4.8 ArrayList Methods

What you need to know

• Create: ArrayList<String> names = new ArrayList<String>(); (empty, size 0). Import java.util.ArrayList.
• size() — number of elements (a method, with parentheses — unlike array length).
• add(obj) — appends to the end, returns true. add(index, obj) — inserts at index, shifting later elements right; size grows by 1.
• get(index) — returns the element. set(index, obj) — replaces the element at index, returns the old value; size unchanged.
• remove(index) — removes and returns the element at index, shifting later elements left; size shrinks by 1.
• Valid indices are 0 to size() - 1. get(size()) throws IndexOutOfBoundsException. add(size(), obj) is legal (appends).
• ArrayList holds objects only — use wrapper types for numbers (4.7).

Worked example

ArrayList<String> a = new ArrayList<String>();
a.add("x");              // [x]
a.add("y");              // [x, y]
a.add(1, "m");           // [x, m, y]   inserted, y shifted right
a.set(0, "z");           // [z, m, y]   replaced
String r = a.remove(1);  // [z, y]      r = "m"
System.out.println(a.size());     // 2
System.out.println(a.get(1));     // y
// a.get(2);             // IndexOutOfBoundsException

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Declaration: ArrayList<Type> name = new ArrayList<Type>();. The type parameter is required for the exam. new ArrayList<>() (diamond) is also fine. Type must be a class: String, Integer, Double, your own class.
• The six methods on the reference sheet: int size(); boolean add(E obj) (appends); void add(int index, E obj) (inserts, shifts right); E get(int index); E set(int index, E obj) (replaces, returns old); E remove(int index) (removes, returns removed, shifts left).
• Index rules: get/set/remove need 0 ≤ index < size. add(index, obj) allows 0 ≤ index ≤ size — index == size appends. get(size()) throws IndexOutOfBoundsException.
• Return values matter: remove and set return the element that was there. Questions use this: String s = list.remove(0); or if (list.set(1, "x").equals("y")).
• Size changes: add (either form) +1, remove −1, set 0, get 0. Track size when tracing.
• Printing an ArrayList shows [a, b, c] — brackets, commas, spaces. Printing an array shows a memory address. The exam prints ArrayLists directly.
• ArrayList vs array: resizable vs fixed; size() vs length; get(i) vs [i]; objects only vs any type. Same 0-based indexing.

Mistakes that cost points

• Not rewriting the list after add/remove. Indices shift. Write it out.
• get(size()). One past the end. Exception.
• Using [] on an ArrayList. Compile error. get and set.
• Ignoring set's/remove's return value when the question uses it.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(1, 15);
list.remove(2);
System.out.println(list);