Array Unexpected Behavior

Question

So I am trying to do a very simple task of taking two identical arrays and setting the first value of the first array to the last value of the second array.

The code below shows me trying to do so with the numberArray and duplicateArray. The bulk of the code however is me trying to figure out why I am getting a weird result.

System.out.print("Normal array 1: ");
for (int l = 0 ; l < numberArray.length ; l++){
    System.out.print(numberArray[l]);
}

System.out.print("Duplicate array 1: ");
for (int l = 0 ; l < duplicateArray.length ; l++){
    System.out.print(duplicateArray[l]);
}
System.out.println();

numberArray[0] = duplicateArray[duplicateArray.length - 1];

System.out.print("Duplicate array 2: ");
for (int l = 0 ; l < duplicateArray.length ; l++){
    System.out.print(duplicateArray[l]);
}
System.out.println();

System.out.print("Normal array 2: ");
for (int l = 0 ; l < numberArray.length ; l++){
    System.out.print(numberArray[l]);
}

This outputs:

Normal array 1: 123
Duplicate array 1: 123
Duplicate array 2: 323
Normal array 2: 323

What makes no sense to me is that even though I am supposedly setting the value one object to another, both are changing.

My understanding of Java is this:

array[x] = array2[y];
//value at index x of "array" now equals value at index y of "array 2"
//however, array2 should not be changed at all since it is just referenced in the above line of code, not mutated

Am I missing something really simple? I've been racking my brains trying to figure this out. Any help would be greatly appreciated.

EDIT:

I have been asked to provide the declaration of my two arrays so here they are:

int[] numberArray = {1, 2, 3};
int[] duplicateArray = numberArray;

This shouldn't make a difference right? Setting one array to another shouldn't link them (or so I think).

Can you provide us declaration of this arrays? I guess that they both reference to the same object — Andrii Abramov, Sep 24 '16 at 12:35
This is called **Aliasing** and happens, when two arrays point to the same reference. — QBrute, Sep 24 '16 at 12:44
Possible duplicate of [Is Java "pass-by-reference" or "pass-by-value"?](http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value) — Oleg Estekhin, Sep 24 '16 at 13:02

Pavneet_Singh · Accepted Answer · 2016-09-24T13:56:11.310

3

As i commented , your both array pointing to same array in memory

int[] numberArray = {1, 2, 3};
int[] duplicateArray = numberArray; // duplicateArray is just a another name for numberArray

so if you change any value with either of names duplicateArray or numberArray mean you are changing the array in memory which is pointed by both.

So you need to create a new array by adding elements of numberArray to duplicateArray using System.arraycopy;

like this

int[] numberArray = {1, 2, 3};
int[] duplicateArray = new int[numberArray.length];
System.arraycopy( numberArray , 0, duplicateArray , 0, numberArray .length );

View the image for visual view

edited Sep 24 '16 at 13:56

answered Sep 24 '16 at 12:39

Pavneet_Singh

36,884
5
53
68

@Benjamin Lowry letme add something more , your will like it and your welcome Mr. Benjamin Lowry – Pavneet_Singh Sep 24 '16 at 12:48
If you want an exact copy of an array, you should use `array.clone()` instead of manually copying the elements. – Clashsoft Sep 24 '16 at 12:53
@PavneetSingh thanks for the visual! That makes a lot of sense now. – Benjamin Lowry Sep 24 '16 at 12:57
1

@Tom ok i gonna do it, it feels good to be courteous but you are 100% right , it's done – Pavneet_Singh Sep 24 '16 at 13:07
@Clashsoft array.clone() can cause the same problem as well cuz it just copy the reference of one to another not element – Pavneet_Singh Sep 24 '16 at 13:17

score 0 · Answer 2 · answered Sep 24 '16 at 12:46

0

Previous answer is correct, to clarify, although the integer elements of the array are immutable, the array itself is not. Essentially, you have one array at two references; the 'duplicate' is not a duplicate object, just a duplicate reference.

answered Sep 24 '16 at 12:46

DanzaBarr

13
6

Array Unexpected Behavior

2 Answers2