Exception in thread "main" java.lang.NullPointerException

[bram van gemert]

Hi everybody Just wrote this script:

import java.sql.*;
import java.util.ArrayList;



public class Database {
	private static  ArrayList<String> notes;
  
    public Database()   {
notes= new ArrayList<String>();

    }
    public static void main(String[] args) throws ClassNotFoundException,
    SQLException {
    	String username = "root";
    	String password = "password";
    	String tablename = "cust";
    	String fieldname = "Column1";
    String query = "SELECT " + fieldname + " FROM " + tablename + ";";
    String url = "jdbc:mysql://127.0.0.1/demo";
    Class.forName("com.mysql.jdbc.Driver");
    Connection con = DriverManager.getConnection(url, username, password);
    Statement stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next())
    notes.add(rs.getString(1));
    rs.close();
    stmt.close();
    con.close();
    
    }
public void main(){
	System.out.println(""+notes.get(0));
}
}

When I compile there are no errors.
But when i run i get this error:

Exception in thread "main" java.lang.NullPointerException
at Database.main(Database.java:29)


Dont understand why.
Tia,
Bram

[Adam Cripps]

bram van gemert wrote:
Hi everybody Just wrote this script:

import java.sql.*;
import java.util.ArrayList;



public class Database {
	private static  ArrayList<String> notes;
  
    public Database()   {
notes= new ArrayList<String>();

    }
    public static void main(String[] args) throws ClassNotFoundException,
    SQLException {
    	String username = "root";
    	String password = "password";
    	String tablename = "cust";
    	String fieldname = "Column1";
    String query = "SELECT " + fieldname + " FROM " + tablename + ";";
    String url = "jdbc:mysql://127.0.0.1/demo";
    Class.forName("com.mysql.jdbc.Driver");
    Connection con = DriverManager.getConnection(url, username, password);
    Statement stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next())
    notes.add(rs.getString(1));
    rs.close();
    stmt.close();
    con.close();
    
    }
public void main(){
	System.out.println(""+notes.get(0));
}
}

When I compile there are no errors.
But when i run i get this error:

Exception in thread "main" java.lang.NullPointerException
at Database.main(Database.java:29)


Dont understand why.
Tia,
Bram

Is line 25 meant to be a useless while? It looks odd to me at that point. Are you missing curly braces?

[Matthew Brown]

The notes variable is initialized in the constructor. But no Database object is ever instantiated, so notes is still null. It's a static variable - why are you initializing it in a constructor?