NullPointerException with multi-dimentional array.

Newbie & Debugging Questions
#1

Hello everyone,

What im trying to do is that the MySQL class retrieves characters from a certain account id and inserts them into a multi-dimentional array. It does this inside the getCharacters function. But when i insert the values into the multi-dimentional array and run my application it give’s a null pointer exception on a certain line. This is the code:

public String[][] getCharacters(int accId) {
		String[][] characters = null;
		try {
			Statement st = con.createStatement();
			ResultSet rs = st.executeQuery("SELECT * FROM `characters` WHERE `account_id`="+ accId);
			int i = 1;
			while(rs.next()) {
				String username = rs.getString("username");
				int level = rs.getInt("level");
				System.out.println("I: "+ i +" User:"+ username +", Lvl: "+ level);
				characters[i][0] = username;
				characters[i][1] = ""+ level;
				i++;
			}
			rs.close();
			st.close();
			
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			System.out.println("[Login Server][Error]: Error with retrieving characters: "+ e.getMessage());
		} catch (Exception e) {
			System.out.println("");
		}
		return characters;
	}

NullPointerException happens on the following line: “characters[i ][0 ] = username;” (Note there are no spaces between the brackets but the forum views it as BB code)

Iv been stuck with this problem for 4 days now and im sirreusly stressing alot over it, iv asked a few application developers at my school if they could help out but they couldnt find the problem either. I hope someone know’s what im doing wrong here and if possible send me a link or type some text informing me on what i did wrong and what i could do to prevent this from happening again…

Thanks in advance :slight_smile:

#2

You haven’t initialized your array.

#3

String[][] array=new String[12][];
array[0]=new String[3];
or
String[][] array=new String[12][10];
or
String[][] array=new String[][]{{“one”,“two”},{“three”,“four”}};

#4

But what if i have no limit to how big it can get?

The first array is the character, the array in the array is the character info so a player could have a bunch of diffrent characters. Well ill prob set a limit to it but i still would like to know how i could have an unlimited amount of value’s in the array (new String[][] doesnt work :frowning: )

#5

Use an ArrayList.

#6

ArrayList doesnt have multiple dimentions

#7

When you know the limit in advance you can do something like this:
int limit1=…;
int limit2=…;
String[][] array=new String[limit1][limit2];
or
int limit1=…;
String[][] array=new String[limit1][];
int limit2=…;
array[…]=new String[limit2];

array lists:
ArrayList<ArrayList> arrayList=new ArrayList<>();
arrayList.add(new ArrayList<>());

#8

Looking at OP’s original code, it seems like he only needs 2 strings per entry, so a ArrayList<String[]> would probably be the best.

#9

yess.

#10

It has only 2 string right now yes, but i’ll be expanding it alot in the future, im still in very low stages of my project :stuck_out_tongue:

Also, ill just try out ArrayList then, ill edit it if it works

#11

You have to ask the question: which dimensions will need to be dynamic? You seem to be programming something that is TOO dynamic/flexible if you need an unlimited 2D array and it’s not map/level data.

#12

Ye you might be right, iv been trying to make everything as dynamic as possible, maybe im trying to hard and its best to just limit certain things.

#13

Also, a small eclipse problem (for people who use the eclipse IDE), when i type an object E.G: ArrayList. after the ‘.’ it should show a tool tip like feature with suggestions of possible functions inside that class but since i switched over to ubuntu it doesnt show anymore, anyone know where i can make it work again (i googled it but i didnt get any results, mainly because i cant think of a decent search query for my problem)

#14

Init your array after execute query, so you can get number of rows of result and use it.

#15

Try Googling on ‘eclipse intellisense not working ubuntu’.

#16

Thanks alot got the eclipse thing working :slight_smile:

Also got my code working after i switched to ArrayList<string[]> thanks alot everyone responding quickly :slight_smile: i appriciate all the help.