4.8 ArrayList Methods · CSA Exam Prep

Jump to topic

Unit 4 · Data Collections · Topic 4.8

ArrayList Methods

Skill 2.BSkill 3.BSkill 3.D

Big idea: An ArrayList is a resizable list of objects. Adding or removing in the middle shifts the later elements, which changes their indexes.

Learning objectives

Develop code for collections of related objects using ArrayList objects and determine the result of calling methods on these objects.

Creating a list

ArrayList is in java.util. The type in angle brackets says what it holds, and the compiler rejects anything else. A new list is empty: size() is 0.

Code

ArrayList<String> names = new ArrayList<String>();
System.out.println(names.size());

Output

The six methods

Method	Does	Returns
`size()`	Number of elements	`int`
`add(obj)`	Appends to the end	`true`
`add(i, obj)`	Inserts at `i`, shifting later elements right (0 ≤ i ≤ size)	nothing
`get(i)`	Element at `i`	the element
`set(i, obj)`	Replaces element at `i`	the old element
`remove(i)`	Removes element at `i`, shifting later elements left	the removed element

Tracing a sequence of calls

Code

ArrayList<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
list.add(1, "X");
System.out.println(list);
String old = list.set(2, "Y");
System.out.println(old + " " + list);
String gone = list.remove(0);
System.out.println(gone + " " + list + " " + list.size());

Output

[A, X, B, C]
B [A, X, Y, C]
A [X, Y, C] 3

Index errors

Valid indexes for get, set, and remove are 0 to size() - 1. add(i, obj) also allows i == size() (appending). Anything else throws IndexOutOfBoundsException.

Code

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(5);
nums.add(nums.size(), 9);
System.out.println(nums);
nums.get(2);

Output

[5, 9]
throws IndexOutOfBoundsException

Key points

An ArrayList resizes itself and holds object references.
new ArrayList<E>() creates an empty list; E lets the compiler catch type errors.
In java.util; needs an import.
Indexes run 0 to size() - 1.

Watch out

add(size(), x) is legal (appends); add(size() + 1, x) throws.
set and remove both return the element that was there before.
Printing an ArrayList shows [a, b, c].

Quick check

list is [10, 20, 30]. After list.add(0, 5); list.remove(2); what is list?

Show answer

[5, 10, 30]. Insert gives [5, 10, 20, 30], then index 2 (20) is removed.

Practice 4.8 (11 questions)