1. File class

java.io.File class is an abstract representation of file and directory pathnames. It is mainly used for the creation, search and deletion of files and directories.

1.1 construction method

• public File(String pathname): create a new File instance by converting the given pathname string to an abstract pathname.
• public File(String parent, String child): creates a new File instance from the parent pathname string and the child pathname string.
• Public File (File parent, string child): creates a new File instance from the parent abstract pathname and child pathname strings.

Code example:

// File pathname
String pathname = "D:\\aaa.txt";
File file1 = new File(pathname);
// File pathname
String pathname2 = "D:\\aaa\\bbb.txt";
File file2 = new File(pathname2);
// Pass parent and child path strings
String parent = "d:\\aaa";
String child = "bbb.txt";
File file3 = new File(parent, child);
// Through the parent File object and child path string
File parentDir = new File("d:\\aaa");
String child = "bbb.txt";
File file4 = new File(parentDir, child);

be careful:

1. A File object represents a File or directory that actually exists in the hard disk.
2. Whether there is a File or directory in this path does not affect the creation of the File object.

1.2 common methods

1.2.1 method of obtaining document information

• public String getAbsolutePath(): returns the absolute pathname string of this File.
• public String getPath(): convert this File to the pathname string provided when creating the File.
• public String getName(): returns the name of the File or directory represented by this File.
• public long length(): returns the length of the File represented by this File.

Code example:

public static void main(String[] args) {
    File f = new File("d:/aaa/bbb.java");
    System.out.println("File absolute path:"+f.getAbsolutePath());
    //Returns the pathname string provided when constructing the file
    //Returned when f = new file (". / aaa/bbb.java")/ aaa/bbb.java
    System.out.println("File construction path:"+f.getPath());
    System.out.println("File name:"+f.getName());
    System.out.println("file length:"+f.length()+"byte");
    File f2 = new File("d:/aaa");
    System.out.println("Directory absolute path:"+f2.getAbsolutePath());
    System.out.println("Directory construction path:"+f2.getPath());
    System.out.println("Directory name:"+f2.getName());
    System.out.println("Directory length:"+f2.length());
}
Output result:
File absolute path:d:\aaa\bbb.java
 File construction path:d:\aaa\bbb.java
 File name:bbb.java
 file length:636 byte
 Directory absolute path:d:\aaa
 Directory construction path:d:\aaa
 Directory name:aaa

API Description: length(), indicating the length of the File. However, if the File object represents a directory, the return value is not specified.

1.2.2 document judgment method

• public boolean exists(): whether the File or directory represented by this File actually exists.
• public boolean isDirectory(): this File indicates whether it is a directory.
• public boolean isFile(): this File indicates whether it is a File.

Code example:

public static void main(String[] args) {
    File f = new File("d:\\aaa\\bbb.java");
    File f2 = new File("d:\\aaa");
    // Judge whether it exists
    System.out.println("d:\\aaa\\bbb.java Does it exist:"+f.exists());
    System.out.println("d:\\aaa Does it exist:"+f2.exists());
    // Determine whether it is a file or directory
    System.out.println("d:\\aaa file?:"+f2.isFile());
    System.out.println("d:\\aaa catalogue?:"+f2.isDirectory());
}
Output result:
d:\aaa\bbb.java Does it exist:true
d:\aaa Does it exist:true
d:\aaa It's a file?:false
d:\aaa It's a directory?:true

1.2.3 method of operating documents

• public boolean createNewFile(): creates a new empty file if and only if the file with this name does not exist yet.
• public boolean delete(): delete the File or directory represented by this File.
• public boolean mkdir(): create the directory represented by this File.
• public boolean mkdirs(): create the directory represented by this File, including any required but nonexistent parent directory.

