Add variables to Arraylist and it replaces all previous elements , while variables are not static

Question

My question is similar to this one Add elements to Arraylist and it replaces all previous elements in Java . Though my variables are not static. Still everytime I add one, the other values will be that value.

Important code:

int counter = 1;
// Threshold the image to get a binary image
image.threshold(44);
image.showImage();
int[] directionFIRST = new int[2];
// Get the first white pixel on the boundary
int[] pixelFIRST = image.getFirstBoundaryPixel();

image.updatePicture(pixelFIRST[0], pixelFIRST[1]);

directionFIRST = getInitialDirection(image, pixelFIRST);

//Create an array for the output.  It will hold the (x,y) coordinates of
//every pixel around the border of the region to be contour-traced.  The
//directions are also saved for the chain code.
List<int[]> listCONTOUR = new ArrayList<int[]>();
List<int[]> listDIRECTION = new ArrayList<int[]>();

// Create a variable which will be used to tell the algorithm when to stop:
boolean stopCondition = false;
int[][] ROTmatrix90 = new int[][]{{0, 1}, {-1, 0}};
int[][] ROTmatrix180 = new int[][]{{-1, 0}, {0, -1}};

int[] tempPIX = pixelFIRST;
int[] tempDIR = directionFIRST;

while (!stopCondition) {

    //Take the direction opposit the current direction
    tempDIR = multiply(ROTmatrix180, tempDIR);

    tempPIX[0] = tempPIX[0] + tempDIR[0];
    tempPIX[1] = tempPIX[1] + tempDIR[1];
    if (image.get(tempPIX[0], tempPIX[1]) == 1) {
        listCONTOUR.add(tempPIX);
        listDIRECTION.add(tempDIR);
    } else {
        tempDIR = multiply(ROTmatrix90, tempDIR);
        tempPIX[0] = tempPIX[0] + tempDIR[0];
        tempPIX[1] = tempPIX[1] + tempDIR[1];
        if (image.get(tempPIX[0], tempPIX[1]) == 1) {
            listCONTOUR.add(tempPIX);
            listDIRECTION.add(tempDIR);
        } else {
            tempDIR = multiply(ROTmatrix90, tempDIR);
            tempPIX[0] = tempPIX[0] + tempDIR[0];
            tempPIX[1] = tempPIX[1] + tempDIR[1];
            if (image.get(tempPIX[0], tempPIX[1]) == 1) {
                listCONTOUR.add(tempPIX);
                listDIRECTION.add(tempDIR);
            } else {
                tempDIR = multiply(ROTmatrix90, tempDIR);
                tempPIX[0] = tempPIX[0] + tempDIR[0];
                tempPIX[1] = tempPIX[1] + tempDIR[1];
                if (image.get(tempPIX[0], tempPIX[1]) == 1) {
                    listCONTOUR.add(tempPIX);
                    listDIRECTION.add(tempDIR);
                } else {
                    tempDIR = multiply(ROTmatrix90, tempDIR);
                    tempPIX[0] = tempPIX[0] + tempDIR[0];
                    tempPIX[1] = tempPIX[1] + tempDIR[1];
                    if (image.get(tempPIX[0], tempPIX[1]) == 1) {
                        listCONTOUR.add(tempPIX);
                        listDIRECTION.add(tempDIR);
                    }
                }
            }
        }
    }

    counter++;
    image.updatePicture(tempPIX[0], tempPIX[1]);
    System.out.println(tempPIX[0] + " , " + tempPIX[1]);

    if(tempPIX[0]== tempPIX[1]){
        System.out.println("test");
    }

    if ((listCONTOUR.size() > 2) && (tempPIX[0] == listCONTOUR.get(1)[0]) && (tempPIX[0] == listCONTOUR.get(1)[1])) {
        stopCondition = true;
        listCONTOUR.remove(listCONTOUR.get(listCONTOUR.size() - 1));
        listDIRECTION.remove(listDIRECTION.get(listDIRECTION.size() - 1));
    }
}

When I check the values of listCONTOUR after lets say 5 runs of the loop, all the values are the same, which is not possible. I searched for a solution, but all solutions are pointing towards the fact that the variable is static. While in my case it isn't. It is just a simple local variable which is initiated in a function and used within 1 function.

