File类的使用

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类的实例作为的构造器参数,然后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

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);
        }
    }
}
Posted in: IO