Getting NullPointerException in Java SQLite

Question

I code this method to fetch result from SQLite table in java.

    public String[][] select(String query, String table) throws ClassNotFoundException, SQLException
{
    Connection con;
    Statement st;
    Class.forName("org.sqlite.JDBC");
    con = DriverManager.getConnection("jdbc:sqlite:"+table);
    con.setAutoCommit(false);
    st = con.createStatement();
    ResultSet rs = st.executeQuery(query);
    String[][] result = new String[100][4];
    int j = 0;
    while ( rs.next() ) {
        String  name = rs.getString("fullname");
        String  username = rs.getString("username");
        String  password = rs.getString("password");
        String  email = rs.getString("email");
        result[j][0] = name; // line number 63
        result[j][1] = username;
        result[j][2] = password;
        result[j][3] = email;
        j++;
    }
    rs.close();
    st.close();
    con.close();
    return result;
}

From my main method i instantiated this method:

    try{
        String[][] data = db.select("SELECT * FROM user", "fliplist.db"); // line number 37
        System.out.println(data);
    } catch (ClassNotFoundException | SQLException e){
        System.out.println(e.getMessage());
    }

This gives me this error:

run:
Exception in thread "main" java.lang.NullPointerException
at FlipList.Database.select(Database.java:63)
at FlipList.FlipList.main(FlipList.java:37)
Java Result: 1
BUILD SUCCESSFUL (total time: 2 seconds)

How it should be in order to fetch the result?

Update

I have initialized the second dimension of the array, and now it showing this error(most likely garbez value):

run:
[[Ljava.lang.String;@2077d4de
BUILD SUCCESSFUL (total time: 2 seconds)

How i can return an array?

Answer 1

(Thanks to @lea for this; if they've posted one please let me know)

You need to initialize the inner arrays, too. In your while loop, put something like this at the top, and it should work.

data[j] = new String[4];

For your new error, you want to iterate through the array instead of just printing it. For example:

for (int i = 0; i < data.length; i++) {
    for (int j = 0; j < data[0].length; j++) {
        System.out.print(data[i][j] + "\t\t");
    }
    System.out.println();
}

There are also a couple of other tips that I'd like to give you:

1. Move the j++ into the while loop, just before the closing bracket. That way it increments with every loop instead of just once, after every loop. Fixed!

2. You might want to use an ArrayList<String[]> instead of String[][] if you won't always have exactly a hundred items. That way, you can go over 100 if you need, and if you use less, it will save memory. To do that, you'll want to do something like this for your while loop:

   while ( rs.next() ) {
       String[] nextRow = new String[4];
       String  name = rs.getString("fullname");
       String  username = rs.getString("username");
       String  password = rs.getString("password");
       String  email = rs.getString("email");
       nextRow[0] = name; // line number 63
       nextRow[1] = username;
       nextRow[2] = password;
       nextRow[3] = email;
       result.add(nextRow);
   }
   

You'll also want to change the declaration of data to ArrayList<String[]> result = new ArrayList<String[]>() instead of String[][] result = ....

Answer 2

i redefined the ArrayList as the list of Strings rather than Arrays.

public void tester()
    {
        ArrayList result = new ArrayList();


    try
    {
    Connection connect = DriverManager.getConnection(host, username, password);

    String sql="select * from FRIENDS";

    Statement stmt=connect.createStatement();

    ResultSet rs=stmt.executeQuery(sql);



    String[] nextRow = new String[4];
    while ( rs.next() ) {


            String  firstname = rs.getString("FIRSTNAME");
            String  lastusername = rs.getString("LASTNAME");
            String  nickname = rs.getString("NICKNAME");
            String  friendsince = rs.getString("FRIENDSINCE");
            String  email = rs.getString("EMAIL");

            System.out.print("Firstname :- "+firstname+" lastname :- "+lastusername+" nickname :-  "+nickname+" friendsince :- "+friendsince+" Email :- "+email);

            nextRow[0] = firstname; 
            nextRow[1] = lastusername;
            nextRow[2] = nickname;
            nextRow[3] = email;

            System.out.println("The values in array are :- "+nextRow);




            int i=0;
            for(i=0;i<nextRow.length;i++)
            {
                String add1=nextRow[i];

                result.add(add1);
            }



           }

    }

Hope , this helps.

Answer 3

1. Something very basic . Please execute the query on the DB and see what it returns(Sometimes the query might not be correct e.g the table name can be 'users' instead of 'user', just a guess) :-

SELECT * FROM user

2. If above is fine , then can you please try extracting the rows using 'columnIndex' instead of the 'columnName'. e.g.

   String  name = rs.getString(1);
   String  username = rs.getString(2);
   String  password = rs.getString(3);
   String  email = rs.getString(4);
   

Here i am assuming that the name , username , password and email belong to 1st , 2nd ,3rd and 4th column respectively.