Thread: Odd behaviour in map of coordinates

Hi,
I have a HashMap<Character, int[]> full of coordinates of letters in 5x5 table (char[5][5]), which I'm using for a cipher program I'm writing. Basically in order to encode each letter I take the coordinates of the letter and select a letter at different coordinates based on certain rules, but it wasn't working correctly for a reason which took me quite a while to work out. The problem was that I was creating an int array of the original coords, and then editing that array, thinking that it was just a copy and it should have no effect on the map, but it was affecting the map. I've recreated a very simple version of this problem, hopefully someone can tell me why this is happening.

import java.util.*;
 
public class Cipher
{
   public void test()
   {
       Map<Character, int[]> m = new HashMap<>();
        int[] v = new int[2];
        v[0] = 0; v[1] = 0;
        m.put('A', v);
 
        // coord should be a copy of the  map value 'A' right?
        int[] coord = m.get('A');
 
        // prints A : 0,0 as expected
        System.out.println(String.format("A : %d,%d", coord[0], coord[1]));
 
        // editing coord shouldn't affect the value in the map
        coord[0] = 1; coord[1] = 1;
 
        int[] newCoord = m.get('A');
 
        // Map value for 'A' is now 1,1
        System.out.println(String.format("A : %d, %d", newCoord[0], newCoord[1]));
   }
 
   public static void main(String[] args)
   {
      Cipher c = new Cipher();
      c.test();
   }
}

This only happens when the value is an array. When i tried this with a Map<Character, Integer>, the map wasn't affected, as expected.

        Map<Character, Integer> m2 = new HashMap<>();
        m2.put('A', 0);
        int c = m2.get('A');
        c = 1;
        // the value of 'A' is still 0
        System.out.println(String.format("A : %d", m2.get('A')));

What is happening here? Why is the value of the first map being changed? Thanks in advance!

I think i get it. Is this kind of like in C, where if say you send an array to a function, you're actually sending a pointer to the memory location of the first element? And when I assigned the array coord to map.get('A'), instead of copying it like I thought it would do, it assigned the memory location of the map value of 'A'?

OK great, thank for your help