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[#]: subject: (Programming 101: Input and output with Java)
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[#]: via: (https://opensource.com/article/21/3/io-java)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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[#]: collector: (lujun9972)
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[#]: translator: (piaoshi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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Programming 101: Input and output with Java
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======
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Learn how Java handles reading and writing data.
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![Coffee beans and a cup of coffee][1]
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When you write a program, your application may need to read from and write to files stored on the user's computer. This is common in situations when you want to load or store configuration options, you need to create log files, or your user wants to save work for later. Every language handles this task a little differently. This article demonstrates how to handle data files with Java.
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### Installing Java
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Regardless of your computer's platform, you can install Java from [AdoptOpenJDK][2]. This site offers safe and open source builds of Java. On Linux, you may also find AdoptOpenJDK builds in your software repository.
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I recommend using the latest long-term support (LTS) release. The latest non-LTS release is best for developers looking to try the latest Java features, but it likely outpaces what most users have installed—either by default on their system or installed previously for some other Java application. Using the LTS release ensures you're up-to-date with what most users have installed.
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Once you have Java installed, open your favorite text editor and get ready to code. You might also want to investigate an [integrated development environment for Java][3]. BlueJ is ideal for new programmers, while Eclipse and Netbeans are nice for intermediate and experienced coders.
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### Reading a file with Java
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Java uses the `File` library to load files.
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This example creates a class called `Ingest` to read data from a file. When you open a file in Java, you create a `Scanner` object, which scans the file you provide, line by line. In fact, a `Scanner` is the same concept as a cursor in a text editor, and you can control that "cursor" for reading and writing with `Scanner` methods like `nextLine`:
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```
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import java.io.File;
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import java.util.Scanner;
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import java.io.FileNotFoundException;
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public class Ingest {
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public static void main([String][4][] args) {
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try {
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[File][5] myFile = new [File][5]("example.txt");
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Scanner myScanner = new Scanner(myFile);
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while (myScanner.hasNextLine()) {
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[String][4] line = myScanner.nextLine();
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[System][6].out.println(line);
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}
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myScanner.close();
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} catch ([FileNotFoundException][7] ex) {
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ex.printStackTrace();
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} //try
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} //main
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} //class
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```
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This code creates the variable `myfile` under the assumption that a file named `example.txt` exists. If that file does not exist, Java "throws an exception" (this means it found an error in what you attempted to do and says so), which is "caught" by the very specific `FileNotFoundException` library. The fact that there's a library specific to this exact error betrays how common this error is.
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Next, it creates a `Scanner` and loads the file into it. I call it `myScanner` to differentiate it from its generic class template. A `while` loop sends `myScanner` over the file, line by line, for as long as there _is_ a next line. That's what the `hasNextLine` method does: it detects whether there's any data after the "cursor." You can simulate this by opening a file in a text editor: Your cursor starts at the very beginning of the file, and you can use the keyboard to scan through the file with the cursor until you run out of lines.
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The `while` loop creates a variable `line` and assigns it the data of the current line. Then it prints the contents of `line` just to provide feedback. A more useful program would probably parse each line to extract whatever important data it contains.
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At the end of the process, the `myScanner` object closes.
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### Running the code
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Save your code as `Ingest.java` (it's a Java convention to give classes an initial capital letter and name the file to match). If you try to run this simple application, you will probably receive an error because there is no `example.txt` for the application to load yet:
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```
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$ java ./Ingest.java
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java.io.[FileNotFoundException][7]:
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example.txt (No such file or directory)
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```
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What a perfect opportunity to write a Java application that writes data to a file!
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### Writing data to a file with Java
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Whether you're storing data that your user creates with your application or just metadata about what a user did in an application (for instance, game saves or recent songs played), there are lots of good reasons to store data for later use. In Java, this is achieved through the `FileWriter` library, this time by opening a file, writing data into it, and then closing the file:
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```
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import java.io.FileWriter;
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import java.io.IOException;
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public class Exgest {
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public static void main([String][4][] args) {
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try {
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[FileWriter][8] myFileWriter = new [FileWriter][8]("example.txt", true);
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myFileWriter.write("Hello world\n");
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myFileWriter.close();
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} catch ([IOException][9] ex) {
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[System][6].out.println(ex);
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} // try
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} // main
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}
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```
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The logic and flow of this class are similar to reading a file. Instead of a `Scanner`, it creates a `FileWriter` object with the name of a file. The `true` flag at the end of the `FileWriter` statement tells `FileWriter` to _append_ text to the end of the file. To overwrite a file's contents, remove the `true`:
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```
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`FileWriter myFileWriter = new FileWriter("example.txt", true);`
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```
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Because I'm writing plain text into a file, I added my own newline character (`\n`) at the end of the data (`Hello world`) written into the file.
