first commit on CP 17.1
This commit is contained in:
Abel-Huang
parent
9f9b8e446d
commit
a4a0995995
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@ -20,3 +20,6 @@ _book
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._*
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*/.DS_Store
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*/._*
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# IDEA
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/.idea/*
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@ -2,17 +2,310 @@
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<!-- File -->
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# 第十七章 文件
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在丑陋的Java I/O编程方式诞生多年以后，Java终于简化了文件读写的基本操作。这种"困难方式"的全部细节都在[Appendix: I/O Streams]()。如果你读过这个部分，就会认同Java设计者毫不在意他们的使用者的体验这一观念。打开并读取文件对于大多数编程语言来是非常常用的，由于I/O糟糕的设计以至于
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很少有人能够在不依赖其他参考代码的情况下完成打开文件的操作。
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好像Java设计者终于意识到了Java使用者多年来的痛苦，在Java7中对此引入了巨大的改进。这些新元素被放在**java.nio.file**包下面，过去人们通常把**nio**中的**n**理解为**new**即新的**io**，现在更应该当成是**non-blocking**非阻塞**io**(**io**就是*input/output输入/输出*)。**java.nio.file**库终于将Java文件操作带到与其他编程语言相同的水平。最重要的是Java8新增的streams与文件结合使得文件操作编程变得更加优雅。我们将看一下文件操作的两个基本组件：
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1. 文件或者目录的路径；
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2. 文件本身。
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<!-- File and Directory Paths -->
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## 文件和目录路径
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### `Paths`
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一个**Path**对象表示一个文件或者目录的路径，是一个跨操作系统（OS）和文件系统的抽象，目的是在构造路径时不必关注底层操作系统，代码可以在不进行修改的情况下运行在不同的操作系统上。**java.nio.file.Paths**类包含一个重载方法**static get()**，该方法方法接受一系列**Strings**字符串或一个*统一资源标识符*(URI)作为参数，并且进行转换返回一个**Path**对象：
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```java
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// files/PathInfo.java
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import java.nio.file.*;
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import java.net.URI;
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import java.io.File;
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import java.io.IOException;
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public class PathInfo {
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static void show(String id, Object p) {
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System.out.println(id + ": " + p);
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}
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static void info(Path p) {
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show("toString", p);
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show("Exists", Files.exists(p));
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show("RegularFile", Files.isRegularFile(p));
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show("Directory", Files.isDirectory(p));
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show("Absolute", p.isAbsolute());
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show("FileName", p.getFileName());
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show("Parent", p.getParent());
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show("Root", p.getRoot());
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System.out.println("******************");
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}
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public static void main(String[] args) {
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System.out.println(System.getProperty("os.name"));
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info(Paths.get("C:", "path", "to", "nowhere", "NoFile.txt"));
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Path p = Paths.get("PathInfo.java");
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info(p);
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Path ap = p.toAbsolutePath();
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info(ap);
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info(ap.getParent());
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try {
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info(p.toRealPath());
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} catch(IOException e) {
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System.out.println(e);
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}
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URI u = p.toUri();
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System.out.println("URI: " + u);
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Path puri = Paths.get(u);
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System.out.println(Files.exists(puri));
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File f = ap.toFile(); // Don't be fooled
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}
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}
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/* 输出:
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Windows 10
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toString: C:\path\to\nowhere\NoFile.txt
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Exists: false
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RegularFile: false
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Directory: false
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Absolute: true
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FileName: NoFile.txt
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Parent: C:\path\to\nowhere
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Root: C:\
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******************
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toString: PathInfo.java
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Exists: true
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RegularFile: true
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Directory: false
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Absolute: false
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FileName: PathInfo.java
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Parent: null
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Root: null
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******************
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toString: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files\PathInfo.java
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Exists: true
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RegularFile: true
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Directory: false
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Absolute: true
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FileName: PathInfo.java
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Parent: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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Root: C:\
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******************
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toString: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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Exists: true
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RegularFile: false
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Directory: true
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Absolute: true
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FileName: files
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Parent: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples
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Root: C:\
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******************
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toString: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files\PathInfo.java
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Exists: true
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RegularFile: true
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Directory: false
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Absolute: true
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FileName: PathInfo.java
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Parent: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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Root: C:\
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******************
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URI: file:///C:/Users/Bruce/Documents/GitHub/onjava/
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ExtractedExamples/files/PathInfo.java
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true
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*/
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```
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我已经在这一章第一个程序的**main()**方法添加了第一行用于展示操作系统的名称，因此你可以看到不同操作系统之间存在哪些差异。理想情况下，差别会相对较小，并且使用**/**或者**\\**路径分隔符进行分隔。你可以看到我运行在Windows 10上的程序输出。
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当**toString()**方法生成完整形式的路径，你可以看到**getFileName()** 方法总是返回当前文件名。
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通过使用**Files**工具类(我们接下类将会更多的使用它)，可以测试一个文件是否存在，测试是否是一个"真正"的文件还是一个目录等等。"Nofile.txt"这个示例展示我们描述的文件可能并不在指定的位置；这样可以允许你创建一个新的路径。"PathInfo.java"存在于当前目录中，最初它只是没有路径的文件名，但它仍然被检测为"存在"。一旦我们将其转换为绝对路径，我们将会得到一个从"C:"盘(因为我们是在Windows机器下进行测试)开始的完整路径，现在它也拥有一个父路径。“真实”路径的定义在文档中有点模糊
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因为它取决于具体的文件系统。例如，如果文件名不区分大小写，即使路径由于大小写的缘故而不是完全相同，也可能得到肯定的匹配结果。
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```java
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// files/PartsOfPaths.java
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import java.nio.file.*;
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public class PartsOfPaths {
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public static void main(String[] args) {
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System.out.println(System.getProperty("os.name"));
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Path p = Paths.get("PartsOfPaths.java").toAbsolutePath();
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for(int i = 0; i < p.getNameCount(); i++)
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System.out.println(p.getName(i));
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System.out.println("ends with '.java': " +
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p.endsWith(".java"));
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for(Path pp : p) {
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System.out.print(pp + ": ");
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System.out.print(p.startsWith(pp) + " : ");
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System.out.println(p.endsWith(pp));
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}
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System.out.println("Starts with " + p.getRoot() + " " + p.startsWith(p.getRoot()));
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}
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}
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/* 输出:
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Windows 10
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Users
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Bruce
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Documents
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GitHub
