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Do curso por Duke University

Java Programming: Arrays, Lists, and Structured Data

1262 classificações

Duke University

Java Programming: Arrays, Lists, and Structured Data

1262 classificações

Curso 2 de 4 em Specialization Object Oriented Programming in Java

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.

Na lição

MiniProject: Vigenère Cipher

In this module, you will develop a program to break the Vigenère Cipher, a more complex version of the Caesar Cipher. You will improve your program in three stages: first decrypting messages where you know the language and key length, then adding the capability to handle messages with unknown key length, then extending the program to handle messages in a range of possible languages. Through this project, you will be able (1) to effectively use HashSet and HashMap, (2) to expand an algorithm from solving a simpler problem to handle broader, more complex problems, and (3) to design and modify program code involving a more complex collection of classes, methods, and data.

Introduction2:49

Known Language and Key Length5:06

Unknown Key Length4:40

Unknown Language4:53

Conheça os instrutores

Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering
Robert Duvall
Lecturer
Computer Science
Owen Astrachan
Professor of the Practice
Computer Science
Susan H. Rodger
Professor of the Practice
Computer Science

Okay, great, now you have cracked Vigenere for English messages of

unknown key length, but what if the messages were in other languages?

What if you did not know the language?

Well, you can use the same technique and just try each language.

For each potential language, you would need a list of words in that language and

to know the most common character, which is not E for all languages.

Then you can try breaking the cipher for each language.

You would use the same code you already wrote to find the key length

that gives you the most real words in each language.

This gives you the best choice for that particular language.

Then you just see which language results in the most real words.

That is, which language's best decryption is best overall?

To do this, you're going to read the words in for each language.

You can make use of the readDictionary method that you already wrote.

As you read each dictionary, you will want to put it into a hash map,

whose keys are strings and whose values are hash sets of strings.

In particular, the key is the name of the language and

the value is the dictionary that you read in with read dictionary.

Such a hash map would conceptually look like this here.

Here you have a table where the keys are the names of languages.

The values are then the sets of words in each language.

You might think that this type looks a little complex.

You have multiple sets of angle brackets nested together.

However, it is a great example of an important programming principle.

Do you remember the idea of composition?

If you took our course Programming in the Web for

Beginners, you learned about it in the context of HTML.

The idea that programming languages allow you to put small pieces together

to make larger pieces, and that they obey the same rules when you put them together.

This principle lets you make and understand large and complex things.

In this case you can understand this complicated looking type

by understanding the pieces.

So what exactly do you need to make your Vigenere breaker work for

unknown languages?

You need to write two new methods, one that counts the most common character and

hash set of strings, and one that tries the different languages.

You also need to modify two old methods, the break Vigenere method,

which is what you run from BlueJ, and the breakForLanguage method.

You need to write the mostCommonCharIn method to account for

the fact that E is not the most common letter in all languages.

So you will count the frequency of each letter in the HashSet of strings

which is the list of words for that language.

You have seen and done many problems with counting how often you find something, and

figuring out which of something occurs most often.

So hopefully, you are getting proficient in these skills by now.

The other new method we'll try each language in that keySet

of the HashMap language.

You will want to use breakForLanguage

to do the work of trying to break that one particular language.

You will want to use .get,

to get the word list out of the HashMap to pass into breakForLanguage.

You will then want to see how many words you ended up with

in the string that breakForLanguage returns.

Fortunately, we already wrote a method that does that.

You will then want to pick the best language, the one with the most words, and

its decryption.

Print them out so that you know what your program found.

You will want to make a few changes to breakVigenere(),

the method that you call from BlueJ.

Instead of just reading one language's word list,

you'll want to read many of them.

The other change is that you will want this method to call breakForAllLangs

instead of breakForLanguage, so

that it tries all of the languages that you read in.

You will also want to make one small change to breakForLanguage.

Up to now, you have just passed an E as the most common letter.

However, now you will want to use the mostCommonCharIn method

that you just wrote to find the most common letter in the word list, and

pass that into try key links.

So, now that you know the plan, it's time for

you to devise your algorithms and write your code.

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