Reading Files with Java.nio

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From the course by Duke University

Java Programming: Principles of Software Design

587 оценки

Duke University

Java Programming: Principles of Software Design

587 оценки

Course 4 of 5 in the Specialization Java Programming and Software Engineering Fundamentals

Solve real world problems with Java using multiple classes. Learn how to create programming solutions that scale using Java interfaces. Recognize that software engineering is more than writing code - it also involves logical thinking and design. By the end of this course you will have written a program that analyzes and sorts earthquake data, and developed a predictive text generator. After completing this course, you will be able to: 1. Use sorting appropriately in solving problems; 2. Develop classes that implement the Comparable interface; 3. Use timing data to analyze empirical performance; 4. Break problems into multiple classes, each with their own methods; 5. Determine if a class from the Java API can be used in solving a particular problem; 6. Implement programming solutions using multiple approaches and recognize tradeoffs; 7. Use object-oriented concepts including interfaces and abstract classes when developing programs; 8. Appropriately hide implementation decisions so they are not visible in public methods; and 9. Recognize the limitations of algorithms and Java programs in solving problems. 10. Recognize standard Java classes and idioms including exception-handling, static methods, java.net, and java.io packages.

From the lesson

Java: Tools and Libraries for Everyone

As you reach the end of this series, you may wish to extend your Java experience to tools beyond those provided here. This module will cover some of the Java basic information that is needed to program without BlueJ or the edu.duke library. After completing this module, you will be able to: (1) write a main method to start a Java program in any programming environment, (2) use the “static” keyword to modify fields, (3) find alternative editors to use with Java, (4) use exceptions to debug your program and make it more robust, and (5) reference Java libraries that are necessary to read files without the edu.duke library.

Introduction1:25

Understanding Exceptions4:01

Handling Exceptions2:22

Declaring Exceptions1:13

Throwing Exceptions1:53

Reading Files with Java.nio6:54

Hello Around the World with Eclipse4:15

Meet the Instructors

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

In this lesson we're going to provide reasons for

why we decided to hide some information about how files and URLs work with Java.

And how you can write programs using standard Java libraries.

Having to understand the details about how files and URLs are used in programming

can easily get in the way of solving higher level problems.

That's why we provided the edu.duke library and the class FileResource

to help you more quickly solve real problems by programming.

All languages and all libraries insulate programmers

from complex details that can get in the way of solving problems.

The libraries we provide are a great insulation and protection for those first

learning Java, but even the regular Java classes provide insulation and protection.

We've worked to empower you to solve problems, to think creatively about

writing programs that will help you learn about programming and

problem solving together.

Now, we'll show you how to write the hello world program

without using the FileResource or URLResource classes.

See how this is done could help you implement your own classes

to insulate common issues you have while programming.

What for us was two classes in the edu.duke package

plus a third exception class that some of you may have encountered

will be at least six classes in three different packages

that are harder to use than the classes we designed.

Using the Java libraries will require understanding exceptions and

several programming idioms that are more complex than the simple four each loops

we started.

Let's look at one of the first Java programs we used

to print hello world in several languages.

This code uses the .edu.FileResource class, and

the method run hello, as you can see here.

This method prints hello world in different spoken and

written languages by reading the text from any languages and printing the words read.

The code first creates a FileResource object to reference and

read the file containing the words we want to print.

Then the method loops over each line in the file using a for

each loop as shown here.

Reading a string and printing the string in the body of the loop.

Let's look at the same code that uses Java not our libraries but

rather a way to do this with just the standard Java libraries.

This code uses seven classes in three packages.

Both programs use the system class.

There are many ways to do this, but the one that we're showing will highlight some

of the flexibility these extra classes provide to reduce duplicated code.

The first step is to create a path object, in the code we've named this object, P.

It represents a path to a file in the file system.

Here we've used the Paths.get, note the extra s, to get the path.

The idea of a path is the same with a file or a URL, something that goes from folder,

to folder, to the file in the actual file system.

One standard way to read a file in Java is using a BufferedReader

from the java.io package.

The paths and path classes are in a different package, as we'll soon see.

To create a BufferedReader, we first ask the files object

again not the s in files to create one for us to use in our program.

We'll see other ways to create BufferedReaders

when we use a URL instead of a path to a file.

We use a loop and the reader dot read line method

to read each line of the open file until a null reference is returned by read line.

That means the file has been completely read and the loop exits.

Both this code and the code with our file resource object use loops but

the loops are different.

The two main packages we use to read files in Java are java.io and java.nio.

The n in nio is for new, though the package is actually old.

But it is newer than the java.io package.

The i stands for input, reading, and the o stands for output, writing.

In other words these are the input output packages.

The class's paths, files, and path are in java.nio.

These classes, files and

paths use static methods, which you have recently learned about.

You know that these methods do not belong to a particular instance of a class, but

rather the class as a whole.

So you do not need to create new objects to use them.

Many of the methods for doing reading and

writing, as well as some constructors can throw exceptions.

Now that you have learned about the basics of exceptions, you know that you should

handle these with try catch blocks, and that if you do not handle them,

you must add a throws to the appropriate exception type to your method declaration.

The java.io package provides other classes for reading and writing.

The buffered reader classes often used when reading sources,

buffering what is read for good performance.

The java.io.io exception class is thrown by many input and output methods.

Using these classes requires reading the documentation, and looking at examples,

because there are many ways of reading files, raw bytes,

or objects, or parsing information.

We created the FileResource class to make these things simpler.

Let's look at one more example,

how to read URLs without using the URL resource by using the Java.net package.

Reading URLs with this package is similar to how reading works

with our edu.duke.URL resource class.

First you create a URL object by providing the string represents the Website.

You'll then use this URL object to create a buffered reader, from the java.io class.

But that takes several steps.

The first step is to open a connection to the associated URL, and

get a stream representing its data.

Then use this stream to create an InputStreamReader.

They use the InputStreamReader to create a buffer reader.

That's three step to replace a one step we had in creating a URLs resource object.

The purpose of this extra steps is to provide different ways to create an object

that implements the reader interface.

So that you can read the data from a file or a URL using the same code.

So, you read using the same BufferedReader loop that we used in the last example.

This kind of reuse is harder to do with our simplified resource classes, and

one of the reasons it is useful to take the time to learn more

about these complex classes.

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