From here:
ArrayList is internally backed by Array in Java, any resize operation in ArrayList will slow down performance as it involves creating new Array and copying content from old array to new array.
In terms of performance Array and ArrayList provides similar performance in terms of constant time for adding or getting element if you know index. Though automatic resize of ArrayList may slow down insertion a bit Both Array and ArrayList is core concept of Java and any serious Java programmer must be familiar with these differences between Array and ArrayList or in more general Array vs List.