[Java] ArrayList element operation issue

[paprikman]

Ok, so we're all familiar with ArrayList stuff in java, but I've faced an issue with it.

Long-shorlty:

ArrayList<int[]> x = new ArrayList<int[]>();
x.add(new int[] { 0, 0 });
// breakpoint 1
int[] y = new int[2];
y = x.get(0);
y[0] = 1;
y[1] = 2;
// breakpoint 2

The values of this single element of x will be different on breakpoints 1 & 2, obviously.

The issue is: I need to form a list first, then get any element and work with it, but not to have this element changed in original list. Kind 'a wanna deal with this thing.

I hope the explanation is clear.

Thanks.

[highonquack]

Try this

ArrayList<int[]> x = new ArrayList<int[]>();
x.add(new int[] { 0, 0 });
// breakpoint 1
int[] y = new int[2];
y[0] = x.get(0)[0];
y[1] = x.get(0)[1];
// breakpoint 2

Objects are passed by reference in Java. So when you set y equal to the first array in the list, it passed you a pointer/reference to the array that was inside of the ArrayList.

[paprikman]

9 minutes ago, highonquack said:

Try this

ArrayList<int[]> x = new ArrayList<int[]>();
x.add(new int[] { 0, 0 });
// breakpoint 1
int[] y = new int[2];
y[0] = x.get(0)[0];
y[1] = x.get(0)[1];
// breakpoint 2

Objects are passed by reference in Java. So when you set y equal to the first array in the list, it passed you a pointer/reference to the array that was inside of the ArrayList.

Yeah, that's it.

I realized what the core problem is, but didn't really see any other workaround with it (thought about more elegant way).

So I'm gonna do it that way, thanks.

[Blade of Grass]

I would suggest against manually copying the array and instead suggest you use Arrays.copyOf. It internally calls System.arrayCopy which will generally be multiple times faster than the alternative. 

 

https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html

 

ArrayList<int[]> x = new ArrayList<>();
x.add(new int[] { 0, 0 });

// If you don't know the length of the array.
int[] y = Arrays.copyOf(x.get(0), x.get(0).length);

// Using a known length
int xArrLen = 2;
int[] y = Arrays.copyOf(x.get(0), xArrLen);

 

[paprikman]

57 minutes ago, Blade of Grass said:

I would suggest against manually copying the array and instead suggest you use Arrays.copyOf. It internally calls System.arrayCopy which will generally be multiple times faster than the alternative. 

 

https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html

 

ArrayList<int[]> x = new ArrayList<>();
x.add(new int[] { 0, 0 });

// If you don't know the length of the array.
int[] y = Arrays.copyOf(x.get(0), x.get(0).length);

// Using a known length
int xArrLen = 2;
int[] y = Arrays.copyOf(x.get(0), xArrLen);

 

Thanks, that's more of what I was looking for. Due to highly probabilistic character of my program, I can't really measure what improvement will be, but it's way more elegant.