📓7.2: ArrayList Methods

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 7.2

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.

NOTE: 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.

  • int size() returns the number of elements in the list

  • boolean add(E obj) appends obj to the end of the list, and also returns true

  • void add(int index, E obj) inserts obj at the index, moves any current objects at index or beyond to the right (to a higher index), and also returns the replaced element

  • E remove(int index) removes the item at the index, shifts remaining items to the left (to a lower index), and also returns the replaced element

  • 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, and also returns the replaced element

size()

As we saw in the last lesson, 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. The number of items in an empty ArrayList is 0.

AP EXAM NOTE: You will not be penalized if you mix up length and size() in the CSA exam.

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.

Try the code below to see how the list changes as each object is added to the end.

ArrayList<String> nameList = new ArrayList<String>();
nameList.add("Diego");
System.out.println(nameList);
nameList.add("Grace");
System.out.println(nameList);
System.out.println(nameList.size());

Can you add your name to the list and then print out the list?

When adding Integers to a list, you can use the Integer object constructor like add(new Integer(5))… but you can also just add the primitive int value directly, like add(5), and it will be changed into an Integer object automatically.

📦 This is called autoboxing. When you pull an int value out of a list of Integers, that is called unboxing.

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 Creatures:

ArrayList<Creature> zoo = new ArrayList<Creature>();
zoo.add(new Creature("Bella", "Unicorn", 15));
zoo.add(new Creature("Bobby", "Bear", 8));

add(index,obj)

There are actually two different add methods in the ArrayList class:

  1. The add(obj) method adds the passed object to the end of the list.
  2. The add(index,obj) method inserts the passed object at the passed index, but first shifts over any existing values to higher indices to make room for the new object.

💬 DISCUSS: What will the code below print out? Try figuring it out before running it.

Remember that ArrayLists start at index 0 and that the add(index,obj) always takes the index as the first argument.

ArrayList<Integer> list1 = new ArrayList<Integer>();
list1.add(1);
// adds the number 2 to the end of the list
list1.add(2);
// This will add the number 3 at index 1
list1.add(1, 3);
// This will add the number 4 at index 1
list1.add(1, 4);
System.out.println(list1);
System.out.println(list1.size());

remove(index)

You can also remove values from an ArrayList using the remove(index) method. It removes the item located at the specified index, which affects the underlying array in two ways: all of the other items after that index shift to a lower index, and the size of the ArrayList is decreased by 1.

Note that this method is NON-VOID: It also returns the item that was removed… in case you need to see it (but usually you don’t).

💬 DISCUSS: What will the code below print out? Try figuring it out before running it.

ArrayList<Integer> list2 = new ArrayList<Integer>();
list2.add(1);
list2.add(2);
list2.add(3);
list2.remove(1);
System.out.println(list2);

The remove(int index) method 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.

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 an object class with methods, not a built in type with special support in the language like arrays are.

💬 DISCUSS: What will the code below print out? Try figuring it out before running it.

ArrayList<String> nameList = new ArrayList<String>();
nameList.add("Diego");
nameList.add("Grace");
nameList.add("Deja");
System.out.println(nameList.get(0));
System.out.println(nameList.get(1));
nameList.set(1, "John");
System.out.println(nameList);

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?

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 ArrayList\ s. Note that ArrayList\ s have a method size() instead of a length property, and ArrayList\ s use get/set methods instead of the index operator ([]).

Operation array ArrayList
Number of Items array.length list.size()
Access value = array[index]; value = list.get(index);
Modify array[index] = value; list.set(index,value)

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

💻 In-Class Activity

💬 DISCUSS: Why might an ArrayList be a more appropriate data structure than an array for a To Do list?

For each of the following tasks, rewrite the given code to use an ArrayList instead of an array.

  1. Create and populate the data structure: 
      String[] toDoList = new String[3];
  toDoList[0] = "Do homework";
  toDoList[1] = "Help make dinner";
  toDoList[2] = "Call grandma";
  2. Replace an element at a specified index: 
      toDoList[1] = "Order pizza";
  3. Check the number of items: 
      System.out.println(toDoList.length + " things to do!");
  4. Access a value at a specified index: 
      System.out.println("Here's the first thing to do: " + toDoList[0]);
  5. Remove the first item and shift everything down: 
      toDoList[0] = toDoList[1];
  toDoList[1] = toDoList[2];
  toDoList[2] = "";

    HINT: this can all be done in one method call with ArrayList!

⭐️ Summary

  • int size() returns the number of elements in the list

  • boolean add(E obj) appends obj to the end of the list, and also returns true

  • void add(int index, E obj) inserts obj at the index, moves any current objects at index or beyond to the right (to a higher index), and also returns the replaced element

  • E remove(int index) removes the item at the index, shifts remaining items to the left (to a lower index), and also returns the replaced element

  • 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, and also returns the replaced element

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.