Topic 6 – Introduction to ArrayLists

In a previous unit, we learned about arrays to hold collections of related data. But arrays have limitations. 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 add and remove items from the collection and change the size of the collection while the program is running? 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.

Luckily, Java has a class called ArrayList which is a re-sizable array. An ArrayList has an underlying array that grows or shrinks as needed. 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 changing the array’s size dynamically during run time. An ArrayList is mutable, meaning it can change during runtime by adding and removing objects from it.

Importing ArrayList

The ArrayList class is in the java.util package. A package is a set or library of related classes. The java.lang package is the main Java language classes that you get automatically without importing it. The java.util package has a lot of utility classes that you can use if you import the package.

You can import just the classes you need from a package as shown below. Just provide an import statement for each class that you want to use.

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

Another option is to import everything at the same level in a package using import packageName.*.

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

Declaring and Creating ArrayLists

To declare a ArrayList use ArrayList<Type> name Change the Type to be whatever type of objects you want to store in the ArrayList, for example String as shown in the code below. You don’t have to specify the generic type <Type>, since it will default to Object, but it is good practice to specify it to restrict what you allow in your ArrayList. The generic type ArrayList<Type> is preferred over just the keyword ArrayList because it allows the compiler to find errors that would otherwise be found at run-time.

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

Declaring a ArrayList doesn’t actually create a ArrayList. It only creates a variable that can refer to a ArrayList. To actually create a ArrayList use new ArrayList<Type>(). If you leave off the <Type> it will default to Object, which means this could hold any type.

.size() and NullPointerException

You can get the number of items in a ArrayList using the size() method. Notice that an empty ArrayList has a size of 0 because the ArrayList constructor constructs an empty list. Also notice that you can’t get the size of a ArrayList that is currently set to null on line 9. You will get a NullPointerException instead, which means that you tried to do something with an object reference that was null (doesn’t exist).

You can also create ArrayLists of integer values. However, you have to use Integer as the type 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. You can actually put in any kind of Objects in an ArrayList, even for a class that you wrote in previous units.

.add()

You can add values to an ArrayList by using its add method, described in detail in the next lesson. Try the code below. Note that the type of the ArrayList, String or Integer, also determines the type of parameters and return types for all of its methods, so add and print work for any type of ArrayList.

Programming Challenge

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 mod 10 which returns the remainder after dividing by 10? Try a different number and guess what it will print and then run to check.

Change the code below to 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.

Next, 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. You can use a special method called Collections.reverse(digitsList); to reverse the order of the digits in the ArrayList after the loop to get them in the right order

Summary

• ArrayList are re-sizable arrays that allow adding and removing items to change their size during run time.
• The ArrayList class is in the java.util package. You must import java.util.* to use it.
• An ArrayList object contains object references and is mutable, meaning it can change (by adding and removing items from it).
• The ArrayList constructor ArrayList() constructs an empty list of size 0.
• Java allows the generic type ArrayList<E>, where the generic type E specifies the type of the elements, like String or Integer. Without it, the type will be Object.
• ArrayList<E> is preferred over ArrayList because it allows the compiler to find errors that would otherwise be found at run-time.
• When ArrayList<E> is specified, the types of the reference parameters and return type when using its methods are type E.
• 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.