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on-java
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ExtractedExamples
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files
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PartsOfPaths.java
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ends with '.java': false
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Users: false : false
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Bruce: false : false
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Documents: false : false
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GitHub: false : false
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on-java: false : false
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ExtractedExamples: false : false
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files: false : false
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PartsOfPaths.java: false : true
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Starts with C:\ true
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*/
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```
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```java
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// files/PathAnalysis.java
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import java.nio.file.*;
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import java.io.IOException;
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public class PathAnalysis {
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static void say(String id, Object result) {
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System.out.print(id + ": ");
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System.out.println(result);
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}
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public static void main(String[] args) throws IOException {
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System.out.println(System.getProperty("os.name"));
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Path p = Paths.get("PathAnalysis.java").toAbsolutePath();
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say("Exists", Files.exists(p));
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say("Directory", Files.isDirectory(p));
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say("Executable", Files.isExecutable(p));
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say("Readable", Files.isReadable(p));
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say("RegularFile", Files.isRegularFile(p));
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say("Writable", Files.isWritable(p));
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say("notExists", Files.notExists(p));
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say("Hidden", Files.isHidden(p));
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say("size", Files.size(p));
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say("FileStore", Files.getFileStore(p));
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say("LastModified: ", Files.getLastModifiedTime(p));
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say("Owner", Files.getOwner(p));
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say("ContentType", Files.probeContentType(p));
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say("SymbolicLink", Files.isSymbolicLink(p));
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if(Files.isSymbolicLink(p))
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say("SymbolicLink", Files.readSymbolicLink(p));
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if(FileSystems.getDefault().supportedFileAttributeViews().contains("posix"))
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say("PosixFilePermissions",
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Files.getPosixFilePermissions(p));
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}
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}
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/* 输出:
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Windows 10
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Exists: true
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Directory: false
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Executable: true
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Readable: true
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RegularFile: true
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Writable: true
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notExists: false
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Hidden: false
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size: 1631
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FileStore: SSD (C:)
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LastModified: : 2017-05-09T12:07:00.428366Z
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Owner: MINDVIEWTOSHIBA\Bruce (User)
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ContentType: null
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SymbolicLink: false
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*/
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```
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```java
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// files/AddAndSubtractPaths.java
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import java.nio.file.*;
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import java.io.IOException;
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public class AddAndSubtractPaths {
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static Path base = Paths.get("..", "..", "..").toAbsolutePath().normalize();
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static void show(int id, Path result) {
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if(result.isAbsolute())
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System.out.println("(" + id + ")r " + base.relativize(result));
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else
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System.out.println("(" + id + ") " + result);
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try {
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System.out.println("RealPath: " + result.toRealPath());
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} catch(IOException e) {
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System.out.println(e);
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}
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}
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public static void main(String[] args) {
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System.out.println(System.getProperty("os.name"));
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System.out.println(base);
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Path p = Paths.get("AddAndSubtractPaths.java").toAbsolutePath();
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show(1, p);
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Path convoluted = p.getParent().getParent()
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.resolve("strings").resolve("..")
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.resolve(p.getParent().getFileName());
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show(2, convoluted);
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show(3, convoluted.normalize());
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Path p2 = Paths.get("..", "..");
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show(4, p2);
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show(5, p2.normalize());
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show(6, p2.toAbsolutePath().normalize());
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Path p3 = Paths.get(".").toAbsolutePath();
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Path p4 = p3.resolve(p2);
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show(7, p4);
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show(8, p4.normalize());
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Path p5 = Paths.get("").toAbsolutePath();
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show(9, p5);
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show(10, p5.resolveSibling("strings"));
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show(11, Paths.get("nonexistent"));
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}
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}
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/* 输出:
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Windows 10
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C:\Users\Bruce\Documents\GitHub
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(1)r onjava\
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ExtractedExamples\files\AddAndSubtractPaths.java
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RealPath: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files\AddAndSubtractPaths.java
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(2)r on-java\ExtractedExamples\strings\..\files
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RealPath: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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(3)r on-java\ExtractedExamples\files
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RealPath: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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(4) ..\..
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RealPath: C:\Users\Bruce\Documents\GitHub\on-java
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(5) ..\..
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RealPath: C:\Users\Bruce\Documents\GitHub\on-java
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(6)r on-java
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RealPath: C:\Users\Bruce\Documents\GitHub\on-java
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(7)r on-java\ExtractedExamples\files\.\..\..
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RealPath: C:\Users\Bruce\Documents\GitHub\on-java
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(8)r on-java
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RealPath: C:\Users\Bruce\Documents\GitHub\on-java
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(9)r on-java\ExtractedExamples\files
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RealPath: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files
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(10)r on-java\ExtractedExamples\strings
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RealPath: C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\strings
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(11) nonexistent
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java.nio.file.NoSuchFileException:
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C:\Users\Bruce\Documents\GitHub\onjava\
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ExtractedExamples\files\nonexistent
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*/
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```
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<!-- File Systems -->
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<!-- Directories -->
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## 目录
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<!-- File Systems -->
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## 文件系统
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