next value, etc...

Answer 6

I agree with BalusC. Allowing an Iterator to escape from your DAO method is going to make it difficult to close any Connection resources. You will be forced to know about the connection lifecycle outside of your DAO, which leads to cumbersome code and potential connection leaks.

However, one choice that I've used is to pass a Function or Procedure type into the DAO method. Basically, pass in some sort of callback interface that will take each row in your result set.

For example, maybe something like this:

public class MyDao {

    public void iterateResults(Procedure<ResultSet> proc, Object... params)
           throws Exception {

        Connection c = getConnection();
        try {
            Statement s = c.createStatement(query);
            ResultSet rs = s.executeQuery();
            while (rs.next()) {
                proc.execute(rs);
            }

        } finally {
            // close other resources too
            c.close();
        }
    }

}


public interface Procedure<T> {
   void execute(T t) throws Exception;
}


public class ResultSetOutputStreamProcedure implements Procedure<ResultSet> {
    private final OutputStream outputStream;
    public ResultSetOutputStreamProcedure(OutputStream outputStream) {
        this.outputStream = outputStream;
    }

    @Override
    public void execute(ResultSet rs) throws SQLException {
        MyBean bean = getMyBeanFromResultSet(rs);
        writeMyBeanToOutputStream(bean);
    }    
}

In this way, you keep your database connection resources inside your DAO, which is proper. But, you are not necessarily required to fill a Collection if memory is a concern.

Hope this helps.