Collections and Generics
Java collections
Homogeneous list

A homogeneous list is where the elements are restricted to a specific type such as a list of Person objects, a list of String objects, a list of Book objects, and so forth. Specifying the element type improves the program reliability because an error such as trying to add a wrong type of object to alist can be caught during the compile time.

To specify a homogeneous list, we must include the type elements in the declaration and creation statements.

List<Person> friends;
...
friends = new ArrayList<Person>();

The general syntax for the declaration is

interface-or-class-name <element-type> identifier;

And the general syntax for the creation is

identifier = new class-name <element-type> (parameters);

We can combine the two into a single statement as

interface-or-class-name <element-type> identifier
	= new class-name <element-type> (parameters);

List<Person> friends = new ArrayList<Person>();
Person person;
person = new Person("Jane", 10, 'F');
friends.add(person);
person = new Person("Jack", 16, 'M');
friends.add(person);
person = new Person("Jill", 8, 'F');
friends.add(person);
person = new Person("John", 12, 'M');
friends.add(person);

Create a homogeneous ArrayList that contains only Book objects

import java.util.*;

ArrayList<Book> bookList = new ArrayList<Book>();

bookList.add(new Book("Jane Austin"));
bookList.add(new Book("Charles Dickens"));
bookList.add(new Book("henry James"));

The following would result in a compile time error:

bookList.add(new String("Java"));

Here is how we access the elements of bookList via an iterator:

Iterator<Book> itr = bookList.iterator();
while (itr.hasNext()) {
	Book book = itr.next(); // typecast is not required
	System.out.println(book.getAuthor());

for (Book book = bookList) {
	System.out.println(book.getAuthor());
}