File类的使用 | 桂子飘香

java.io.File类是文件和文件目录路径的抽象表示形式，与具体的操作系统无关。

File类提供了新建、删除、重命名文件和目录的功能，但File不能访问文件内容本身以及想文件中写入内容。如果需要访问文件内容本身，则需要使用输入/输出流。通常将File类的实例作为流的构造器参数，然后流就可以读取或写入数据到文件中。

想要在Java程序中表示一个真实存在的文件或目录，那么必须有一个File对象，但是Java程序中的一个File对象，可能没有一个真实存在的文件或目录。

File file = new File("1.txt"); // 当前moudle下没有1.txt文件
System.out.println(file);  // 1.txt

Table of Contents

常用构造器

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.

以pathname为路径创建File对象，可以是绝对路径或者相对路径，如果pathname是相对路径，则默认的当前路径在系统属性user.dir中存储。（Idea编辑器中，在main方法里，相对路径为当前工程。若是@Test，则为当前的module）

// type1
File file = new File("1.txt");
File file2 = new File("D:\GWX\1.md");

// type2
File file3 = new File("D:\GWX", "1.md");

// type3
File file4 = new File("D:\GWX", "dir");
File file5 = new File(file4, "1.txt");

路径分隔符

路径中的每级目录之间用一个路径分隔符隔开。

windows中路径分隔符\
unix中路径分隔符/

Java程序支持跨平台运行，因此路径分隔符要慎用。为了解决这个问题，File提供了一个类常量，

public static final String separator。根据操作系统，动态的提供分隔符

File file = new File("dir1" + File.separator + "1.md");

常用方法

File提供了非常多的方法，并且绝大部分都非常的实用。下面从几个方面来介绍这些常用方法：

属性相关

Modifier and Type	Method	Description
`File`	`getAbsoluteFile()`	Returns the absolute form of this abstract pathname.
`String`	`getAbsolutePath()`	Returns the absolute pathname string of this abstract pathname.
`long`	`getFreeSpace()`	Returns the number of unallocated bytes in the partition named by this abstract path name.
`String`	`getPath()`	Converts this abstract pathname into a pathname string.
`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.
`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.

getPath、getName用法示例：

File file3 = new File("D:\GWX", "1.md");
System.out.println(file3.getPath());  // D:\GWX.md
System.out.println(file3.getName());  // 1.md

File file4 = new File("dir1\dir2", "1.md");
System.out.println(file4.getPath());  // dir1\dir2.md
System.out.println(file4.getName());  // 1.md

重命名

bool renameTo(File dest) 若目标文件不存在，则可以重命名成功，否则，则不会成功。该方法名应该叫move更加合适，因为该方法可以移动文件并修改文件名。

File file = new File("1.txt");

if (file.renameTo(new File("dir/hello.txt"))) {
    System.out.println("文件移动成功");
} else {
    System.out.println("文件移动失败");
}

判断相关

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.
`boolean`	`exists()`	Tests whether the file or directory denoted by this abstract pathname exists.
`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.

创建、删除

Modifier and Type	Method	Description
`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.
`boolean`	`delete()`	Deletes the file or directory denoted by this abstract pathname.
`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.

练习

习题1：判断指定目录下是否有后缀名为.png的文件，如果有，就输出该文件名称

File dir = new File("dir");

if (!dir.isDirectory()) {
    throw new IOException("the file is not a dir");
}

String[] list = dir.list((dirfile, filename)->{
    return filename.endsWith(".png");
});

for (String filename : list)
    System.out.println(filename);

习题2.遍历指定目录所有文件名称，包括子文件目录中的文件。

@Test
void test6 () throws IOException {
    File dir = new File("../IO");

    if (!dir.isDirectory()) {
        throw new IOException("the file is not a dir");
    }

    printFile(dir, 0);
}

private static void printFile (File file, int length)
{
    for (int i=0; i<length; i++) {
        System.out.print(" ");
    }

    System.out.println(file.getName());

    if (file.isDirectory()) {
        File[] files = file.listFiles();

        for (File f : files) {
            printFile(f, length + 2);
        }
    }
}