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 o

Answer 1

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)

Answer 2

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