4.8 ArrayList and its Methods

4.8 ArrayList and its Methods

Each coding exercise has its own folder. Open its .java file and press Run to try it. Run Tests checks every graded exercise in this lesson at once.

A couple of lists

At the beginning of this unit, we learned about using arrays to hold collections of related data. However arrays are not very flexible. Most notably, the size of an array is established at the time of creation and cannot be changed. What if you don’t know how big the collection of data will be? What if you want to both add and remove items from a collection? For example, if you wanted to represent a shopping list, you might add to the list throughout the week and remove things from the list while you are shopping. You probably would not know how many items will be on the list at the beginning of the week.

For cases like this, Java has a class called ArrayList which is a re-sizable list. It is called ArrayList because it stores the items that have been added to it in an underlying array. But it also takes care of keeping track of how many items have been added to the array and it will create a new bigger array under the covers when needed to hold more items.

You can use ArrayList instead of arrays whenever you don’t know the size of the array you need or you know that you will add and remove items and may need to change the array’s size dynamically during run time. An ArrayList is mutable in size and contains object references, meaning it can change during run time by adding and removing objects from it.

An ArrayList is often called just a list. Prior to 2020 the AP CSA curriculum included interfaces which are somewhat like classes and the interface List was often used to declare a variable that would refer to an ArrayList. Interfaces are no longer on the exam, but if you see List being used in an old exam question just assume it’s an ArrayList.

Check your understanding: Which of the following is a reason to use an ArrayList instead of an array? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qloopList

import java.util.ArrayList

The ArrayList class is in the java.util package. A package is a set or library of related classes. The classes we have used until now, such as String and Math, are in the special package java.lang whose classes are always available in any Java program. Other packages, such as java.util, provide classes that can only be used either by importing them or (much more rarely) by referring to them by their full name which includes the package as a prefix. The full name of ArrayList is thus java.util.ArrayList but rather than type that out all the time, in any class where we want to use ArrayList we will usually import it with an import statement.

Import statements have to come before the class definition in a Java source file and serve to tell Java which class you mean when you use a short name like ArrayList. To import just one class we use a single import of the fully-qualified name of the class like this:

// Import just the ArrayList class from java.util
import java.util.ArrayList;

After such an import statement, anywhere ArrayList is used as a class name in the file it will be taken to mean java.util.ArrayList.

Another option is to import all the classes in a package with a “wildcard” import:

// Import everything in java.util including ArrayList
import java.util.*;

This import statement will also cause ArrayList to refer to java.util.ArrayList. But many other names of classes defined in the java.util package will also be available whether you use them or not. (One that you have probably used by now is Scanner which can be used to read input a user types at the command line.) Using wildcard imports can cause conflicts if you import all the classes from two different packages and they have class names in common but usually that’s not a problem, at least with packages that are part of Java itself.

Don’t worry about adding import statements on the AP CSA exam. Any that you need will be provided for you.

Check your understanding: Which of the following is true about import statements? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qlib_1

Declaring and Creating ArrayLists

To declare an ArrayList use ArrayList<Type> name where Type, called a type parameter is the type of objects you want to store in the ArrayList. For example a variable naming an ArrayList meant to hold Strings is declared as ArrayList<String> as shown in the code below. Programmers use the letter E and call it the generic type for an Element. ArrayList<E>, where the generic type E specifies the type of the elements. (Without it, the type will be Object). When ArrayList<E> is specified, the types of the reference parameters and return type when using its methods are type E. ArrayList<E>, where the generic type E specifies the type of the elements. So if E is String, then the type of the reference parameters and return type when using its methods are type String.

You can declare a variable to just be of type ArrayList, with no type parameter, and it’ll be approximately the same as if you had declared ArrayList<Object>, but it is good practice to specify the type of objects you intend to store in an ArrayList as it allows the compiler to find errors (that are specific to that to Strings or ints or whatever type you put in) that would otherwise be missed until run time.

// ArrayList<Type> name = new ArrayList<Type>();
// An ArrayList of Strings:
ArrayList<String> shoppingList = new ArrayList<String>();

As with other reference types, declaring a ArrayList variable doesn’t actually create a ArrayList object. It only creates a variable that can refer to a ArrayList or null. To actually create a ArrayList we must invoke a constructor such as new ArrayList<String>(). The ArrayList constructor ArrayList() constructs an empty list.

