Shift elements of an array

I'm writing a code that moves all even numbers to the front of the array and odd numbers to the back. So when an array looks like this:

int[] a={1,3,2,5, 4, 7, 8, 6};

the output should look like:

int[] b={2,4,8,6, 1, 3, 5, 7};

my problem is that the programme should return an array of positions that every even number moved to get to these positions. with my example the output should look like this: [2, 3, 4, 4]

shiftArray = {2,3,4,4};

as number 2 moved two positions, number 4 three positions etc.

here is what i have so far:

int temp=0;
int a=0;
for(int i=0;i<arr.length;i++){
    if(arr[i]%2==0){

        for (int j=i;j>a;j--){
            temp=arr[j-1];
            arr[j-1]=arr[j];
            arr[j]=temp;
        }
        a++;
    }
return arr;
}

how can i use what i have so far to get my desired output?

"How many times does this number need to shift" in this case is actually just "how many odd numbers in total are before this number", because you are trying to move all the even numbers to the beginning without changing the relative positions of the even numbers. Each even number needs to "move over" however many odd numbers there are before it.

So you just need to keep a cumulative count:

int[] a = { 1, 3, 2, 5, 4, 7, 8, 6 };
List<Integer> shifts = new ArrayList<>();
int oddNumberCount = 0;
for (int i : a) {
    if (i % 2 == 0) {
        shifts.add(oddNumberCount);
    } else {
        oddNumberCount++;
    }
}

I don't know how many even numbers there will be, so I used a list. If you require an array as the output you can convert it to a list simply like this (See Also):

int[] result = shifts.stream().mapToInt(x -> x).toArray();

If you do know how many even numbers there will be, you can of course just start with an array of that given size.

What are you lacking in your code is you don't track what's the current position of the even numbers correctly:

public static void main(String...arg){
    int[] a={1,3,2,5, 4, 7, 8, 6,0}; //the array to be shifted
    
    int indexEvenNumber = 0;//where should we put the even number if we found it.
    for (int i = 0; i < a.length; i++) {
        if (a[i] % 2 == 0) {
            int temp = a[i];
            a[i] = a[indexEvenNumber];
            a[indexEvenNumber] = temp;
            indexEvenNumber++;
        }
    }
    // print the shifted array to confirm results
    Arrays.stream(a).forEach(System.out::println);

}