How to stop two arrays having the same memory

Question

I have two identical 2D array's of characters however I want to make it so that when I make changes to one it doesn't effect the other. I am assuming it is because they are sharing the same space in memory and was wondering how to stop this.

private char[][] a;
private char[][] b;

a = new char[8][];
b = new char[8][];

while(file.hasNext()) //reads 8 lines from a file and adds it to text to both arrays
{
    char[] constant =  file.nextLine().toCharArray();
    a[i] = constant;
    b[i] = constant;
    i++;
}

then if i were to run the code

a[0][0] = 't';

then it also changes b[0][0] to 't'

@EJP _Technically_, they might be stored in the same index of a variable table. (busting balls) — Sotirios Delimanolis, Feb 17 '14 at 00:21
@SotiriosDelimanolis That would implicitly violate both the Java Language Specification #4.5 and the Java Virtual Machine Specification #2.5 (same wording in each). — user207421, Feb 17 '14 at 00:58
@EJP JLS Chapter 4.5 is about parameterized types. Can you check again? Or clarify. — Sotirios Delimanolis, Feb 17 '14 at 01:01
@EJP Maybe I should specify _local_ variables. See [chapter 2.6 of JVM Spec.](http://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf). Also, take a look at [this answer.](http://stackoverflow.com/questions/21437699/outofmemoryerror-when-seemingly-unrelated-code-block-commented-out) — Sotirios Delimanolis, Feb 17 '14 at 01:34

Sergey Kalinichenko · Answer 1 · 2014-02-17T00:22:42.737

4

Arrays are objects. When you assign them like this

int[][] firstArray = getArray();  // Get your array from somewhere
int[][] secondArray = firstArray; // Assign an array

you have only one array, but you have a second variable that references it. Essentially, secondArray[i][j] and firstArray[i][j] refer to the same object in memory by two different names.

To fix this, make a copy of the array. A simple way of doing it is to call the clone() method on the original array, like this:

int[][] secondArray = new int[firstArray.length][];
for (int i = 0 ; i != secondArray.length ; i++) {
    secondArray[i] = (int[])firstArray[i].clone();
}

Note: clone() creates a shallow copy. If you

edited Feb 17 '14 at 00:22

answered Feb 17 '14 at 00:13

Sergey Kalinichenko

714,442
84
1,110
1,523

2

Be aware that clone() creates a shallow copy of the object. In your example it seems that clone() creates a deep copy. – ROT13 Feb 17 '14 at 00:16
I have just found out that this doesn't work with an array that has more then two dimensions. However if you loop it for each nested array it works – i cant do java but i can do vb Feb 17 '14 at 00:33
1

@icantdojavabuticandovb Correct - you can `clone()` only the last dimension; you need loops for all the remaining ones. – Sergey Kalinichenko Feb 17 '14 at 00:38

ROT13 · Accepted Answer · 2014-02-17T00:30:04.897

My first idea was:

char[] orig = // something;
char[] copy = orig.clone();

But be aware: clone() only creates a shallow copy of an array. That means for a 2D-Array with chars you have to clone each subarray.

For further information about clone() click here.

Your code should look something like this

char[][] original = new char[3][3];
char[][] copy = new char[orignal.length][0];

for(int i = 0; i < original.length; i++){
    copy[i] = original[i].clone();
}

score 0 · Answer 3 · answered Feb 17 '14 at 00:15

I have two identical 2D array's of characters however I want to make it so that when I make changes to one it doesn't effect the other. I am assuming it is because they are sharing the same space in memory and was wondering how to stop this.

Arrays don't share memory. Nor do any other variables in Java.

Ergo if changes to one affect the other, you don't have two arrays at all. You only have one, with two references to it. Updating the array via one reference is visible via the other reference.

The solution is therefore to create two arrays, not just one.

How to stop two arrays having the same memory

3 Answers3