Basic operation of file

java.io.File

• File Each instance of is used to represent a file or directory on the hard disk . Use File Sure :
• 1: Access the attribute information of the file directory ( name , size , Change time, etc )
• 2: Create or delete file directories
• 3: Access all children in a directory

File When creating, you need to specify the path information of the file or directory it represents , There are two ways to write a path
Absolute path :
for example :D:/jas2009/JSD2009/demo.txt
Relative paths :
for example :./demo.txt

The advantage of absolute path is clarity , But the position is fixed , Cross platform disadvantage .
The advantage of relative path is good adaptability , Facilitate cross platform . But the disadvantage is not clear , need It depends on the running environment of the current program .
stay idea(eclipse Also the same ) The location of the current directory in is the directory where the current program is located , This is the equivalent of JSD2009_SE This directory

   File file = new File("./demo.txt");
    String name = file.getName();  // Get the name of the file 
    System.out.println(name);

    long length = file.length();  // file size 
    System.out.println(length+" byte ");

    boolean cr = file.canRead();    // Is the file readable 
    boolean cw = file.canWrite();   // Is the document writable 
    System.out.println(" Can be read :"+cr);
    System.out.println(" Can write :"+cw);
    boolean ih = file.isHidden();   // Whether the file can be hidden 
    System.out.println(" Can I hide :"+ih);

createNewFile(): create a file

// Create a new file in the current directory :test.txt
    File file = new File("./test.txt");
    //boolean exists() The method is to judge File Indicates whether the file or directory already exists 
    if(file.exists()){
    
        System.out.println(" The file already exists ");
    }else{
    
        file.createNewFile();
        System.out.println(" File created ");
    }

delete(): Delete file

 File file = new File("test.txt");
    if(file.exists()){
    
        file.delete();
        System.out.println(" The file has been deleted ");
    }else {
    
        System.out.println(" The file does not exist ");
    }

To create a directory, you usually use mkdirs

     File dir = new File("a/b/c/d/e/f");
        if(dir.exists()){
    
            System.out.println(" The directory already exists !");
        }else{
    
   // dir.mkdir(); // The directory where the created directory is located must exist , Otherwise, the creation will not succeed 
            dir.mkdirs();    // Will create the nonexistent negative directory together 
            System.out.println(" The directory has been created !");
        }

delete(): Delete directory :

File file = new File("./demo");
    if(file.exists()){
    
        file.delete();  // When deleting a directory, it is required that the directory must be empty 
        System.out.println(" The directory has been deleted ");
    }else {
    
        System.out.println(" The directory does not exist ");
    }

  // Get all the children of the current directory 
    File dir = new File(".");
    /* boolean isFile() Judge the present File Indicates whether it is a file  boolean isDirectory() Determine whether it is a directory  */
      if(dir.isDirectory()){
    
                   /* Files listFile()  Get current File Object represents all children in a directory , Each subitem is represented by a  File Object representation is finally returned as an array  */
       File[] subs = dir.listFiles();
        System.out.println(" Sub item total :"+subs.length+" individual ");
        for (int i=0;i<subs.length;i++){
    
            System.out.println(subs[i].getName());
        }
    }else {
    
        System.out.println(" Not a directory ");
    }

Get the current directory containing a Children of

   File dir = new File(".");
        if(dir.isDirectory()){
    
            File[] subs = dir.listFiles(new FileFilter() {
    
                public boolean accept(File file) {
    
                    String name = file.getName();
                    int index = name.indexOf("a");
                    return index!=-1;
                }
            });
            System.out.println(" altogether :"+subs.length+" individual ");
        }