Introduction to using collections in Java

What is a list?

OK, it might sound a dumb question, but when we refer to a list in Java, we mean specifically a collection of objects that:

has a fixed order (when we add an object to the list, it will stay in the place we put it);

allows objects to be referred to by position, so we can get or set the "4th element in the list";

generally speaking, does not have a fixed number of elements, unlike an array.

A plain old array has the first two of these properties, but not the third: when we create an array, we always have to say how many elements it has, and we can't change this number once the array is created. The third property isn't strictly speaking a necessary property of a Java List. But in practice, the most commonly used types of List are used instead of a plain old array because they have this "unbounded" property.

Using a List in Java

As with Java collections generally, there isn't actually a single Java class called List¹. Instead, there is an interface called List, with various implementations. So a typical way to create a list would look like this:

import java.util.*;

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

In the above example, we import all classes from java.util, which is where List and ArrayList reside. Unless otherwise stated, assume that all of the collections classes we mention from now on are in the java.util package, and you will need to import them.

In this example, the actual collection object that we create is an ArrayList. This is a list implementation that stores objects inside an array. Although arrays have to be fixed-size, ArrayList internally handles this by creating a new, bigger array where necessary and copying the previous elements into the new array. So to the caller, what we effectively get is a list with no fixed size which we can add to and remove from arbitrarily.

Notice that we also state, inside angle brackets, that the list will specifically hold objects of type String. If you haven't come across it before, this syntax was introduced in Java 5, part of what is technically called the generics framework; we'll introduce more details as we go along.

Now we have our list, we can start doing interesting things with it such as adding and removing elements. Any List provides various methods, including add(), get(), size():

myList.add("Hello");
myList.add("world");
System.out.println("The list contains " +
   myList.size() + "elements, and " +
   "the first is " + myList.get(0));

Interfaces

A key feature of Java used in the collections framework is the notion of interfaces and implementations. An interface is a "blueprint" for a class: it specifies a series of methods, but does not contain any code inside those methods. For example, the List interface includes the methods add() and size(). This means that any class that wants to be a list implementation must include an add() and a size() method, but doesn't state how individual classes should implement these methods (i.e. what code goes in the method). That's up to individual classes. For example, the ArrayList class implements its add() method by putting the object in its underlying array. The LinkedList class implements its add() method by adding the object to the end of its linked 'chain of objects'. The advantage of having interfaces is that we can write code that is more 'general': we can write code that operates on "any sort of list", always referring to the object in question as a List (the name of the interface), without having to worry about whether it's specifically an ArrayList or a LinkedList that we're dealing with in any one case.

For more information, see the separate section on interfaces in Java.

Lists provide various other useful methods, such as for removing items from the list, adding at a specific position, or determining the first or last position of a particular item in the list.

Next...

On the next pages, we continue by looking at:

Enumerating the contents of a list;

Sorting a Java list;

Sets, a kind of unordered list in which items are simply "present or not".

1. OK, there's an AWT component also called List, but that's not what we're talking about here...!

Using collections in Java (1)

What is a list?

Using a List in Java

Interfaces

Next...