Java 输入/输出——File类

　　File类是java.io包下代表与平台无关的文件和目录，也就是说，如果希望在程序中操作文件和目录，都可以通过File类来完成。值得指出的是，不管是文件还是目录都是使用File来操作的，File能新建、删除、重命名文件和目录，File不能访问文件内容本身。如果需要访问文件内容本身，则需要使用输入/输出流。

　　File类相关的方法参考链接：https://docs.oracle.com/javase/9/docs/api/overview-summary.html

• Field Summary

  Fields

  Modifier and Type	Field	Description
  static String	pathSeparator	The system-dependent path-separator character, represented as a string for convenience.
  static char	pathSeparatorChar	The system-dependent path-separator character.
  static String	separator	The system-dependent default name-separator character, represented as a string for convenience.
  static char	separatorChar	The system-dependent default name-separator character.

• Constructor Summary

  Constructors

  Constructor	Description
  File​(File parent, String child)	Creates a new File instance from a parent abstract pathname and a child pathname string.
  File​(String pathname)	Creates a new File instance by converting the given pathname string into an abstract pathname.
  File​(String parent, String child)	Creates a new File instance from a parent pathname string and a child pathname string.
  File​(URI uri)	Creates a new File instance by converting the given file: URI into an abstract pathname.

• Method Summary

  All MethodsStatic MethodsInstance MethodsConcrete MethodsDeprecated Methods

  Modifier and Type	Method	Description
  boolean	canExecute​()	Tests whether the application can execute the file denoted by this abstract pathname.
  boolean	canRead​()	Tests whether the application can read the file denoted by this abstract pathname.
  boolean	canWrite​()	Tests whether the application can modify the file denoted by this abstract pathname.
  int	compareTo​(File pathname)	Compares two abstract pathnames lexicographically.
  boolean	createNewFile​()	Atomically creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist.
  static File	createTempFile​(String prefix,String suffix)	Creates an empty file in the default temporary-file directory, using the given prefix and suffix to generate its name.
  static File	createTempFile​(String prefix,String suffix,File directory)	Creates a new empty file in the specified directory, using the given prefix and suffix strings to generate its name.
  boolean	delete​()	Deletes the file or directory denoted by this abstract pathname.
  void	deleteOnExit​()	Requests that the file or directory denoted by this abstract pathname be deleted when the virtual machine terminates.
  boolean	equals​(Object obj)	Tests this abstract pathname for equality with the given object.
  boolean	exists​()	Tests whether the file or directory denoted by this abstract pathname exists.
  File	getAbsoluteFile​()	Returns the absolute form of this abstract pathname.
  String	getAbsolutePath​()	Returns the absolute pathname string of this abstract pathname.
  File	getCanonicalFile​()	Returns the canonical form of this abstract pathname.
  String	getCanonicalPath​()	Returns the canonical pathname string of this abstract pathname.
  long	getFreeSpace​()	Returns the number of unallocated bytes in the partition named by this abstract path name.
  String	getName​()	Returns the name of the file or directory denoted by this abstract pathname.
  String	getParent​()	Returns the pathname string of this abstract pathname's parent, or null if this pathname does not name a parent directory.
  File	getParentFile​()	Returns the abstract pathname of this abstract pathname's parent, or null if this pathname does not name a parent directory.
  String	getPath​()	Converts this abstract pathname into a pathname string.
  long	getTotalSpace​()	Returns the size of the partition named by this abstract pathname.
  long	getUsableSpace​()	Returns the number of bytes available to this virtual machine on the partition named by this abstract pathname.
  int	hashCode​()	Computes a hash code for this abstract pathname.
  boolean	isAbsolute​()	Tests whether this abstract pathname is absolute.
  boolean	isDirectory​()	Tests whether the file denoted by this abstract pathname is a directory.
  boolean	isFile​()	Tests whether the file denoted by this abstract pathname is a normal file.
  boolean	isHidden​()	Tests whether the file named by this abstract pathname is a hidden file.
  long	lastModified​()	Returns the time that the file denoted by this abstract pathname was last modified.
  long	length​()	Returns the length of the file denoted by this abstract pathname.
  String[]	list​()	Returns an array of strings naming the files and directories in the directory denoted by this abstract pathname.
  String[]	list​(FilenameFilter filter)	Returns an array of strings naming the files and directories in the directory denoted by this abstract pathname that satisfy the specified filter.
  File[]	listFiles​()	Returns an array of abstract pathnames denoting the files in the directory denoted by this abstract pathname.
  File[]	listFiles​(FileFilter filter)	Returns an array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname that satisfy the specified filter.
  File[]	listFiles​(FilenameFilter filter)	Returns an array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname that satisfy the specified filter.
  static File[]	listRoots​()	List the available filesystem roots.
  boolean	mkdir​()	Creates the directory named by this abstract pathname.
  boolean	mkdirs​()	Creates the directory named by this abstract pathname, including any necessary but nonexistent parent directories.
  boolean	renameTo​(File dest)	Renames the file denoted by this abstract pathname.
  boolean	setExecutable​(boolean executable)	A convenience method to set the owner's execute permission for this abstract pathname.
  boolean	setExecutable​(boolean executable, boolean ownerOnly)	Sets the owner's or everybody's execute permission for this abstract pathname.
  boolean	setLastModified​(long time)	Sets the last-modified time of the file or directory named by this abstract pathname.
  boolean	setReadable​(boolean readable)	A convenience method to set the owner's read permission for this abstract pathname.
  boolean	setReadable​(boolean readable, boolean ownerOnly)	Sets the owner's or everybody's read permission for this abstract pathname.
  boolean	setReadOnly​()	Marks the file or directory named by this abstract pathname so that only read operations are allowed.
  boolean	setWritable​(boolean writable)	A convenience method to set the owner's write permission for this abstract pathname.
  boolean	setWritable​(boolean writable, boolean ownerOnly)	Sets the owner's or everybody's write permission for this abstract pathname.
  Path	toPath​()	Returns a java.nio.file.Path object constructed from the this abstract path.
  String	toString​()	Returns the pathname string of this abstract pathname.
  URI	toURI​()	Constructs a file: URI that represents this abstract pathname.
  URL	toURL​()	Deprecated. This method does not automatically escape characters that are illegal in URLs. It is recommended that new code convert an abstract pathname into a URL by first converting it into a URI, via the toURImethod, and then converting the URI into a URL via the URI.toURL method.

