【】 Java IO (1) - 类 File
In Java, File class java.io package is subject only mapping a disk file itself . File class can obtain information about the file (see the file name, path, access, modification date, etc.), but the File class can not read information from and write information to a file in a file .
1. Three File class constructor
1 // File (File parent, String child): Creates a new File instance from a parent abstract pathname and a child pathname string. 2 File parent = new new File ( "E: \\ Test" ); . 3 File Child = new new File (parent, "demo.txt" ); . 4 . 5 // File (String pathname): given by the path name string converted into an abstract pathname to create a new File instance. If the given string is a null string, the result is empty abstract pathname. 6 File File = new new File ( "E: \\ \\ demo.txt the Test" ); 7 8 // File (String parent, String child): Creates a new File from a parent pathname string and a child pathname string instance. . 9 String parent = "E: \\ \\ Test" ; 10 String = Child "demo.txt"11 File file = new File(parent,child);
Note: Windows operating system path, such as E: \ demo \ hello.java, in a Java program, its path is written as E: /demo/hello.java or E: \\ demo \\ hello.java
The main function 2. File class
(1) get the file attributes
. 1 Import java.io.File; 2 Import java.util.Date; . 3 . 4 public class File_test { . 5 public static void main (String [] args) { . 6 File F = new new File ( "E: \\ \\ Hello Test .doc "); // abstract path name" E: \\ test \\ hello.doc " 7 System.out.println (" get file E: \\ test \\ hello.doc of information: " ); 8 System.out.println ( "-------------------------------" ); 9 System.out.println ( "file length: "+ f.length () +" bytes " ); 10 System.out.println ( "judgment of the File object is not a file:" + (f.isFile () "is a file":? "Is not a file" )); 11 System.out.println ( "File object is not the judge directory: "+ (? f.isDirectory () " directory ":" not a directory " )); 12 System.out.println (" is readable: "? + (f.canRead () " can be read ": "unreadable" )); 13 System.out.println ( "can write:" + (f.canWrite () "can be written":? "writing" )); 14 System.out.println ( "whether to hide:" + (? f.isHidden () " is a hidden file": "No hidden file" )); 15 System.out.println ( "last modified:" + new new a date (f.lastModified ()) ); 16 System.out.println ( "File name: "+ f.getName ()); . 17 System.out.println (" Parent File Path: "+ f.getParent ()); 18 is System.out.println ( "abstract pathname into the path name string:" + f.getPath ()); // equivalent f.toString () . 19 System.out.println ( "determines whether the abstract pathname absolute path: "+ (f.isAbsolute ()? " is the absolute path ":" is not an absolute path " )); 20 System.out.println (" absolute file path: "+ f.getAbsolutePath ()); 21 System .out.println ( "-------------------------------" ); 22 is } 23 is }
operation result
Get File E: \ test \ hello.doc of information: ------------------------------- file length: 9216 bytes determine whether the file object is not a file: the file to determine the file object is not a directory: not a directory is readable: to read is writable: writable is hidden: hidden files are not the last modified date: Mon Jul 01 21:20 : 47 CST 2019 file name: hello.doc file parent path: E: \ test abstract pathname into a pathname string: E: \ test \ hello.doc determine whether abstract pathname is absolute path: absolute path absolute file path: E: \ the Test \ hello.doc -------------------------------
boolean setReadOnly public () : Set the file can only be read, not write.
. 1 public static void main (String [] args) { 2 File F = new new File ( "E: \\ test \\ hello.doc"); // abstract path name "E: \\ test \\ hello.doc " . 3 f.setReadOnly (); . 4 System.out.println (" is readable: "+ (f.canRead ()? " readable ":" unreadable " )); . 5 System.out.println ( "can write:" + (f.canWrite () "can be written":? "writing" )); 6 }
operation result
It is readable: readable
is writable: unwritable
(2) create and delete files, create and delete directories, directory traversal list () and list ()
String [] List () : This method returns an array of strings represents all files in a directory and subdirectory names consisting of the File object, if the call is not a directory File object, null is returned.
