Question

I have an application that iterates over an amount of resultSets and queries additional information for every row from a different table.

The rough structure is like this:

public void main(String[] args) {
    ResultSet result = database.connection.createStatement()
                           .executeQuery("SELECT * FROM entities");
    ArrayList<Entity> entities = new ArrayList<Entity>();
    while (result.next() {
        Entity entity = EntityFactory.createById(result.getInt("id"));
        entities.add(entity);
    }
}

// EntityFactory
public static Entity createById(int id) {
    StringBuilder sql = new StringBuilder("SELECT * FROM sampling_data WHERE id = ")
                            .append(id);
    ResultSet result = database.connection.createStatement()
                            .executeQuery(sql.toString());
    result.first();
    EntityData data = new EntityData(25);

    for (int sample = 1; sample <= 25; sample++) {
        String sample_R = new StringBuilder("sample_")
                              .append(sample).append("_R").toString();
        String sample_G = new StringBuilder("sample_")
                              .append(sample).append("_G").toString();
        String sample_B = new StringBuilder("sample_")
                              .append(sample).append("_B").toString();

        int r = resultSet.getInt(sample_R);
        int g = resultSet.getInt(sample_G);
        int b = resultSet.getInt(sample_B);

        data.add(r, g, b);
    }

    return new Entity(data);
}

Which results in a OutOfMemoryException.

How can I make the loop (or the whole methods) more memory efficient?

Answer 1

The largest problem was (I think) that createById did not close result set and statement.
Further it makes sense to not have nx1 queries.

    try {
        Statement stmt = database.connection.createStatement();
        ResultSet result = stmt.executeQuery("SELECT * FROM sampling_data d"
            + " WHERE EXISTS(SELECT * FROM entities e WHERE e.id = d.id)");
        List<Entity> entities = new ArrayList<Entity>();
        while (result.next()) {
            Entity entity = EntityFactory.createById(result);
            entities.add(entity);
        }
        result.close();
        stmt.close();
    } catch (SQLException ex) {
        Logger.getLogger(Test1.class.getName()).log(Level.SEVERE, null, ex);
    }

And

// EntityFactory
public static Entity createById(ResultSet resultSet) {
    EntityData data = new EntityData(25);
    for (int sample = 1; sample <= 25; sample++) {
        String sample_R = new StringBuilder("sample_").append(sample).append("_R").toString();
        String sample_G = new StringBuilder("sample_").append(sample).append("_G").toString();
        String sample_B = new StringBuilder("sample_").append(sample).append("_B").toString();
        int r = resultSet.getInt(sample_R);
        int g = resultSet.getInt(sample_G);
        int b = resultSet.getInt(sample_B);

        data.add(r, g, b);
    }

    return new Entity(data);
}