Overview

java.io.FileClasses are abstract representations of file and directory pathnames, and are mainly used for operations such as file and directory creation, search, and deletion.

Construction method

• public File(String pathname): Creates 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 child pathname string .
• public File(File parent, String child): Creates a new File instance from the parent abstract pathname and child pathname strings .
• The construction example, the code is as follows:

// File path name
String pathname = "D:\\aaa.txt";
File file1 = new File(pathname); 

// File path name
String pathname2 = "D:\\aaa\\bbb.txt";
File file2 = new File(pathname2); 

// By parent path and child path string
 String parent = "d:\\aaa";
 String child = "bbb.txt";
 File file3 = new File(parent, child);

// By parent File object and child path string
File parentDir = new File("d:\\aaa");
String child = "bbb.txt";
File file4 = new File(parentDir, child);

💡Tips:

1. A File object represents a file or directory that actually exists on the hard disk.
2. Regardless of whether there is a file or directory in the path, it does not affect the creation of the File object.

Common method

How to get features

• public String getAbsolutePath(): Returns the absolute pathname string of this File.
• public String getPath(): Convert this File to a pathname string.
• 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.
  Method demonstration, the code is as follows:

public class FileGet {
    public static void main(String[] args) {
        File f = new File("d:/aaa/bbb.java");     
        System.out.println("Absolute path to the file:"+f.getAbsolutePath());
        System.out.println("File construction path:"+f.getPath());
        System.out.println("File name: "+f.getName()); 
        System.out.println("File length: "+f.length()+"bytes");

        File f2 = new File("d:/aaa");     
        System.out.println("Absolute path to directory:"+f2.getAbsolutePath());
        System.out.println("Directory construction path:"+f2.getPath());
        System.out.println("Catalog Name:"+f2.getName());
        System.out.println("Directory length:"+f2.length());
    }
}

Output results.
File absolute path :d:\aaa\bbb. java
File construction path :d:\aaa\bbb. java
File name :bbb. java
File length :636 bytes

Directory absolute path :d:\aaa
Directory construction path :d:\aaa
Directory name :aaa
Directory length:4096

Description in the API: length(), which indicates the length of the file. But the File object represents a directory, the return value is unspecified.

Absolute and relative paths

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

public class FilePath {
    public static void main(String[] args) {
      	//The bbb.java file under the D drive
        File f = new File("D:\\bbb.java");
        System.out.println(f.getAbsolutePath());
      	
		// The bbb.java file under the project
        File f2 = new File("bbb.java");
        System.out.println(f2.getAbsolutePath());
    }
}
Output results.
D:\bbb.java
D:\idea_project_test4\bbb.java

How to judge function

• public boolean exists(): Whether the file or directory represented by this File actually exists.
• public boolean isDirectory(): Whether this File represents a directory.
• public boolean isFile(): Whether this File represents a file or not.

Method demonstration, the code is as follows:

public class FileIs {
    public static void main(String[] args) {
        File f = new File("d:\\aaa\\bbb.java");
        File f2 = new File("d:\\aaa");
        System.out.println("d:\\aaa\\bbb.java Does it exist:"+f.exists());
        System.out.println("d:\\aaa Does it exist:"+f2.exists());
      	// Determine if it is a file or a directory
        System.out.println("d:\\aaa file?:"+f2.isFile());
        System.out.println("d:\\aaa Catalog?:"+f2.isDirectory());
    }
}
Outcome:
d:\aaa\bbb.java Does it exist:true
d:\aaa Does it exist:true
d:\aaa file?:false
d:\aaa Catalog?:true

How to create a delete function

• public boolean createNewFile(): Creates a new empty file if and only if a file with that name does not already exist.
• public boolean delete(): Deletes the file or directory represented by this File.
• public boolean mkdir(): Creates the directory represented by this File.
• public boolean mkdirs(): Creates the directory represented by this File, including any required but non-existing parent directories.

Method demonstration, the code is as follows:

public class FileCreateDelete {
    public static void main(String[] args) throws IOException {
        File f = new File("aaa.txt");
        System.out.println("Does it exist:"+f.exists()); // false
        System.out.println("Whether to create:"+f.createNewFile()); // true
        System.out.println("Does it exist:"+f.exists()); // true
		
      	File f2= new File("newDir");	
        System.out.println("Does it exist:"+f2.exists());// false
        System.out.println("Whether to create:"+f2.mkdir());	// true
        System.out.println("Does it exist:"+f2.exists());// true

      	File f3= new File("newDira\\newDirb");
        System.out.println(f3.mkdir());// false
        File f4= new File("newDira\\newDirb");
        System.out.println(f4.mkdirs());// true
      
       	System.out.println(f.delete());// true
      
        System.out.println(f2.delete());// true
        System.out.println(f4.delete());// false
    }
}

💡Description in API: delete method, if this File represents a directory, the directory must be empty to delete.

directory traversal

• public String[] list(): Returns a String array representing all subfiles or directories in the File directory.
• public File[] listFiles(): Returns a File array, representing all subfiles or directories in the File directory.

public class FileFor {
    public static void main(String[] args) {
        File dir = new File("d:\\java_code");
      
      	//Get the names of the files and folders in the current directory.
		String[] names = dir.list();
		for(String name : names){
			System.out.println(name);
		}
        //Get the file and folder objects in the current directory, as long as you get the file object, then you can get more information
        File[] files = dir.listFiles();
        for (File file : files) {
            System.out.println(file);
        }
    }
}

💡Tips:
The File object that calls the listFiles method must represent the actual directory, otherwise it returns null and cannot be traversed.