Trigger

I teach Java at school, so I made it from the materials at that time.

Explanation of words

For the time being, explain the words and phrases.

What is FileIO?

You can do I (Input / Input) and O (Output / Output) of the file system!

What is a file system? !!

A file system is a world created by a file that stores data in the OS and a directory that manages it collectively (Directory, Folder).

Introduction

What can you do with FileIO?

For files and directories, you can do everything we do (probably) in the OS, such as delete, create, exist,…, write, read, etc. !!

So how do you do that in Java?

→ NewIO, a Java standard library, is recommended!

In the past, it was mainly composed of a class called File (java.io series), but now it is convenient and optimized, so it uses a package called New I / O (java.nio series). What we do is the mainstream in the future.

Let's use NewIO

Getting started with NewIO

In NewIO, the following two (three) classes generally have a basic role.

Path:A class that converts a String to represent the path of a file
Paths:Class for creating Path
Files:Class that actually operates files

Note that the File class is not NewIO, but an old one (an old IO system that is not NewIO).

Get Path

Get Path (Example)… File location!

Path path = Paths.get(“C:\\”, “a”);

Path cannot be created with new!
Paths get method is a static method!
The argument of the get method is a variable length argument, and each argument can represent the file hierarchy.
To get the path (normal character string), select the file and Shift + right-click to "copy the path".
Be careful of escape sequences and double quotation marks when passing.

Examples of using Files

Example of using Files ... Perform operations on files / directories.

Files.exsit(path);

The return value is boolean, so you can use this in an if statement!

Reading and writing to files

Basically, you should use the following two classes. (Although most of the other classes are basically old, please note that they are often used in other people's programs even if they are old.)

BufferedWriter:A class that writes to a file efficiently using a buffer
BufferedReader:A class that efficiently reads a file using a buffer

What is Buffer?

Data that is temporarily placed in the memory space. Also known as tmp / temp.

BufferedWriter / BufferedReader (instance) generation

Old way

Maybe I don't use it much (in NewIO).

BufferedWriter bw = new BufferedWriter(Writer);
BufferedReader br = new BufferedReader(Reader);

Generation of BufferedWriter / BufferedReader using NewIO

BufferedWriter bw = Files.newBufferedWriter(path);

Use of BufferedWriter / BufferedReader

bw.write(String);
String line = br.readLine();

Bw and br can only be accessed by calling the close () method. (Not output unless close () is called with why-> bw, call flush ())

try {
	BufferedWriter bw = Files.newBufferedWriter(path);
	bw.write(“A”);
	bw.close();
} catch (IOException ex) {
	ex.printStackTrace();
}

In addition, an exception may occur in close, and exception handling for close is also necessary. Therefore, it is necessary to do as follows.

try {
	BufferedWriter bw = Files.newBufferedWriter(path);
	bw.write(“A”);
} catch (IOException ex) {
	ex.printStackTrace();
} finally {
	try {
		bw.close();
	} catch (IOException ex) {
		ex.printStackTrace();
	}
}

Fucking long

Use of BufferedWriter / BufferedReader (improvement) ~ try-catch-resouce!

It's very long to write like before. Therefore, the try-catch-resouce syntax was added from Java 7, and it became possible to write concisely as follows.

try (BufferedWriter bw = Files.newBufferedWriter(path)) {
	bw.write(“A”);
}

very good. Reference: https://qiita.com/Takmiy/items/a0f65c58b407dbc0ca99

How to start a new line? (Use of write ())

Just calling br.write (text) will not break the line,

br.write("AA");
br.write("BB");

↓

AABB

It will be something like.

To start a newline, add a newline character at the end of the line string.

bw.write(text + System.lineSeparator());

System.lineSeparator (); is a method that returns the correct newline character for each OS. You don't have to be careful about the difference between CRLF (\ r \ n) and LF (\ n)! (Let's find out about newline characters)

Task

Write a string to the file
Multi-line version of "1." (using List)
Read the file
Multi-line version of "3." (line expression using List), all lines may be used, or any range of lines may be used.

column

About the usual syntax we've used so far ...

System.out.println(“Hello world!”);

If you understand FileIO, you will think that it looks like FileIO.

What do you mean? It will be.

In fact, System.out (out) is an OutputStream! That is. (That is, for output)

For the time being, Stream is an abstraction of FileIO. (maybe) Similarly, there is also System.in, which is an InputStream. (That is, for input)

However, this is natural when you think about it, and writing characters on the console is not much different from writing characters on a file (I thought it was interesting to have this kind of commonality).

Reference / Reference

-Oracle official commentary pages -Rewrite the code of java.io.File using java.nio.Path and java.nio.Files -Organize Java file I / O related classes / interfaces

Let's use Java New FileIO! (Introduction, for beginners)