You can get the number of items in a ArrayList using the size() method. Notice that a newly constructed ArrayList is empty and thus has a size of 0. Also remember that you can’t call methods on null so trying to call size on the value of list2 at line 10 below causes a NullPointerException.

Exercise 1

The code demonstrates a NullPointerException. Change the list2 declaration so that it creates a new ArrayList to remove the NullPointerException.

Open the 01-ArrayListCreateStr folder and edit ArrayListCreateStr.java. This one is not graded, so check it yourself.

You can also create ArrayLists of integer and double values. However, you have to use the wrapper classes Integer or Double as the type parameter because ArrayLists can only hold objects, not primitive values. All primitive types must be wrapped in objects before they are added to an ArrayList. For example, int values can be wrapped in Integer objects, double values can be wrapped in Double objects. However this normally happens automatically thanks to autoboxing.

You can actually put in any kind of objects in an ArrayList, including instances of classes that you write, such as the Student, Person, or Pet classes.

// An ArrayList of Integers:
ArrayList<Integer> numList = new ArrayList<Integer>();
// An ArrayList of Student objects:
ArrayList<Student> roster = new ArrayList<Student>();

Check your understanding: Which of the following is the correct way to create an ArrayList of integers? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qArrayListInteger

ArrayList Methods

The following are the ArrayList methods that you need to know for the AP CSA exam. These are included on the AP CSA Java Quick Reference Sheet that you will receive during the exam so you do not need to memorize them. The E in the method headers below stands for the type of the element in the ArrayList; this type E can be any Object type. We will look at how these methods work below.

• int size() returns the number of elements in the list
• boolean add(E obj) appends obj to the end of the list and returns true
• E remove(int index) removes the item at the index and shifts remaining items to the left (to a lower index)
• void add(int index, E obj) moves any current objects at index or beyond to the right (to a higher index) and inserts obj at the index
• E get(int index) returns the item in the list at the index
• E set(int index, E obj) replaces the item at index with obj

size()

You can get the number of items in a ArrayList using its size() method. The ArrayList starts out empty with a size of 0.

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

With arrays, you use the length field to get the number of items in the array. But, with an ArrayList you use the size() method to get the number of items in the ArrayList. You will not be penalized if you mix up length and size() in the CSA exam. The number of items in an empty ArrayList is 0.

add(obj)

You can add values to an ArrayList using the method add(obj) which will add the object to the end of the list, just like you would join the end of the line to board a bus. Note that we can add objects of any type to an ArrayList. The following code has a String list and an Integer list.

Exercise 2

Can you add another item to the shopping list and print out the new list?

Open the 02-listAdd folder and edit Shopping.java. Press Run to try it; Run Tests checks it (1 test).

Primitive types like int and double are automatically converted to their corresponding wrapper classes Integer and Double using autoboxing when added to an ArrayList. When you pull an int value out of a list of Integers that is called unboxing.

ArrayList<Integer> list = new ArrayList<Integer>();
list.add(new Integer(5)); // this will work in Java 7
list.add(5); // this will work in all Java versions

You can put any kind of objects into an ArrayList. Even instances of a class that you wrote. For example, here is an ArrayList of Students.

Exercise 3

An example of an ArrayList of Student objects. Add a new student with your name and info in it.

Open the 03-StudentArrayList folder and edit StudentList.java. This one is not graded, so check it yourself.

add(index,obj)

There are actually two different add methods in the ArrayList class. The add(obj) method adds the passed object to the end of the list. The add(index,obj) method adds the passed object at the passed index, but first moves over any existing values to higher indices to make room for the new object. The indices for an ArrayList start at 0 and end at the number of elements - 1.

Exercise 4

What will the code print out? Try figuring it out before running it. Remember that ArrayLists start at index 0 and that the add(index,obj) always has the index as the first argument.

Open the 04-listAddInt2 folder and run listAddInt2.java. It is an example, so it has no tests.

ArrayLists like arrays start numbering their elements from 0.

