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I am trying to adjust the size of an array using Arrays.copyOf with import java.util.Arrays but for some reason even though the Console is outputting that the array length is 4 like it is supposed to be when i try and assign a value to anything within array row number 4 it is giving me a nullPointerException on the line where it attempts to assign a value to something in row number 4. Can anyone explain this to me?

import java.util.Arrays;
import static java.lang.System.out;

public class Main {
    static int TTT[][] = new int[3][3];

    public static void main(String[] args) {
        TTT = Arrays.copyOf(TTT, 4);
        out.print(TTT.length);
        TTT[3][0] = 2;
    }
}

this gives me an output of:

4Exception in thread "main" java.lang.NullPointerException
    at Main.main(Main.java:13)

Does anyone know why this is happen. Also on a side note is there a better way for me too make an array where the size can be changed? If so can you give me some example code please!

share|improve this question
    
Which is line 13? I only see 12 –  rickyman20 Jul 9 '12 at 23:51
    
out.print(TTT.length); or System.out.print(TTT.length);? –  Lion Jul 9 '12 at 23:51
    
out.print(TTT.length) is fine because he is importing the specific inner reference. –  Jack Jul 9 '12 at 23:54
    
As to your side note, if you want a resizable data structure which is like an array, I would advise an ArrayList in the java.util package. You'll find the documentation at docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html helpful –  CosmicComputer Jul 10 '12 at 0:08
    
Its line 13 because there was other lines that made absolutely no difference to the problem i was trying to solve that were running with it. –  semicolon Jul 10 '12 at 1:52
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1 Answer

up vote 2 down vote accepted

From the documentation:

Copies the specified array, truncating or padding with nulls (if necessary) so the copy has the specified length. For any indices that are valid in the copy but not the original, the copy will contain null. Such indices will exist if and only if the specified length is greater than that of the original array. The resulting array is of exactly the same class as the original array.

What happens is that the array size is increased, so a new reference to int[3] is created at position TTT[3] and it is initialized with null as the documentation states. The fact that the array has a primitive type doesn't mean that it should initialize inner arrays by itself.

Try with:

TTT = Arrays.copyOf(TTT, 4);
TTT[3] = new int[3];
share|improve this answer
    
Thank you that worked perfectly –  semicolon Jul 10 '12 at 0:46
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