The lists are stored contiguously in a piece of an ArrayList, specified by the indexes at their boundaries.

For example, here is a picture of the two sorted lists of card values that we merged in class on Wednesday 2019-05-08, preceded by 2 positions whose values are to be ignored, and followed by 3 positions whose values are to be ignored. The values to be ignored are represented here by _.

values:  _ _ "4" "5" "6" "9" "2" "3" "4" "6" "7" "J" "Q" "K" _  _  _
indexes: 0 1  2   3   4   5   6   7   8   9   10  11  12  13 14 15 16 

The boundaries (check my counting!) are represented with half-open intervals:

• list0 starts at index 2 with the String value "4"
• list1 starts at index 6 with the String value "2". Since the lists are "stored contiguously", that 6 also marks the position just past the end of list0. The last value in list0 is therefore "9".
• list1 ends just before index 14, with a last value of "K".

The resulting, merged list is to occupy the original range. In the example above that means the interval [2,14).

We need not merge in place. (Doing so is apparently complicated.) So start your merge procedure by allocating storage to hold a copy of the data. Copy the data, then merge from the copy into the range whence it arrived.

You are of course welcome to look into in-place merges. But get this merge working first.