Code example:

public static void main(String[] args) throws IOException {
    // File creation
    File f = new File("aaa.txt");
    System.out.println("Does it exist:"+f.exists()); // false
    System.out.println("Create:"+f.createNewFile()); // true
    System.out.println("Does it exist:"+f.exists()); // true
    // Directory creation
    File f2= new File("newDir");
    System.out.println("Does it exist:"+f2.exists());// false
    System.out.println("Create:"+f2.mkdir()); // true
    System.out.println("Does it exist:"+f2.exists());// true
    // Create multi-level directory
    File f3= new File("newDira\\newDirb");
    System.out.println(f3.mkdir());// false, mkdirs for multi-level directory
    File f4= new File("newDira\\newDirb");
    System.out.println(f4.mkdirs());// true
    // Deletion of files
    System.out.println(f.delete());// true
    // Deletion of directory
    System.out.println(f2.delete());// true
    System.out.println(f4.delete());// false, the directory must be empty
}

1.3 absolute path and relative path

• Absolute path: the path starting from the drive letter. This is a complete path.
• Relative path: relative to the path of the project directory, this is a convenient path, which is often used in development.

Code example:

public static void main(String[] args) {
    // BBB under disk D Java file
    File f = new File("D:\\bbb.java");
    System.out.println(f.getAbsolutePath());
    // BBB under Project Java file
    File f2 = new File("bbb.java");
    System.out.println(f2.getAbsolutePath());
}

Output result:
D:\bbb.java
D:\myproject\bbb.java

2. Recursion

2.1 general

There are two kinds of recursion, direct recursion and indirect recursion.

• Direct recursion is called method calling itself.
• Indirect recursion can use method A to call method B, method B to call method C, and method C to call method A.

Precautions for using recursion:

• Recursion must be conditionally limited to ensure that recursion can stop, otherwise stack memory overflow will occur.
• Although there are restrictions in recursion, the number of recursions cannot be too many. Otherwise, stack memory overflow will also occur.
• Construction method. Recursion is prohibited.

2.2 recursive printing of multi-level directories

When printing multi-level directories of files, we generally need to use recursive implementation.

Code example:

//Recursive Method 
public static void printDir(File dir) {
    // Get sub files and directories
    File[] files = dir.listFiles();
    // Cyclic printing
    for (File file : files) {
        // judge
        if (file.isFile()) {
            // Is the absolute path of the output file
            System.out.println("file name:"+ file.getAbsolutePath());
        } else {
            // Is the directory, and the absolute path of the output directory
            System.out.println("catalogue:"+file.getAbsolutePath());
            // Continue to traverse and call printDir to form recursion
            printDir(file);
        }
    }
}

2.3 recursive search for files

When we are looking for all suffixes under a directory java files, you can use recursive implementation.

Code example:

public static void printDir(File dir) {
    // Get sub files and directories
    File[] files = dir.listFiles();
    // Cyclic printing
    for (File file : files) {
        if (file.isFile()) {
            // Is a file. Judge the file name and output the absolute path of the file
            if (file.getName().endsWith(".java")) {
                System.out.println("file name:" + file.getAbsolutePath());
            }
        } else {
            // It is a directory. Continue to traverse to form recursion
            printDir(file);
        }
	}
}

3. Document filter

java.io.FileFilter is an interface, which is the filter of File. The object of this interface can be passed to listFiles(FileFilter) of the File class as a parameter. There is only one method in the FileFilter interface:

boolean accept(File pathname): the parameter is File, which indicates all sub files and subdirectories under the current File. If it is retained, it returns true; if it is filtered out, it returns false.

Let's take the above file search as an example to filter out the non - documents java for the suffix file, and then print all java file.

Code example:

public static void printDir2(File dir) {
    // Create filter subclass objects by anonymous inner class
    File[] files = dir.listFiles(new FileFilter() {
        @Override
        public boolean accept(File pathname) {
            return pathname.getName().endsWith(".java")||pathname.isDirectory();
        }
    });
    // Cycle through all java file
    for (File file : files) {
        if (file.isFile()) {
            System.out.println("file name:" + file.getAbsolutePath());
        } else {
            printDir2(file);
        }
    }
}