Inserting, Replacing, and Removing ArrayList Elements

A music app keeps a playlist in an ArrayList, in the order the songs will play.

ArrayList<String> songs = new ArrayList<String>();
songs.add("Intro");
songs.add("Rain");
songs.add("Static");
songs.add("Neon");
songs.add("Echo");

The calls below run one after another, so each one starts from the list the one before it left. The table after the three calls shows every state.

songs.add(2, "Bridge");

This version of add puts "Bridge" at index 2. The elements that were at index 2 and above each shift one place to the right. So "Static" goes from index 2 to 3, "Neon" from 3 to 4, and "Echo" from 4 to 5. The size goes from 5 to 6, and nothing is lost.

String old = songs.set(0, "Opening");

The call stores "Opening" at index 0 and returns the element that was there, so old is "Intro". No element changes its index, and the size stays 6.

String gone = songs.remove(3);

Before the insertion, index 3 held "Neon". The insertion shifted "Static" there, so this call removes "Static" and gone refers to it. The elements after index 3 each shift one place to the left, so "Neon" goes back to index 3 and "Echo" to index 4. The size drops to 5.

The methods get, set, and remove name an element that already exists, so their index must be from 0 through size() - 1. The two-argument add names the place a new element will go. So it also accepts size() itself, which puts the new element last. Any index outside the allowed range throws an IndexOutOfBoundsException.