Check your understanding: What will print when the following code executes?

ArrayList<Integer> list1 = new ArrayList<Integer>();
list1.add(1);
list1.add(2);
list1.add(3);
list1.add(2, 4);
list1.add(5);
System.out.println(list1);

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qalAdd1

You can step through the code above by clicking on this Java Visualizer.

Check your understanding: What will print when the following code executes?

ArrayList<String> list1 = new ArrayList<String>();
list1.add("Anaya");
list1.add("Layla");
list1.add("Sharrie");
list1.add(1, "Sarah");
System.out.println(list1);

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qalAdd2

You can step through the code above by clicking on the following Java Visualizer.

remove(index)

You can also remove values from an ArrayList using the remove(index) method. It removes and returns the item at the given index. This will move all the other items over in the underlying array and decrease the size of the ArrayList by 1.

Exercise 5

What will the code print out? Try to guess before you run it. Were you surprised? Read the note below.

Open the 05-listRem folder and run listRem.java. It is an example, so it has no tests.

The remove(int index) method will remove the object at the given index and shift left any values to the right of that index. It doesn’t remove the object that matches the integer value given. In the example above it doesn’t remove the value 1. It removes the value 2 at index 1.

Check your understanding: What will print when the following code executes?

List<Integer> list1 = new ArrayList<Integer>();
list1.add(1);
list1.add(2);
list1.add(3);
list1.remove(2);
System.out.println(list1);

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qListRem

You can step through the code above by clicking on the following RemoveExample.

get(index) and set(index, obj)

You can get the object at an index using obj = listName.get(index) and set the object at an index using listName.set(index,obj). Both methods require that the index argument refer to an existing element of the list, i.e. the index must be greater than or equal to 0 and less than the size() of the list.

Notice that ArrayLists use get and set methods instead of the index operator that we use with arrays: array[index]. This is because ArrayList is a class with methods, not a built in type with special support in the language like arrays.

Exercise 6

Try to guess what the code will print before running it. Can you get the last element in the nameList to print it out? Can you set the first element in the list to your name and print out the list?

Open the 06-listGetSet folder and edit listGetSet.java. Press Run to try it; Run Tests checks it (2 tests).

Check your understanding: What will print when the following code executes?

List<Integer> list1 = new ArrayList<Integer>();
list1.add(1);
list1.add(2);
list1.add(3);
list1.set(2, 4);
list1.add(2, 5);
list1.add(6);
System.out.println(list1);

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qListRem1

You can step through the code above by clicking on the following Example1.

Check your understanding: What will print when the following code executes?

List<String> list1 = new ArrayList<String>();
list1.add("Anaya");
list1.add("Layla");
list1.add("Sharrie");
list1.set(1, "Destini");
list1.add(1, "Sarah");
System.out.println(list1);

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-4-8-arraylists.html#qListRem2

You can step through the code above by clicking on the following Example2.

Comparing arrays and ArrayLists

When do you use arrays and when do you use ArrayLists? Use an array when you want to store several items of the same type and you know how many items will be in the array and the items in the array won’t change in order or number. Use an ArrayList when you want to store several items of the same type and you don’t know how many items you will need in the list or when you want to remove items from the list or add items to the list while the program is running.

Here is a comparison of how to create arrays and ArrayLists:

// arrays must specify a size!
int[] highScores = new int[5];
String[] names = new String[5];

// ArrayLists are empty to start with
ArrayList<Integer> highScoreList = new ArrayList<Integer>();
ArrayList<String> nameList = new ArrayList<String>();

Here is a comparison of how to access and change elements in arrays and ArrayLists. Note that ArrayLists have a method size() instead of a length property, and ArrayLists use get/set methods instead of the index operator ([]).

Note that the ArrayList methods add and remove do not have a simple equivalent in arrays because they change the number of elements in the list and may shift the positions of other elements.

Here is a comparison handout of the basic operations to access 1-dimensional and 2-dimensional arrays (which we will see in the next lessons), ArrayLists, and Strings made by AP CSA teacher Sam Procopio of Bishop Blanchet High School.

Exercise 7

Rewrite the code that uses an array to use an ArrayList instead. In the comments write why you think an ArrayList is a better data structure to use than an array for this problem.

