How can I copy an array by value in Java?

Question

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Make copy of array Java

I'm a beginner at Java and I need to copy the contents of one array into another variable. However, Java always passes the array by reference instead of by value.

Here's what I mean if that was confusing:

int test[]={1,2,3,4};
int test2[];
test2=test;
test2[2]=8;
for(int i=0;i<test2.length;i++)
    System.out.print(test[i]); // Prints 1284 instead of 1234

In this example, I don't want the value of test to change. Is this possible without using any of the more advanced features of Java such as ArrayList and Vectors?

Edit: I tried System.ArrayCopy and test.clone(), but they still don't seem to work. Here's my actual code:

temp_image=image.clone();
for(int a=0;a<image.length;a++)
    for(int b=0;b<image[0].length;b++)
        image[a][b]=temp_image[image.length-1-a][b];

Basically I'm trying to flip the "image" upside down. Is there an error somewhere in the code?

System.arraycopy: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html — Luiggi Mendoza, Apr 13 '12 at 02:34
@LuiggiMendoza you might want to start using more recent JavaDocs; [Java 1.4 is 3 versions (10 years) behind](http://en.wikipedia.org/wiki/Java_version_history). — Matt Ball, Apr 13 '12 at 02:37
@MДΓΓБДLL well, if you check the link in your comment, it refers to `System.arraycopy` too! — Luiggi Mendoza, Apr 13 '12 at 02:50
@LuiggiMendoza all I meant was that you should link to [non-obsolete JavaDocs](http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#arraycopy%28java.lang.Object,%20int,%20java.lang.Object,%20int,%20int%29). — Matt Ball, Apr 13 '12 at 03:49

score 5 · Accepted Answer · answered Apr 13 '12 at 02:32

5

You need to clone your array.

test2=test.clone();

answered Apr 13 '12 at 02:32

kba

19,333
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Kaleb Brasee · Answer 2 · 2012-04-13T02:45:16.443

2

Starting in Java 6 you can use Arrays.copyOf:

test2 = Arrays.copyOf(test, test.length);

For what you're looking to do, test.clone() is fine. But if you wanted to do a resize, copyOf allows you to do that. I think in terms of performance it

System.arraycopy would give even more options if you needed them.

edited Apr 13 '12 at 02:45

answered Apr 13 '12 at 02:39

Kaleb Brasee

51,193
8
108
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score 0 · Answer 3 · answered Apr 13 '12 at 02:41

Because test and test2 are both pointers to the same array, you are changing the value of both test and test2 with your statement test2[2]=8

A solution would be to copy the contents of test into test 2 and change the value at the specific index of test2.

    for (int i=0,i<test.length,i++)
        test2[i]=test[i]
    //Now both arrays have the same values

    test2[2]=8

    for (int j=0,j<test.length,j++)
        System.out.print(test[i])
        System.out.println()
        System.out.print(test2[i])

Will output

    1 2 3 4
    1 2 8 4

How can I copy an array by value in Java?

3 Answers3