Rewriting with Encapsulation

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Java Programming: Arrays, Lists, and Structured Data

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Duke University

Java Programming: Arrays, Lists, and Structured Data

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Cours 2 sur 4 dans 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.

À partir de la leçon

Cryptography: Keeping Information Secret

In this module, you will learn about the basics of cryptography, the science of keeping information private and secure. You will learn about simpler cryptographic systems, which were used from the Roman Empire through the early 1900s. You will learn how to implement these ciphers, as well as how to break them. To solve these problems, you will work more with processing Strings, but also learn about arrays—a way to store an indexable sequence of elements. You will be able to: (1) combine Strings using concatenation; (2) build Strings within a Java program using StringBuilder; (3) use arrays to store and manipulate collections of data; (4) refactor your programs for improved organization using object-oriented principles; (5) and practice effective algorithm design.

Introduction2:33

Rewriting with Encapsulation3:38

Constructors3:53

Rencontrer les enseignants

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

Welcome back.

To start learning concepts of object oriented programming,

it is useful to take an existing piece of code and

see how you could write it differently in a more object oriented way.

Here is the code you wrote in a previous lesson to perform encryption with

a CaesarCipher.

As you may recall one of the parameters to this method

is the encryption key that you wanna use.

And the first thing you did with this code was to compute the shifted alphabet based

on this key.

Here's a different way to write the CaesarCipher class, which does exactly

the same thing but makes better use of the object oriented nature of Java.

This class has two fields.

A field is a special kind of variable which lives inside of an object

instead of inside of a method.

Here, the two fields are the strings for the alphabet and

the shifted alphabet and they have been moved outside of the encrypt method.

Notice that they are now declared inside of the class, but outside of any method.

These are now data that are encapsulated in your object.

When you make a CaesarCipher object, it will have these two fields,

which any code inside that class can refer to by name.

Next, notice that there is some code here which looks like a method,

where we forgot to write the return type, and named it the same as the class.

This code is actually a constructor, which means code that gets run to initialize

an object when it is created using new.

This constructor takes in the key as a parameter and initializes the alphabet and

shifted alphabet fields using the same approach you used before.

If you look further down in this implementation of the CaesarCipher,

the encrypt method looks mostly the same as before but

it no longer takes the key as a parameter, nor does it compute the shifted alphabet.

But the rest of the code is the same.

In fact, the code uses the alphabet and shifted alphabet fields in the object,

even though these are not declared inside the method.

This code is allowed to use them because it is inside the object.

So it can use any fields within the object.

An illustration helps understand the differences between these two approaches.

In the old code, a CaesarCipher object held no data.

When you do new CaesarCipher, you pass in no arguments and

create an object with nothing in it.

When you call encrypt, you pass the message and

the key and the method returns the encrypted message.

In the new way, each CaesarCipher object contains a key.

Now when you do new CaesarCipher, you pass in the key and

the object you have created stores that key inside of itself.

When you call encrypt on such an object you pass in only the message.

The key is already in the object and

it still returns the same encrypted message as before.

So if these two implementations produce the same result

what are the benefits of an object oriented approach?

When you encapsulate the key inside of the cipher object,

you have one thing that is capable of taking a message and encrypting it.

That makes a nice logical unit to think about, a thing that does a task.

You do not need to separately track the key and pass it in.

For small programs, such as the ones you have written so

far, that may not seem like a big deal.

However, as you solve larger programs with more complex code,

this design idea will help you immensely.

Now that you have seen an example of these ideas in action, the rest of the lesson

will teach you about the details of the technique you just saw.

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