Open the 07-array-to-arraylist folder and edit ToDoList.java. Press Run to try it; Run Tests checks it (4 tests).

Although it is not on the AP exam, you can convert an array to a List using the static method asList from the Arrays helper class: Arrays.asList(arrayname). Note that ArrayList has a toString method that is automatically called to print the list in a nice format.

Exercise 8

Example code creating an ArrayList from an array.

Open the 08-ArrayListFromArray folder and run ArrayListFromArray.java. It is an example, so it has no tests.

Coding Challenge: FRQ Digits

This coding challenge is based on the 2017 Free Response Question part 1a on the 2017 AP CSA exam. In this question, you are asked to write a constructor for a class called Digits. This constructor takes an integer number as its argument and divides it up into its digits and puts the digits into an ArrayList. For example, new Digits(154) creates an ArrayList with the digits [1, 5, 4].

First, let’s discuss how to break up a number into its digits. Try the code below. What happens if you divide an integer by 10? Remember that in integer division the result truncates (cuts off) everything to the right of the decimal point. Which digit can you get by using % 10 which returns the remainder after dividing by 10? Try a different number and guess what it will print and then run to check.

Exercise 9

Set number to a different number and guess what number / and % will return. Which operator gives you a digit in number?

Open the 09-divideby10 folder and edit DivideBy10.java. Press Run to try it; Run Tests checks it (1 test). Its test checks your code, not what it prints: “Code has been changed”.

We can use a while loop to print out each digit in reverse order starting from the right (4, 5, 1 for the number 154) while dividing it by 10. You can try it in the exercise above. Here is the pseudocode:

• while number is greater than 0
  • print out the last digit using %
  • change the number to cut off the last digit using /

Now, let’s write a constructor for the Digits class that uses this loop and adds each found digit to the ArrayList instead of printing it out.

Note that this will create the digit list in reverse order. To get the digits in the right order, you can use the add(index, obj) method to add the digit to the beginning of the ArrayList instead of the end.

Exercise 10

Complete the challenge below to put the digits of a number in an ArrayList.

Open the 10-challenge-digits folder and edit Digits.java. Press Run to try it; Run Tests checks it (2 tests).

Summary

• ArrayLists are re-sizable lists that allow adding and removing items to change their size during run time.
• (AP 4.8.A.4) The ArrayList class is part of the java.util package. An import statement can be used to make this class available for use in the program (import java.util.ArrayList or java.util.*).
• (AP 4.8.A.1) An ArrayList object is mutable in size and contains object references. (Mutable means that it can change by adding and removing items from it.)
• (AP 4.8.A.2) The ArrayList constructor ArrayList() constructs an empty list (of size 0).
• (AP 4.8.A.3) Java allows the generic type ArrayList<E>, where the generic type E specifies the type of the elements. (Without it, the type will be Object). When ArrayList<E> is specified, the types of the reference parameters and return type when using its methods are type E.
• (AP 4.8.A.3) ArrayList<E> is preferred over ArrayList (which creates a list of type Object). For example, ArrayList<String> names = new ArrayList<String>(); allows the compiler to find errors that would otherwise be found at run time.
• ArrayLists cannot hold primitive types like int or double, so you must use the wrapper classes Integer or Double to put numerical values into an ArrayList. However autoboxing usually takes care of that for you.
• (AP 4.8.A.6) The indices for an ArrayList start at 0 and end at the number of elements - 1.
• (AP 4.8.A.5) The following ArrayList methods, including what they do and when they are used, are part of the Java Quick Reference:
  • int size() : Returns the number of elements in the list
  • boolean add(E obj) : Appends obj to end of list; returns true
  • void add(int index, E obj) : Inserts obj at position index (0 <= index <= size), moving elements at position index and higher to the right (adds 1 to their indices) and adds 1 to size
  • remove(int index) — Removes element from position index, moving elements at position index + 1 and higher to the left (subtracts 1 from their indices) and subtracts 1 from size; returns the element formerly at position index
  • E get(int index) : Returns the element at position index in the list
  • E set(int index, E obj) : Replaces the element at position index with obj; returns the element formerly at position index