Problema eccezioni nullpointerexception

package data;

import java.io.Serializable;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

import javax.swing.text.AttributeSet;

import database.DatabaseConnectionException;
import database.DbAccess;
import database.EmptySetException;
import database.Example;
import database.NoValueException;
import database.QUERY_TYPE;
import database.TableData;
import database.TableSchema;
import database.TableSchema.Column;

public class Data {

	private List<Example> data;
	private int numberOfExamples;// cardinalita' dell'insieme di transazioni (numero di righe in data)
	private List<Attribute> attributeSet;// un vettore degli attributi in ciascuna tupla (schema della tabella di dati)

	public Data(String table) throws EmptyDatasetException, DatabaseConnectionException,SQLException, EmptySetException, NoValueException{

		//data
		data = new ArrayList<Example>();
		DbAccess db = new DbAccess();
		TableData tableData=new TableData(db);
		data = tableData.getDistinctTransazioni(table);

		int index=0;
		db.initConnection();
		
		TableSchema sch;
		
		try{
			sch = new TableSchema(new DbAccess(), table);
			data = tableData.getDistinctTransazioni(table);
			numberOfExamples = data.size();
			attributeSet = new LinkedList<Attribute>();
			

			for (Column c: sch){ //exception...at data.Data.<init>(Data.java:59)
				if (c.isNumber()){
					attributeSet.add(new ContinuousAttribute(c.getColumnName(),index, (Double) tableData.getAggregateColumnValue(table, c, QUERY_TYPE.MIN), (Double) tableData.getAggregateColumnValue(table, c, QUERY_TYPE.MAX)));
					index++;
				} else{ 
					String[] x = (String[]) tableData.getDistinctColumnValues(table, c).toArray(new String[0]);
					attributeSet.add(new DiscreteAttribute(c.getColumnName(),index, x));
					index++;
				}
			}
			
		} catch (SQLException e){
			e.printStackTrace();}
		
		DbAccess.closeConnection();
	}	

	public int getNumberOfExamples(){
		return numberOfExamples;
	}

	public int getNumberOfExplanatoryAttributes(){
		return attributeSet.size();
	}

	int getNumberOfAttributes(){
		return attributeSet.size();
	}


	public Object getAttributeValue(Object array, int attributeIndex){
		return data.get((Integer) array).get(attributeIndex);
	}

	Attribute getAttribute(int index){
		return attributeSet.get(index); 
	}

	public Tuple getItemSet(int index){
		Tuple tuple = new Tuple(attributeSet.size()); 
		for(int i = 0; i < attributeSet.size(); i++){
			if(attributeSet.get(i) instanceof DiscreteAttribute)
			// tuple.add(new DiscreteItem((DiscreteAttribute) attributeSet.get(i), (String)getAttributeValue(index,i)) ,i);
			   tuple.add(new DiscreteItem((DiscreteAttribute)(attributeSet.get(i)),(String)(data.get(index).get(i))), i);
			else
			// tuple.add(new ContinuousItem((ContinuousAttribute) attributeSet.get(i), (double)getAttributeValue(index,i)) ,i);
				tuple.add(new ContinuousItem((ContinuousAttribute)(attributeSet.get(i)), (double)(data.get(index).get(i))), i);
		}
		return tuple;
	}

	public String toString(){
		String container = new String();
		for(Attribute a: attributeSet){
			container = container + a + " ";
		}
		container += "\n";
		for(int i=0; i<getNumberOfExamples(); i++)
		{
			container += i;
			container += ": ";
			// for(int j=0; j<attributeSet.size(); j++){
			for(int j = 0; j<getNumberOfExplanatoryAttributes(); j++){
				container += data.get(i).get(j);
				if(j < getNumberOfExplanatoryAttributes() - 1){
					container+= ",";
				}
			}
			container += "\n";
		}
		return container;
	}

	public static void main(String args[]) throws DatabaseConnectionException, SQLException, EmptySetException, NoValueException{
		Data trainingSet;
		try {
			trainingSet = new Data("playtennis");
			System.out.println(trainingSet); //exception...at data.Data.main(Data.java:207)
		} catch (EmptyDatasetException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}			
	}
}