Question

I am trying use a from a multi-dimensional array that I create in another classes method. Below is my main method:

public class main {

public static void main(String[] args) throws Exception {

    sql test = new sql();
    String[][] test2 = test.getDb();
    System.out.print(test2[0][0]);
}

Now here is the class that returns an multi-dimensional array.

import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;

import javax.swing.JLabel;
import javax.swing.JTextField;

import com.mysql.jdbc.Statement;

public class sql {

java.sql.Connection con = null;
PreparedStatement pst = null;
ResultSet rs = null;
String url = "jdbc:mysql://localhost:8889/deliveryEarn";
String user = "root";
String password = "root";

ArrayList<String> sqlCol1 = new ArrayList<String>();
ArrayList<String> sqlCol2 = new ArrayList<String>();
ArrayList<String> sqlCol3 = new ArrayList<String>();
ArrayList<String> sqlCol4 = new ArrayList<String>();
ArrayList<String> sqlCol5 = new ArrayList<String>();
ArrayList<String> sqlCol6 = new ArrayList<String>();
ArrayList<String> sqlCol7 = new ArrayList<String>();
String sqlArray[][] = new String[7][7];

public sql() {
}

public String[][] getDb() {
    try {
         con = DriverManager.getConnection(url, user, password);
         pst = con.prepareStatement("select * from incomeCalc");
         rs = pst.executeQuery();

    while (rs.next()) {

        sqlCol1.add(rs.getString(1));
        int i1=0;
        for(String s: sqlCol1){
            sqlArray[i1++][0] = s;
        }

        sqlCol2.add(rs.getString(2));
        int i2=0;
        for(String s: sqlCol2){
            sqlArray[i2++][1] = s;
        }

        sqlCol3.add(rs.getString(3));
        int i3=0;
        for(String s: sqlCol3){
            sqlArray[i3++][2] = s;
        }

        sqlCol4.add(rs.getString(4));
        int i4=0;
        for(String s: sqlCol4){
            sqlArray[i4++][3] = s;
        }

        sqlCol5.add(rs.getString(5));
        int i5=0;
        for(String s: sqlCol5){
            sqlArray[i5++][4] = s;
        }

        sqlCol6.add(rs.getString(6));
        int i6=0;
        for(String s: sqlCol6){
            sqlArray[i6++][5] = s;
        }
        sqlCol7.add(rs.getString(7));
        int i7=0;
        for(String s: sqlCol7){
            sqlArray[i7++][6] = s;
        }
    }
    }

catch( Exception E ) { 
    System.out.println( E.getMessage() );   
}
    return sqlArray;
}
}

Here is the screenshot of the MySQL database.

Edit: It appears I wasn’t clear with my question. I apologize. I am getting a runtime error at this line:

System.out.print(test2[0][0]);

What am I doing wrong? Also, for correct OOP, is it better to use a constructor or a method to pull from or input to a database? THis is my first program so sorry if it seems trivial.

Edit2: Here is the error:

Exception in thread “main” java.lang.NullPointerException
at main.main(main.java:17)

Answer 1

As to why you’ve got an error, it would be nicer to know the error, however…

Personally, I’d drop the contents of the result set into a “Data Object”…

public class Income {
    // Column decelerations...

    private long id;
    private int tips;
    private int hours;
    private int gas;
    private double hourly;
    private double other;
    private double other2;

    public int getGas() {
        return gas;
    }

    public double getHourly() {
        return hourly;
    }

    public int getHours() {
        return hours;
    }

    public long getId() {
        return id;
    }

    public double getOther() {
        return other;
    }

    public double getOther2() {
        return other2;
    }

    public int getTips() {
        return tips;
    }

    public void setGas(int gas) {
        this.gas = gas;
    }

    public void setHourly(double hourly) {
        this.hourly = hourly;
    }

    public void setHours(int hours) {
        this.hours = hours;
    }

    public void setId(long id) {
        this.id = id;
    }

    public void setOther(double other) {
        this.other = other;
    }

    public void setOther2(double other2) {
        this.other2 = other2;
    }

    public void setTips(int tips) {
        this.tips = tips;
    }
}

Then when you load it you could do something like…

public Income[] getIncome() {

    // Call database...

    List<Income> data = new ArrayList<Income>(25);

    while (rs.next()) {

        Income income = new Income();

        income.setID(rs.getInt(1)));
        income.setTips(rs.getInt(2)));
        income.setHours(rs.getInt(3)));
        income.setGas(rs.getInt(4)));
        income.setHourly(rs.getDouble(5)));
        income.setOther(rs.getDouble(6)));
        income.setOther2(rs.getDouble(7)));

        data.add(income);

    }

    return data.toArray(new Income[data.size()]);

}

The you could do things like this…

sql test = new sql();
Income[] incomes = test.getIncome();

System.out.println(incomes[0].getID());

Isn’t that easier to read 😛

Your attempt to use a Factory is probably the best idea. It comes down to a matter of management as to weather you maintain a single instance (Singlton) or allow multiple instances of this Factory to be created. Personally, I prefer to use Singltons in this case where I can, it allows a centralised place to perform operations (saving changes, creating new objects, listing, deleting) and helps manage the resources involved. IMHO