Same Instances header ( arff ) for all my database queries

Question

I am using InstanceQuery , SQL queries, to construct my Instances. But my query results does not come in the same order always as it is normal in SQL. Beacuse of this Instance

Answer 1

I tried various approaches to my problem. But it seems that weka internal API does not allow solution to this problem right now. I modified weka.core.Instances append command line code for my purposes. This code is also given in this answer

According to this, here is my solution. I created a SampleWithKnownHeader.arff file , which contains correct header values. I read this file with following code.

public static Instances getSampleInstances() {
    Instances data = null;
    try {
        BufferedReader reader = new BufferedReader(new FileReader(
                "datas\\SampleWithKnownHeader.arff"));
        data = new Instances(reader);
        reader.close();
        // setting class attribute
        data.setClassIndex(data.numAttributes() - 1);
    }
    catch (Exception e) {
        throw new RuntimeException(e);
    } 
    return data;

}

After that , I use following code to create instances. I had to use StringBuilder and string values of instance, then I save corresponding string to file.

public static void main(String[] args) {

    Instances SampleInstance = MyUtilsForWeka.getSampleInstances();

    DataSource source1 = new DataSource(SampleInstance);

    Instances data2 = InstancesFromDatabase
            .getInstanceDataFromDatabase(DatabaseQueries.WEKALIST_QUESTION1);

    MyUtilsForWeka.saveInstancesToFile(data2, "fromDatabase.arff");

    DataSource source2 = new DataSource(data2);

    Instances structure1;
    Instances structure2;
    StringBuilder sb = new StringBuilder();
    try {
        structure1 = source1.getStructure();
        sb.append(structure1);
        structure2 = source2.getStructure();
        while (source2.hasMoreElements(structure2)) {
            String elementAsString = source2.nextElement(structure2)
                    .toString();
            sb.append(elementAsString);
            sb.append("\n");

        }

    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }

    MyUtilsForWeka.saveInstancesToFile(sb.toString(), "combined.arff");

}

My save instances to file code is as below.

public static void saveInstancesToFile(String contents,String filename) {

     FileWriter fstream;
    try {
        fstream = new FileWriter(filename);
      BufferedWriter out = new BufferedWriter(fstream);
      out.write(contents);
      out.close();
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }

This solves my problem but I wonder if more elegant solution exists.