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Java Programming: Arrays, Lists, and Structured Data

Université Duke

4.7 (1,336 notes) | 59K étudiants inscrits

Cours 2 de 4 dans Object Oriented Programming in Java Spécialisation

Build on the software engineering skills you learned in “Java Programming: Solving Problems with Software” by learning new data structures. Use these data structures to build more complex programs that use Java’s object-oriented features. At the end of the course you will write an encryption program and a program to break your encryption algorithm. After completing this course, you will be able to: 1. Read and write data from/to files; 2. Solve problems involving data files; 3. Perform quantitative analyses of data (e.g., finding maximums, minimums, averages); 4. Store and manipulate data in an array or ArrayList; 5. Combine multiple classes to solve larger problems; 6. Use iterables and collections (including maps) in Java.

Visualiser le programme de cours

Compétences que vous apprendrez

Data Structure, Cryptography, Hash Table, Java Programming

Avis

4.7 (1,336 notes)

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GZ

Aug 20, 2017

The programming exercises are really interesting! They make programming much more fun to study. The lecturers are all great and the lessons are engaging. Enjoyed it greatly! Thank you Duke.

KE

Jan 28, 2016

the course content was great , thank to the teaching staff . i was so much excited about the modules... the projects were also very good , and make me feel that i became a real programmer

À partir de la leçon

Web Server Logs: From Logs to Visits

In this module, you will learn about how web server logs store information about visitors to a website, and you will write programs to access information like user IP address, date and time of access, and more. Using Java programs you write in this module, you will be able (1) to read information from a web server log, (2) to count the number of unique visitors to your website, and (3) to count the number of times each visitor uses your website.

Introduction1:03

Developing an Algorithm4:35

Translating to Code11:08

HashMap for Unique IPs1:58

Enseigné par

Andrew D. Hilton
Associate Professor of the Practice
Robert Duvall
Lecturer
Owen Astrachan
Professor of the Practice
Susan H. Rodger
Professor of the Practice

Transcription

We'd like to find out how many times somebody visits a web site.

So we're gonna look at a log file that has IP addresses and see, for example,

for one IP address, how many times did that IP address appear in the file?

That's how many times that person visited the web site.

So we have a program here, a class called LogAnalyzer.

And we are going to write the method count visits per IP.

So how many times does each IP address visit a page?

And what we have in here is we're going to put log entries in an ArrayList.

It's called records.

So we have a constructor that initializes that ArrayList.

We then have readFile, which is gonna have the file name of the log entries.

And that is going to allow you to select a log entry file.

And then it just goes through and reads through all the lines and

puts them into records.

So we are gonna focus on writing countVisitsPerIP and

we are gonna use a HashMap to do that.

So the first thing we are gonna do is we need to make an empty HashMap.

And we're going to be mapping a string to an integer.

So for each IP address, which is a string, we're gonna map that to the account,

which is the number of times that IP address appears in the file.

So first let's make, let's see.

A HashMap of type String, which is the type of the key,

and of type Integer, which is the type of the value for the HashMap.

We need to give it a name, so we're gonna call it counts and

then we have to create a new one.

So a new HashMap of type

String for the key and

Integer for the value.

There we go.

Now that we have it, we want to iterate over all

the records that we have, so we're gonna use a for

loop for Each log entry.

And we need a variable name so I'm just gonna call that le.

We're going to iterate over the records where we've put all the records from

the file.

Okay.

All right, there's our for loop.

And now we're going to look at each LogEntry, one at a time.

So, first what we'll do from the LogEntry is we'll get the IP address.

Okay, so we need a variable for that, and that is gonna be of type String.

So we'll call that variable, say, IP, which is of type String.

And we will use the log entry to get the IP address.

So if you remember we have getIpAddress.

And that should get us the IP address.

Now that we have the IP address, we have to see if it's already in our HashMap or

not, so we'll have to ask a question about that.

So, we're going to ask if.

Counts.containsKey, okay.

So again, counts is our HashMap and we're using the containsKey function,

and we will ask if that IP address is in there or not.

And I am actually going to ask first if it's not in there.

So we'll put the not there.

And if it's not in there, then that means we want to put it in there for

the first time.

When we put it in there for the first time, its count is gonna be 1.

So we'll add code for that.

So we get the name of the HashMap which is counts.

And we'll use the put.

And we need to put in the IP address, which is the variable IP.

And we put in 1 for the count 1.

Now if it's not in there.

Then we'll want to.

Sorry, if it is in there, then we'll know it's already in there.

We need to get the value out.

We need to add 1 to it and we need to put it back in.

So we'll do that now.

So essentially we're gonna use counts.put but

what we're gonna put in there is we're gonna put the IP address in there again,

or basically replace it with, we have to get the old value out and add 1 to it.

So we'll have to use counts.get to get the old value of it.

And then we'll have to add 1 to that.

And then what else do we have to do?

We come down here.

So if we get to the end of that and we've put all of our log entries into our

HashMap and essentially they're each in there once, with the count of how many

times they appear in the file, then we can return the answer, which is just counts.

So we'll compile this and, let's see,

we gotta spell getIpAddress correctly, so we'll fix that.

And we've compiled it with no syntax errors now.

All right, so now we wanna test this out.

Again, let's just see, countVisitsPerIP is gonna return the HashMap that is all

the mappings of the IP addresses to their counts, so you get the whole thing.

We need to test it out now.

So in order to test it out, we're going to create another class here,

which I've started, called CountTester.

And what we'll do in here is we'll first create a LogAnalyzer,

the class that we just had, so, we'll create a LogAnalyzer object.

We'll just call it la.

And we have to create a new one.

Okay.

So, now we have a LogAnalyzer object.

We'll need to pick a file to read from.

So I'm going to use the readFile and we'll put in.

I happen to have a very small test file to make sure and

convince myself that this actually works.

So it is called short-test_log.

And I'll show you that in a minute, and

then I want to now call count visits per IP.

And remember that's gonna return a HashMap, so

I need to have a HashMap variable to put it in.

So I'll do that.

HashMap of type String Integer.

I'm gonna call it counts and now that is gonna be assigned

the value returned by our LogAnalyzer which is called la and

that's gonna call the method we just wrote which is countVisitsPerIP.

And then once we have that, we can just print out the HashMap that we created.

So I'll just have system.out.println on counts.

Now, let's see if this compiles.

And we're missing something.

Let's see.

We need to use.

We used the wrong kind of quotes.

So, we'll put double quotes here.

And we'll try compiling it again.

And we forgot the semicolon at the end.

All right good, we're good.

So that works, and I want to show you over here my simple file and then we'll run it.

So I have this very short test file, and you can see it's got one, two, three,

four, seven log entries in there.

You can see the second entry which is 152.3.135.44 is in there three times.

Now we're gonna run it.

So we'll come over to BlueJ.

Everything's compiled.

We'll run count tester.

We gotta create the new object first.

And we'll run, and there we go.

And you can see, if I can put both of these up here at the same time.

There we go with one.

So we created a HashMap, and you can see that

the one that starts with 152.3 is listed as being there three times.

That is, we print out the HashMap and

it prints out each IP address equals its count that we came up with.

And so the second entry here is 152.3.135.44 with a 3 and

you can see the 157.55 one is only in there one time and

we got 1 and you can see the 152.3.135 is in there twice.

And you can see, we counted it twice.

So now I'm more convinced that what we wrote is correct.

All right, happy coding.

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