This book is now obsolete Please use CSAwesome instead.

9.9. Declaring Lists

To declare a list use List<Type> name Change the Type to be whatever type of objects you want to store in the list like String to hold strings as shown in the code below. You don’t have to specify a <Type>, since it will default to Object, but it is good practice to specify it to restrict what you allow in your list.

You should declare the variable to be of type List rather than ArrayList so that you can change what type of class you want to use to implement the list interface in the future without having to change your declarations. There are actually several classes in Java that implement the List interface, but you only need to know the ArrayList class for the AP CSA course.

In the code below we are declaring a variable called nameList that can refer to a list of strings, but currently doesn’t refer to any list yet (is set to null).

import java.util.*; // import everything at this level public class Test { public static void main(String[] args) { List<String> nameList = null; System.out.println(nameList); } }

9.10. Creating Lists

Declaring a list doesn’t actually create a list. It only creates a variable that can refer to a list. To actually create a list use new ArrayList<Type>(). If you leave off the <Type> it will default to Object.

When you first create a new list it is empty, meaning that it doesn’t contain any items yet. You can get the number of items in a list using the size() method. Notice that an empty list has a size of 0. Also notice that you can’t get the size of a list that is currently set to null on line 9. You will get a NullPointerException instead, which means that you tried to do something on an object reference that was null meaning that it doesn’t reference an object.

import java.util.*; // import everything at this level public class Test { public static void main(String[] args) { List<String> nameList = new ArrayList<String>(); System.out.println("The size of nameList is: " + nameList.size()); List<String> list2 = null; System.out.println("The size of list2 is: " + list2.size()); } }

Note

You use the length field to get the number of items in an array. But, with an ArrayList you use the size() method to get the number of items in the list. The number of items in an empty list is 0. You can’t get the size of a list that is set to null. You will get a NullPointerException instead.

You can also create lists of integer values. However, you have to use Integer as the type. Integer objects can hold an int value.

You can also declare a list and create it in the same statement as shown below.

import java.util.*; // import everything at this level public class Test { public static void main(String[] args) { List<Integer> numList = new ArrayList<Integer>(); System.out.println(numList.size()); } }

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