  • Methods inherited from class java.lang.Object

    clone, finalize, getClass, notify, notifyAll, wait, wait, wait

 1、访问文件和目录

 package com.zyjhandsome.io;

 import java.io.*;

 public class FileTest {

     public static void main(String[] args) throws IOException
     {
         // 以当前路径来创建一个File对象
         File file = new File(".");
         // 直接获取文件名, 输出一点
         System.out.println(file.getName());
         // 获取相对路径的父路径可能出错, 下面代码输出null
         System.out.println(file.getParent());
         // 获取绝对路径
         System.out.println(file.getAbsoluteFile());
         // 获取绝对路径
         System.out.println(file.getAbsoluteFile().getParent());
         // 当前路径下创建一个临时文件
         File tmpFile = File.createTempFile("aaa", ".txt", file);
         // 指定当JVM退出时候删除该文件
         tmpFile.deleteOnExit();
         // 以系统当前时间作为新文件名来创建新文件
         File newFile = new File(System.currentTimeMillis() + "");
         System.out.println("newFile对象是否存在1: " + newFile.exists());
         // 以指定newFile对象来创建一个文件
         newFile.createNewFile();
         System.out.println("newFile对象是否存在2: " + newFile.exists());
         // 以newFile对象来创建一个目录，因为newFile已经存在，所以下面方法返回false, 即无法创建该目录
         System.out.println("newFile.mkdir():" + newFile.mkdir());
         System.out.println("----------------------");
         // 使用list()方法列出当前路径下的所有文件和路径
         String[] fileList = file.list();
         for (String fileName : fileList)
         {
             System.out.println(fileName);
         }
         System.out.println("----------------------");
         // listRoots()静态方法列出所有的磁盘根路径
         File[] roots = File.listRoots();
         System.out.println("====系统所有根路径如下====");
         for (File root : roots)
         {
             System.out.println(root);
         }
     }
 }

 .
 null
 D:\zhaoyingjun\eclipse-workspace\CollectionTest\.
 D:\zhaoyingjun\eclipse-workspace\CollectionTest
 newFile对象是否存在1: false
 newFile对象是否存在2: true
 newFile.mkdir():false
 ----------------------
 .classpath
 .project
 .settings
 1537712414564
 1537712424492
 1537712893895
 1537712926829
 1537712940906
 1537713158525
 1537713167968
 1537774060515
 aaa1610208071466755969.txt
 bin
 src
 ----------------------
 ====系统所有根路径如下====
 C:\
 D:\

 2、文件过滤器

　　在File类的list()方法中可以接收一个FilenameFilter参数，通过该参数可以只列出符合条件的文件。这里的FilenameFilter接口和javax.swing.filechooser包下的FileFilter抽象类的功能非常相似，可以把FileFilter当成是FilenameFilter的实现类。

　　FilenameFilter接口里包含了一个accept(File dir, String name)方法，该方法将依次对指定File的所有子目录或者文件进行迭代，如果该方法返回true，则list()方法会列出该子目录或者文件。

 package com.zyjhandsome.io;

 import java.io.*;

 public class FilenameFilterTest {

     public static void main(String[] args) {
         // TODO Auto-generated method stub
         File file = new File(".");
         // 使用Lambda表达式(目标类型是FilenameFilter)实现文件过滤器
         // 如果文件名以.java结尾, 或者文件对应一个路径，则返回true
         String[] nameList = file.list((dir, name) ->
             name.endsWith(".java") || new File(name).isDirectory());
         for (String name : nameList)
         {
             System.out.println(name);
         }
     }
 }

 .settings
 bin
 src