Can you post your loop? That may be the reason how does your loop looks like.. — Rohit Jain, Oct 17 '12 at 08:07

MadProgrammer · Accepted Answer · 2012-10-17T08:46:28.683

tempPIX is reference to an int[] array in memory.

Each time you update the array and add it to the list, you are simply adding the same reference to the same array over and over again.

A better solution would be to create a new array on each loop...

int[] tmpAry = new int[2];
tmpAry[0] = ntempPIX[0] + tempDIR[0];
tmpAry[1] = tempPIX[1] + tempDIR[1];

tempPIX = tmpAry; // Reassign the reference so the rest of the code doesn't need to be updated

UPDATED from comments

Well, all I can say is, I don't know what you're doing...

public class TestArrays {

    public static void main(String[] args) {
        List<int[]> listOfValues = new ArrayList<int[]>();
        int[] outter = new int[] {1, 2, 3, 4};

        listOfValues.add(outter);
        dump(outter);
        for (int index = 0; index < 5; index++) {            
            int[] inner = new int[] {
                rand(),
                rand(),
                rand(),
                rand()
            };
            outter = inner;
            dump(outter);
            listOfValues.add(outter);            
        }

        int index = 0;
        for (int[] values : listOfValues) {
            System.out.print("[" + index + "] ");
            dump(values);
            index++;
        }

    }

    public static void dump(int[] values) {
        for (int value : values) {
            System.out.print(value + ", ");
        }
        System.out.println("\b\b"); // Cheeck...;)
    }

    public static int rand() {
        return (int)Math.round(Math.random() * 100);
    }

}

Which outputs something like...

1, 2, 3, 4
44, 35, 76, 9
44, 11, 17, 35
99, 24, 39, 23
20, 31, 9, 66
45, 50, 60, 27
[0] 1, 2, 3, 4
[1] 44, 35, 76, 9
[2] 44, 11, 17, 35
[3] 99, 24, 39, 23
[4] 20, 31, 9, 66
[5] 45, 50, 60, 27

Yeah I thought of this too, though creating a new variable for every loop seems a bit overkill, no ? — Olivier_s_j, Oct 17 '12 at 08:13
Not really, it's a relativly small array of int's, so the memory foot print will still be small, besides, for this you really don't have much choice — MadProgrammer, Oct 17 '12 at 08:14
I've tried this but strangely it does not work. See edit1 for what I did (I expected this to work tbh) — Olivier_s_j, Oct 17 '12 at 08:25
If I copy the arrays by using System.arraycopy(tempPIX, 0, tmpAry, 0, tempPIX.length); it works — Olivier_s_j, Oct 17 '12 at 08:32

Rohit Jain · Answer 2 · 2012-10-17T08:22:13.307

Ok, the problem is that you are changing the same array object and pushing a reference to it into the list. So, when the array object is changed, it will also get reflected in all the references pointing it.

int[] tempPIX = pixelFIRST;

So, you have created this array outside your while loop. And inside the while loop, you are modifying your array and adding to the list. Since, array object are not immutable so, a new array object will not be created when you change the content, rather the change will get reflected in your list also.

What you can do is, create a new array inside your while loop. And copy the content in that array.

int[] temp = new int[2];
temp[0] = tempPIX[0] + tempDIR[0];
temp[1] = tempPIX[1] + tempDIR[1];

tempPIX = temp;

And add this array to your List. You also need to re-assign the new array to your old array to reflect changes there also (As in the 4th line)

Add variables to Arraylist and it replaces all previous elements , while variables are not static

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