Tip: List () method returns an array contains only the file name, and does not include a path. But it does not guarantee the same strings in the resulting array will appear in a specific order, in particular, that they appear in alphabetical order.
. 1 Package test0607; 2 . 3 Import java.io.File; . 4 Import java.io.IOException; . 5 Import java.util.Date; . 6 . 7 public class File_test { . 8 public static void main (String [] args) throws IOException { . 9 of NF = file new new file ( "E: /test/new_file.doc"); // create a link to the file E: /test/new_file.doc file object 10 iF (nf.exists ()) // determine whether the file exists 11 { 12 nf.delete (); // exist delete 13 } 14 nf.createNewFile (); // re-create the file 15 16 File dir = new new File ( "E: / test01"); // create a link to the directory E: / test01 File object . 17 iF (dir.exists ()) // determines whether there is the directory 18 is { . 19 dir.delete (); // exist delete 20 is } 21 is dir.mkdir (); // then create the directory 22 is 23 is File LF = new new File ( "E: / Test /"); //The need to create a File object to traverse the 24- System.out.println ( "file type \ t File size \ t file name" ); 25 System.out.println ( "--------------- --------------- " ); 26 is String the fileList [] = lf.list (); // call list with no parameters () method 27 for ( int I = 0; I <fileList.length; i ++) { // character through the returned array 28 System.out.print (( new new file ( "E: / the Test /", fileList [i])) isFile () "file" +. "? \ t ":" folder "+" \ T " ); 29 of System.out.print (( new new file (" E: / Test / ", the fileList [I])). length () +" bytes "+ "\ t"); 30 System.out.println (the fileList [I]); 31 is // Since the character array list () method returns the name of the file includes only, so in order to get the file type and size, must be converted to a File object then call its methods. 32 } 33 is System.out.println ( "------------------------------" ); 34 is } 35 }
operation result
File Name File Type Size ---------------------------------- folder dir01 0 byte folder word 0 section dir02 file good.bmp 0 byte file hello.doc 9216 byte file hi.txt 0 byte file byte 0 new_file.doc -------------------- --------------
String [] list (the FilenameFilter filter) : the same effect as the method list () method, except that the array returned contains only the files and directories of the filter in line with filter, if the filter is null, then accept all names.
First create a file filter , the filter must implement java.io.FilenameFilter an interface, and allows the specified file type accept () method.
. 1 Import java.io.File; 2 Import java.io.FilenameFilter; . 3 Import java.io.IOException; . 4 Import java.util.Date; . 5 . 6 class DocFilter the implements FilenameFilter . 7 { . 8 // implement FilenameFilter interfaces . 9 @Override 10 public Boolean Accept (file the dir, String name) . 11 { 12 is // specified allowed document type 13 is return name.endsWith ( "TXT.") || name.endsWith ( "BMP." ); 14 } 15 } 16 . 17 public class File_test03 { 18 is public static void main (String [] args) throws IOException { . 19 File LF = new new File ( "E: / Test /"); // Create a File object needs to traverse 20 is the System.out .println ( "file type \ t file size \ t file name" ); 21 System.out.println ( "------------------------- ----- " ); 22 is String the fileList [] = lf.list ( new new DocFilter ()); // call list with no parameters () method 23 is for ( int0 = I; I <fileList.length; I ++) { // iterate return character array 24 of System.out.print (( new new File ( "E: / Test /", the fileList [I])) isFile ().? "file" + "\ t": "folder" + "\ T" ); 25 of System.out.print (( new new file ( "E: / Test /", the fileList [I])). length () + "byte" + "\ T" ); 26 is System.out.println (the fileList [I]); 27 // Since the list () method returns a character array containing only a file name, so in order to get the file type and size, File must be converted to an object and then call its methods. 28 } 29 System.out.println ( "------------------------------" );
operation result
File Name File Type Size ------------------------------ file good.bmp 0 byte file byte 0 hi.txt ------------------------------