Structuring Data with Java - Java - page 240

Using Generic Collections

Problem

You want to store your data in one of the Collection classes defined in Chapter 7 with type
safety, and without having to write downcasts when retrieving data from the collection.

Solution

Use Java's Generic Types mechanism, and declare the Collection with a “type parameter”
of the correct type. The type parameter name appears in angle brackets after the declaration
and instantiation. For example, to declare an ArrayList for holding String object referen-
ces:

List<String> myList = new ArrayList<String>();

Discussion

When you instantiate a Collection (or any other class using Generic Types), the class ap-
pears to be instantiated with the type given in angle brackets becoming the type of arguments
passed in, values returned, and so on. Avoid Casting by Using Generics provides some de-
tails on the implementation. As an example, consider the code in Example 7-1 , which creates
and uses an ArrayList specialized to contain String objects.

Example 7-1. ArrayListGenericDemo.java
public

public class class ArrayListGenericDemo ArrayListGenericDemo {

public public static void main ( String [] args ) {

ArrayList < String > data = new

static void

new ArrayList <>();

data . add ( "hello" );
data . add ( "goodbye" );

// data.add(new Date()); This won't compile!

data . forEach ( s -> System . out . println ( s ));

}

}

As you know from the ArrayList example in Like an Array, but More Dynamic , prior to
Generics, the references obtained from a Collection or Iterator would have to be down-