How would you optimise Bubble Sort?

In Bubble sort, you know that after k passes, the largest k elements are sorted at the k last entries of the array, so the conventional Bubble sort uses:

public static void bubblesort(int[] a) {
  for (int i = 1; i < a.length; i++) {
    boolean is_sorted = true;

    for (int j = 0; j < a.length - i; j++) { // skip the already sorted largest elements, compare to a.length - 1
      if (a[j] > a[j+1]) {
         int temp = a[j];
         a[j] = a[j+1];
         a[j+1] = temp;
         is_sorted = false;
      }
    }

    if(is_sorted) return;
  }
}

Now, that would still do a lot of unnecessary iterations when the array has a long sorted tail of largest elements. If you remember where you made your last swap, you know that after that index, there are the largest elements in order, so:

public static void bubblesort(int[] a) {
  int lastSwap = a.length - 1;
  for (int i = 1; i< a.length; i++) {
    boolean is_sorted = true;
    int currentSwap = -1;

    for (int j = 0; j < lastSwap; j++) { // compare to a.length - i
      if (a[j] > a[j+1]) {
         int temp = a[j];
         a[j] = a[j+1];
         a[j+1] = temp;
         is_sorted = false;
         currentSwap = j;
      }
    }

    if (is_sorted) return;
    lastSwap = currentSwap;
  }
}

This allows to skip over many elements, resulting in about a worst case 50% improvement in comparison count (though no improvement in swap counts), and adds very little complexity.