Welcome back! The rest of the semester is incredibly exciting material. As you begin work on the machine project, we begin building and analyzing new data structures and algorithms. We’ll also introduce new bits of Java syntax along the way.
Algorithms and data structures comprise the core conceptual concerns of computer science. Algorithms are how we do things. Data structures are how we represent things.
The two topics are intertwined. We will implement data structures to support certain algorithms. And we will design algorithms that utilize specific data structure capabilties.
As we proceed, we will spend more time talking about how long certain algorithms take and why—or performing algorithm analysis. To do this we use something called Big-O notation to describe the behavior of algorithms. Let’s define those terms:
Algorithm analysis: the determination of the computational complexity of algorithms, that is the amount of time, storage and/or other resources necessary to execute them.
Big-O notation is a mathematical notation that describes the limiting behavior of a function when the argument tends towards a particular value or infinity.
We’ll take a very high-level view of Big-O as we get started with algorithm analysis. Let’s provide an overview of the different complexity categories that we’ll learn to identify, and some of the code features that are associated with them.
To get some practice with algorithm analysis, over the next few lessons we’ll be implementing a data structure known as a list.
You’ve already been working with Java’s built-in
Lists, so this will give you
a peek at how they are actually implemented.
Lists store a sequence of elements. We already know how to do that using arrays, and we can build an implementation of lists on top of an array. Let’s see how!
OK, this is good start. But so far all we have is a wrapper around an array! That’s not particularly interesting.
Indeed, the key difference between a Java array and list is that the size of the list can change.
But doing this using a list that maintains are array internally requires more work.
Let’s see how, starting with the
(You get to implement
add as this lesson’s homework.)
Let's begin building a simple list implementation that uses arrays to store the values.
Create a class
SimpleArrayList with a public constructor that initializes the list using a passed array of
Object references. assert that the passed array is not
Object get(int), which takes an
intindex and returns the
Objectat that index
void set(int, Object), which takes an
intindex and an
Objectreference and sets that value at the index to the passed reference.
set method should assert that the index passed is valid for that SimpleArrayList.
Here's an example of how your
SimpleArrayList should work:
Don't overthink this! Both
set should be two lines of code (including one for the
Next, let’s take a look at our core list functions and see how they perform.
We’re going to use our new big-O vocabulary and try to understand the performance of
Let's write the
add method for our
First, create a
SimpleArrayList class with a single public constructor that initializes the list with a passed
Object references. You should assert that the passed array is not
Next, implement a getter
getValues() which returns the
Object array that the list uses to store values. (This is
purely for testing.) Also implement
size() which returns the size of this list as an
Now write the
add method, which takes the position to add at as an
int as its first parameter and the Object
reference to add as its second.
add should add the element to the list, increasing the size by one and shifting
elements after the add position backward. You should assert that the passed position is valid for this list. But
note that you should allow adding a new item to the end of the existing list.
When you are done, here is how your
SimpleArrayList class should work:
Need more practice? Head over to the practice page.