Collections.reverse() flips the order of a list:
Collections.reverse(nums);
// [10, 20, 30] becomes [30, 20, 10]
This runs in time and space because it swaps elements from both ends toward the middle.
To sort in descending order, sort first, then reverse. There is no single method for reverse sorting in the basic Collections class.
Collections.shuffle() randomizes the order:
Collections.shuffle(nums);
Each call produces a different arrangement. This is useful when you need to randomize a deck of cards, generate random test data, or pick elements without bias. The shuffle runs in time and space using the Fisher-Yates algorithm.