Generics and the ArrayList Class - Java - page 702

Using the Class

ArrayList

are used in much the same way as arrays, but there are some important
differences. First, the definition of the class

ArrayLists

is not provided automatically.

import
statement

ArrayList

The definition is in the package , and any code that uses the class

java.util ArrayList

must contain the following, normally at the start of the file:

import java.util.ArrayList;

An ArrayList is created and named in the same way as objects of any class, except
that you specify the base type using a new notation. For example,

ArrayList<String> list = new ArrayList<String>(20);

This statement makes list the name of an ArrayList that stores objects of the class
String and that has an initial capacity of 20 items. When we say that an ArrayList has
a certain capacity, we mean that it has been allocated memory for that many items, but if
it needs to hold more items, the system will automatically allocate more memory. By care-
fully choosing the initial capacity of an ArrayList , you can often make your code more
efficient, but this capacity has no effect on how many items the ArrayList can hold. If
you choose your capacity to be large enough, then the system will not need to reallocate
memory too often, and as a result, your program should run faster. On the other hand, if
you make your capacity too large, you will waste storage space. However, no matter what
capacity you choose, you can still do anything you want with the ArrayList .
The type String in the previous ArrayList example is the base type of the ArrayList
class. An ArrayList , that is, an object of the ArrayList class, stores objects of its base
type. You can use any reference type as the base type of an ArrayList class. In particu-
lar, you can use any class or interface type. However, you cannot use a primitive type,
such as int or double , as the base type of an ArrayList class. This is an example of a
type parameter. The ArrayList class is defined as having a type parameter for the type
of the elements in the list. You create a concrete class by specifying, in angular brack-
ets, a class type to be substituted for this type parameter. For example, the following
code, which we saw earlier, substitutes the type String for the type parameter to create
the class ArrayList<String> and an object of this class named list :

capacity

base type

ArrayList<String> list = new ArrayList<String>(20);

ArrayList objects can be used like arrays, but they do not have the array square-
bracket notation. If you would use

a[index] = "Hi Mom!";

for an array of strings a , then the analogous statement for a suitable ArrayList named
list would be

set

list.set(index, "Hi Mom!");