1. The Construction Method of File Class
(1) The role of File classes
Representing a file or directory with an object of the File class
(2)File(String path)
Create a File object by specifying the path of a file or directory
Path: It can be either a relative path or an absolute path.
For example: File noteFile = new File("note.txt");
System.out.println( file.exists() );
Interpretation:
Relative paths in java: The reference is the path where the JVM is started
When running Java programs through Eclipse, the relative path reference is: Engineering directory
The current project catalogue will be used as a reference, that is, note.txt will output true in the current project catalogue.
(3)File(URI uri)
uri: Unified Resource Location Identification
Get URI: Any class name. class.getResource("/abstract path"). toURI()
Search the specified subpath under the classpath environment variable to get the URL object converted to the URI object by the toURI method
Be careful:
You need to start with "/", which represents the classpath path path
Effect:
Customizable search lookup paths, i.e. adding classpath paths without modifying the program
For example:
File uri_File =new File( PathDemo.class.getResource("/res/uri-demo.txt").toURI() );
System.out.println( uri_File.exists() );
Interpretation: The first / representative is the classpath path path path
The classpath path path in eclipse is the src or bin directory under the project file, and res represents the package name, so you can find the file.
2. Common methods of File classes
(1) File class operation file
createNewFile: When the file represented by the file object does not exist, a new file can be created successfully, otherwise it will have no effect.
Delete: delete immediately
deleteOnExit: Delete when the program exits normally
Existence of exists
isFile determines whether it is a file and returns false if the file object does not already exist on disk.
length Gets File Size
getName Gets Files
GettAbsolutePath Gets the Absolute Path
(2) File class operation directory
mkdir: Only one layer of directory can be created. If there are multiple layers, the creation fails.
mkdirs: Single or multiple directories can be created
delete: delete empty directories
Determine whether the directory exists:exists
Determine if the directory is a directory: isDirectory returns false when the directory does not exist?
Get a list of directory contents:
list()
String[] f = file.list(); // Returns all file and directory names in the directory
listFiles()
File[] f = file.listFiles();//Return all files and directories in the directory
listFiles( FileFilter filter )
File[] f = file.listFiles( new FileFilter(){ //File filters, screening out files and directories that meet their own requirements
public boolean accept(File f){
String fileName = f.getName();
if(fileName.toLowerCase().startsWith("a")){
return true;
}
return false;
}
} );(3) Examples: How to delete all directories and files in a non-empty directory
package file;
import java.io.File;
public class FileOprater {
/**
* How to delete all directories and files in a non-empty directory
*/
public static void removeFile(File file) {
/**
* file Either blank or file is not a directory, which does not fit the title.
*/
if(file == null || !(file.isDirectory())) {
throw new RuntimeException(file+"Not a valid directory");
}
File[] files = file.listFiles();
for(File f : files) {
if(f.isFile()) {//If f is a file, delete it directly
f.delete();
}else {
removeFile(f);
}
}
file.delete();//Delete empty directories recursively returned
}
public static void main(String[] args) {
File file = new File("src/a");
removeFile(file);
}
}
3. Random AccessFile Class: Provides the ability to read and write the contents of files randomly (to read content anywhere in the file)
(1) Construction method
RandomAccessFile(File file, String mode)
RandomAccessFile(String name, String mode)
name:File path to access
mode:What model to start with "r" / "w" / "rw"
(2) File pointer
When the object is currently newly created, the file pointer of the object defaults to the first byte.
The function of the file pointer: to mark the location of the next read-write occurrence
See: Move the file pointer to the file header and jump back to the specified number of bytes
SkpBytes: Jump back the specified number of bytes from the current file pointer
readXxx
writeXxx
(3) Examples: video encryption
package file;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
public class CodeMP4 {
public static void main(String[] args){
try {
//Encrypted Video Files
File file = new File("C:\\Users\\sun changxin\\Desktop\\abc.mp4");
RandomAccessFile raf = new RandomAccessFile(file, "rw");
//Number of bytes to get the file
long len = raf.length();
//Set the location of the variable record file pointer
long filePointerPOS = 0;
//Read 8k bytes at a time
byte[] datas = new byte[8*1024];
do {
//First determine whether the file is larger than 8k, read 8K data to datas if it is larger, and change the datas array if it is less than 8k.
if( len - filePointerPOS < datas.length ) {
datas = new byte[ (int)(len - filePointerPOS) ];
}
//Read the data from the file, read the full data array, and put it into the array data
raf.readFully(datas);
filePointerPOS = raf.getFilePointer();
//Modify each number in the data array
for( int i=0;i<datas.length;i++ ) {
datas[i] -= 1;//encryption
}
//Overlay that piece of data in the file with the modified datas array
raf.seek(filePointerPOS - datas.length);
raf.write(datas);
}while( raf.getFilePointer() < len );
raf.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}