StringBuilder insert() vs append() performance?

They have different functionalities and different complexities,

insert:

1. (ensures The Capacity of the backing array, needs to copy the old one if necessary)
2. pushes the elements leading the item at the insertion index (offset)

Where append:

1. (ensures The Capacity of the backing array, needs to copy the old one if necessary)
2. adds the new element to the tail of the array

So if you want to always add to the tail, then the performance will be the same since insert will not push any elements.

So, I would use append, it is just cleaner.

Knowing that:

• An insert at the end of the string representation is equivalent to an append in term of time complexity (O(n)).
• An insert anywhere else than at the end can't be obtained with an append (as they have differents purposes).
• For info, an insert may involve up to 3 System.arraycopy (native) calls, while an append 1.

You can easily conclude:

• If you want to insert at the end of the string representation, use append
• Otherwise, use insert

Doing so, you will have the best performance. But again, these two methods serving two differents purposes (with the exception of inserting at the end), there is no real question here.