Collections

In this lesson we will learn about the Java Collection Framework. There are many abstract classes, concrete classes, and interfaces that make up the framework.

We will look at a class hierarchy showing interfaces, abstract classes, and concrete classes. The point is to see how code is shared by using the abstract classes, and behaviors are provided by classes not necessarily in the same path.

We also want to review the most fundamental data structures.

Lists, Sets and Maps

The most popular Collection classes are Lists, Sets and Maps.

As we’ve seen, a List behaves like a dynamically sized array. One can index into it using integer values (e.g. list.get(3)). The list has order as the items are stored at indices 0 to n.

Index	Element
0	Item0
1	Item1
2	Item2
3	Item3

A Set is a very simple collection of object. It is very much like a set in mathematics. There is no order to the items. The main thing you do with a set is to add and remove items. You can also clear it, get its size, and see if the set contains an item already.

This is a Set
{ Item2, Item1, Item0, Item3 }

A Map behaves like an array where the index must be an Object instead of an int. See Maps

Quick Summary

There are many Collection classes. Here are the top three types.

Collection interface	Description	Example Concrete Class
List	Like an array, but it grows and shrinks, and has helper methods. Indexed with integer values.	ArrayList<String> LinkedLIst<String>
Set	Like a List, but no duplicates are allowed. There is no predictable order in a HashSet One will add to it and check if the set contains something.	HashSet<String>
Map	Like a List, but the index can be any Object Key order depends on the concrete class	HashMap<String> TreeMap<String>

Fundamental Structures

We will use conceptual names that map to interfaces. And we will show the most popular types of classes used to implement that behavior.

Type	Class	Notes
List	ArrayList
	LinkedList
Queue	LinkedList
	ArrayDeque
Stack	Stack	Stack is a concrete class. Slow.
	LinkedList	Acts as both a Stack and a Queue via interface Deque
	ArrayDeque	Acts as both a Stack and a Queue via interface Deque
Set	HashSet
	TreeSet
Map	HashMap
	TreeMap

Here we see the class hierarchy of many classes in the Java Collections Framework.