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### Trying the code
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Save this code as `Exgest.java`, following the Java convention of naming the file to match the class name.
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Now that you have the means to create and read data with Java, you can try your new applications, in reverse order:
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```
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$ java ./Exgest.java
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$ java ./Ingest.java
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Hello world
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$
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```
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Because it appends data to the end, you can repeat your application to write data as many times as you want to add more data to your file:
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```
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$ java ./Exgest.java
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$ java ./Exgest.java
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$ java ./Exgest.java
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$ java ./Ingest.java
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Hello world
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Hello world
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Hello world
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$
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```
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### Java and data
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You're don't write raw text into a file very often; in the real world, you probably use an additional library to write a specific format instead. For instance, you might use an XML library to write complex data, an INI or YAML library to write configuration files, or any number of specialized libraries to write binary formats like images or audio.
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For full information, refer to the [OpenJDK documentation][10].
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/io-java
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/java-coffee-mug.jpg?itok=Bj6rQo8r (Coffee beans and a cup of coffee)
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[2]: https://adoptopenjdk.net
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[3]: https://opensource.com/article/20/7/ide-java
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[4]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+string
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[5]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+file
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[6]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+system
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[7]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+filenotfoundexception
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[8]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+filewriter
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[9]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+ioexception
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[10]: https://access.redhat.com/documentation/en-us/openjdk/11/
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@ -0,0 +1,163 @@
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[#]: subject: (Programming 101: Input and output with Java)
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[#]: via: (https://opensource.com/article/21/3/io-java)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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[#]: collector: (lujun9972)
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[#]: translator: (piaoshi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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编程秘笈：Java 中的输入和输出
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======
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学习 Java 如何外理数据的读与写
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![Coffee beans and a cup of coffee][1]
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当你写一个程序时，你的应用程序可能需要读取和写入存储在用户计算机上的文件。这在你想加载或存储配置选项，你需要创建日志文件，或你的用户想要保存工作以待后用的情况下是很常见的。每种语言处理这项任务的方式都有所不同。本文演示了如何用 Java 处理数据文件。
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### 安装 Java
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不管你的计算机是什么平台，你都可以从 [AdoptOpenJDK][2] 安装 Java。这个网站提供安全和开源的 Java 构建。在 Linux 上，你的软件库中也可能找到 AdvertOpenJDK 的构建。
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我建议你使用最新的长期支持（LTS）版本。最新的非 LTS 版本对希望尝试最新 Java 功能的开发者来说是最好的，但它很可能超过大多数用户所安装的版本——要么是系统上默认安装的，要么是以前为其他 Java 应用安装的。使用 LTS 版本可以确保你与大多数用户所安装的版本保持一致。
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一旦你安装好了 Java，就可以打开你最喜欢的文本编辑器并准备开始写代码了。你可能还想要研究一下 [Java 集成开发环境][3]。BlueJ 是新程序员的理想选择，而 Eclipse 和 Netbeans 对中级和有经验的编码者更友好。
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### 利用 Java 读取文件
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Java 使用 `File` 类来加载文件。
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这个例子创建了一个叫 `Ingest` 的类来读取文件中数据。当你要在 Java 中打开一个文件时，你创建了一个 `Scanner` 对象，它可以逐行扫描你提供的文件。事实上，`Scanner` 与文本编辑器中的光标是相同的概念，这样你可以用 `Scanner` 的一些方法（如 `nextLine`）来控制这个“光标”以进行读写。
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```
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import java.io.File;
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import java.util.Scanner;
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import java.io.FileNotFoundException;
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public class Ingest {
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public static void main([String][4][] args) {
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try {
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[File][5] myFile = new [File][5]("example.txt");
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Scanner myScanner = new Scanner(myFile);
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while (myScanner.hasNextLine()) {
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[String][4] line = myScanner.nextLine();
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[System][6].out.println(line);
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}
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myScanner.close();
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} catch ([FileNotFoundException][7] ex) {
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ex.printStackTrace();
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} //try
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} //main
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} //class
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```
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这段代码首先在假设存在一个名为 `example.txt` 的文件的情况下创建了变量 `myfile`。如果该文件不存在，Java 就会“抛出一个异常”（这意味着它在你试图做的事情中发现了一个错误，并这样告诉你），这个异常是被非常明确的 `FileNotFoundException` 类所“捕获”。事实上，有一个专门的类来处理这个明确的错误，这说明这个错误是多么常见。
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接下来，它创建了一个 `Scanner` 并将文件加载到其中。我把它叫做 `myScanner`，以区别于它的通用类模板。接着，一个 `while` 循环将 `myScanner` 逐行送入文件中，只要_存在_下一行。这就是 `hasNextLine` 方法的作用：它检测“光标”之后是否还有数据。你可以通过在文本编辑器中打开一个文件来模拟这个过程：你的光标从文件的第一行开始，你可以用键盘控制光标来向下扫描文件，直到你用完了所有的行。
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`while` 循环创建了一个变量 `line`，并将文件当前行的数据分配给它。然后将 `line` 的内容打印出来以提供反馈。一个更有用的程序可能会解析每一行的内容，从而提取它所包含的任何重要数据。
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在这个过程结束时，关闭 `myScanner` 对象。
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### 运行代码
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将你的代码保存到 `Ingest.java` 文件（这是一个 Java 惯例，将类名的首字母大写，并以类名来命名相应的文件）。如果你试图运行这个简单的应用程序，你可能会接收到一个错误信息，这是因为还没有 `example.txt` 文件供应用程序加载:
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```
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$ java ./Ingest.java
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java.io.[FileNotFoundException][7]:
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example.txt (No such file or directory)
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```
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正好可以编写一个将数据写入文件的 Java 应用程序，多么完美的时机!
