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I'm working with a program that uses two-dimensional arrays of Strings (probably not that smart to begin with, but eh), and I'd like to write a function that takes one of these arrays (let's say array1), makes an independent copy, and returns it (let's say array2). However, when I then change a value in array2, it seems to be reflected in array1.

My function currently looks something like this:

public static String[][] copy(String[][] matrix, int n) { String[][] out = new String[n+1][n+1]; for (int i = 0; i < n+1; i++) for (int j = 0; j < n+1; j++) { if(matrix[i][j] != null) { String cp = new String(matrix[i][j]); out[i][j] = cp; } } return out; }

I declare a new array of Strings, and then iterate through it, copying each value individually. When that didn't work, I even tried explicitly declaring a new string from each old string and putting that in the array instead.

Can anyone tell me where I'm going wrong?

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  • Seems like it should work to me... can you provide some example code that illustrates the problem you've been having? Commented Apr 1, 2009 at 18:20
  • +1: works fine for me... can you provide the code for your whole test setup? Commented Apr 1, 2009 at 18:28
  • Is there an edit function? Sorry, all, I realized that the problem was actually with my print function. Thanks for your thorough help, though: I cleaned up the function with the suggestions and learned a lot about how Java passes arguments. Commented Apr 1, 2009 at 18:48
  • Actually, there is an edit function ... go ahead, find and use it :-) ... Commented Apr 1, 2009 at 18:57
  • Works for me. Provide a small, complete example of this not working, or it's not a question. Commented Apr 1, 2009 at 19:44

7 Answers 7

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I'm not sure what the n parameter is for, but if I needed such a function, I'd use something like this:

public static String[][] copy(String[][] matrix) { String[][] copy = new String[matrix.length]; for (int idx = 0; idx < matrix.length; ++idx) copy[idx] = matrix[idx].clone(); return copy; }

You don't need to create a copy of the String, because they are immutable. As pointed out by Michael in the comments, the String(String) constructor might be useful if the original string was created as a substring of some very large string. Another use is when you are using String objects as locks (not recommended), and want a private instance to avoid deadlocks.

Also, your check to see whether an element is null before assigning is unnecessary; if you have your loops setup correctly, the element is guaranteed to be null. (And if it's not, what's the harm in overwriting it?)

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3 Comments

Another useful reason to use String(String) could be to force substrings of a large base String to be trimmed down so that the base String's memory can be reclaimed (since substring() usually keeps a reference to the entire base string)
Since String is immutable, you don't need to clone it.
@Michael: good point; will update accordingly. @Steve: no one is cloning any Strings. Could you explain what you are talking about?
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Your method looks like it should work, though passing in n as a parameter makes it brittle, using the input array's length field would be better, and you could even handle jagged arrays that way.

Making a copy of the contents is not necessary, since Strings cannot be changed - which leads to the main question: What kind of changes are you making that seem to be reflected in the copy? Show us the code that does this.

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Have a look at System.arraycopy. That way you can get rid of the inner loop.

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Maybe Arrays.copyOf would be of some use?

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I tried with your code : got exception java.lang.ArrayIndexOutOfBoundsException

It's working for me, please try like this :

 public static String[][] copy(String[][] matrix, int n) { String[][] out = new String[n][n]; for (int i = 0; i < n; i++) for (int j = 0; j < n; j++) { if(matrix[i][j] != null) { String cp = new String(matrix[i][j]); out[i][j] = cp; } } return out; }
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Take a look at this question Is Java pass by reference? It can be a little confusing how Java passes objects, but this would explain why making a change to one array also makes the change to your other array.

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-1 it doesn't explain, for the code given, why making a change to one array also changes the other.
Thats b/c the problem was not with the code given, re-read the first part of the question. From the question comments "I cleaned up the function with the suggestions and learned a lot about how Java passes arguments. – Lily"
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Your use of the 'n' parameter, as noted above is redundant, but also flawed by your code with n+1?? Your code would produce an ArrayIndexoutOfBoundsException if run something like:

  String [][] m1 = { {"A", "B"}, {"C", "D" } }; String [][] m2 = copy(m1, 2);

Which presumably is how you intend it to be invoked?

It also limits your function to square 'matrices' of Strings. But as for the problem you cited, I see no reason why the program should behave that way... I even ran it using the above call (but with n=1???) then changed 

m2[0][1] = "X";

and m1 was unaffected as expected. Even replacing the innermost code line to:

 out[i][j] = matrix[i][j];

does not change this, as the compiler rewrites it to what you originally had. In fact a lot of the String sytax is simply syntactic sugar for StringBuffers (such as concatenation and assignment). eg the compiler will rewrite 

 String s = "Hello "; s += "World"; // Makes it appear that String is builtin type!
to 
 String s = new String("Hello "); s = new StringBuffer(s).append("World").toString();
Which is why is you have lots of string concatenation inside loops they can perform very poorly.

I do not understand why you are having the problem you cited either.

And as you are not modifying the parameter 'matrix', 'Pass By Reference' has nothing to do with it.

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