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Thread: Exception in thread "main" java.lang.NullPointerException

  1. #1
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    Default Exception in thread "main" java.lang.NullPointerException

    I haven't been using java that long, but I'm currently in a class and we've been given this program to complete; I have been working on this for the last for 3 days now and I keep getting this error:

    Selecting the correct constructor
    Exception in thread "main" java.lang.NullPointerException
    at SlotReservation.main(SlotReservation.java:80)

    referenced code:

    while(answer<1750)
    {
    answer++;
    //Selecting a user randomly from the user table such that he is not in the parking lot and has not been used till now.

    ResultSet rs1=stmt.executeQuery("SELECT MIN(car_regnum), start_time,end_time FROM fsbm_data where temp_present='0' and temp_block='0'");
    if (rs1.next())
    {
    arrivalThreadRegnum = Integer.parseInt(rs1.getString(1).trim());
    startTime = Integer.parseInt(rs1.getString(2).trim());
    endTime = Integer.parseInt(rs1.getString(3).trim());
    stmt.executeUpdate("UPDATE fsbm_data set temp_present=1, temp_block=1 where car_regnum="+rs1.getString(1));

    //Calling thread for each new user and sending his registration number and advance reservation spot number

    new FreeSpaceDetection(userPL.summaryVector,userPL.bit Map,arrivalThreadRegnum, startTime,endTime,userPL.defragList);

    slotNumber++;
    try
    {
    Thread.sleep(1000);
    }
    END OF MY CODE:

    I typed everything as it was presented, but I can't get it to run without this error.
    Any suggestions would be much appreciated...


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: Exception in thread "main" java.lang.NullPointerException

    Something on that line is null. What is it? Use a debugger or some print statements to figure that out.
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  3. #3
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    Default Re: Exception in thread "main" java.lang.NullPointerException

    Thanks for those words

    I have tried debugging, and it found nothing.

    this is the entire page of code:

    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    import java.util.ArrayList;
    import java.util.Vector;

    public class SlotReservation {

    Vector<Integer> summaryVector = new Vector<Integer>();
    ArrayList<slotInfo> defragList = new ArrayList<slotInfo>();
    int[][] bitMap=new int [48][500];


    public static void main (String args [])
    {

    SlotReservation userPL = new SlotReservation();
    userPL.summaryVector = new Vector<Integer>();
    userPL.defragList = new ArrayList<slotInfo>();
    int answerLimit = 0;
    //Arrival Thread Variable initialization

    int slotNumber = 0;
    int startTime = 0;
    int endTime = 0;
    int arrivalThreadRegnum = 0;

    //Setting up database connection

    Connection conn = null;

    try
    {

    String userName = "root";
    String password = "";
    String url = "jdbc:mysql://localhost/mysql";
    Class.forName ("com.mysql.jdbc.Driver").newInstance ();
    conn = DriverManager.getConnection (url, userName, password);
    }
    catch (Exception e)
    {
    System.err.println ("Cannot connect to database server");
    }
    finally
    {
    if (conn != null)
    {
    Statement stmt = null;
    int answer=1;
    try
    {
    stmt = conn.createStatement();
    ResultSet rs=stmt.executeQuery("SELECT MIN(car_regnum), start_time,end_time FROM fsbm_data where temp_present='0' and temp_block='0'");
    if (rs.next())
    {
    System.out.println("Selecting the correct constructor");
    new DefragmentAllocations3(userPL);
    //new DefragmentAllocations2(userPL);
    //new DefragmentAllocations3(userPL);
    }


    while(answer<1750)
    {
    answer++;
    //Selecting a user randomly from the user table such that he is not in the parking lot and has not been used till now.

    ResultSet rs1=stmt.executeQuery("SELECT MIN(car_regnum), start_time,end_time FROM fsbm_data where temp_present='0' and temp_block='0'");
    if (rs1.next())
    {

    arrivalThreadRegnum = Integer.parseInt(rs1.getString(1).trim());
    startTime = Integer.parseInt(rs1.getString(2).trim());
    endTime = Integer.parseInt(rs1.getString(3).trim());
    stmt.executeUpdate("UPDATE fsbm_data set temp_present=1, temp_block=1 where car_regnum="+rs1.getString(1));

    //Calling thread for each new user and sending his registration number and advance reservation spot number

    new FreeSpaceDetection(userPL.summaryVector,userPL.bit Map,arrivalThreadRegnum, startTime,endTime,userPL.defragList);
    slotNumber++;
    try
    {
    Thread.sleep(1000);
    }
    catch (InterruptedException e)
    {
    e.printStackTrace();
    }
    }
    else
    System.out.println("No more users available to simulate the arrival threads");
    }
    }
    catch (SQLException e1)
    {
    e1.printStackTrace();
    }

    }
    }
    System.out.println("DEFRAG LIST AT END OF MAIN IS "+userPL.defragList);

    }

    }


    Complete page of code... Error Still occurring after running debugger...

  4. #4
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: Exception in thread "main" java.lang.NullPointerException

    When posting code, please use the highlight tags to preserve formatting. Unformatted code is pretty hard to read.

    A debugger doesn't find or fix anything for you. It simply allows you to examine the values while the code is running. I'd add a break point right before the line that errors, run the debugger, and examine each variable- one of them is null. Alternatively you could use a print statement, like so: http://www.javaprogrammingforums.com...t-println.html
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