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### 利用 Java 将数据写入文件
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无论你是存储用户使用你的应用程序创建的数据，还是仅仅存储关于用户在应用程序中做了什么的元数据（例如，游戏保存或最近播放的歌曲），有很多很好的理由来存储数据供以后使用。在 Java 中，这是通过 `FileWriter` 类实现的，这次先打开一个文件，向其中写入数据，然后关闭该文件。
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```
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import java.io.FileWriter;
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import java.io.IOException;
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public class Exgest {
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public static void main([String][4][] args) {
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try {
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[FileWriter][8] myFileWriter = new [FileWriter][8]("example.txt", true);
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myFileWriter.write("Hello world\n");
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myFileWriter.close();
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} catch ([IOException][9] ex) {
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[System][6].out.println(ex);
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} // try
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} // main
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}
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```
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这个类的逻辑和流程与读取文件类似。但它不是一个 `Scanner`，而是以一个文件的名字为参数创建的一个 `FileWriter` 对象。`FileWriter` 语句末尾的 `true` 标志告诉 `FileWriter` 将文本_追加_到文件的末尾。要覆盖一个文件的内容，请移除 `true` 标志。
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```
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`FileWriter myFileWriter = new FileWriter("example.txt", true);`
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```
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因为我在向文件中写入纯文本，所以我在写入文件的数据（Hello world）的结尾处手动添加了换行符（\n）。
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### 试试代码
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将这段代码保存到 `Exgest.java` 文件，遵循 Java 的惯例，使文件名为与类名相匹配。
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既然你已经掌握了用 Java 创建和读取数据的方法，你可以按相反的顺序尝试运行你的新应用程序。
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```
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$ java ./Exgest.java
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$ java ./Ingest.java
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Hello world
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$
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```
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因为程序是把数据追加到文件末尾，所以你可以重复执行你的应用程序以多次写入数据，只要你想把更多的数据添加到你的文件中。
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```
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$ java ./Exgest.java
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$ java ./Exgest.java
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$ java ./Exgest.java
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$ java ./Ingest.java
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Hello world
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Hello world
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Hello world
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$
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```
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### Java 和数据
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你并不经常向文件中写入原始文本；实事上，你可能会使用一个其它的类库以写入特定的格式。例如，你可能使用 XML 类库来写复杂的数据，使用 INI 或 YAML 类库来写配置文件，或者使用任何数量的专门类库来写二进制格式，如图像或音频。
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||||
更完整的信息，请参阅 [OpenJDK 文档][10]。
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||||
--------------------------------------------------------------------------------
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||||
|
||||
via: https://opensource.com/article/21/3/io-java
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|
||||
作者：[Seth Kenlon][a]
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||||
选题：[lujun9972][b]
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译者：[piaoshi](https://github.com/piaoshi)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/seth
|
||||
[b]: https://github.com/lujun9972
|
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/java-coffee-mug.jpg?itok=Bj6rQo8r (Coffee beans and a cup of coffee)
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[2]: https://adoptopenjdk.net
|
||||
[3]: https://opensource.com/article/20/7/ide-java
|
||||
[4]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+string
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||||
[5]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+file
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||||
[6]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+system
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[7]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+filenotfoundexception
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[8]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+filewriter
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||||
[9]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+ioexception
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||||
[10]: https://access.redhat.com/documentation/en-us/openjdk/11/
|
||||
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