PIC20B File notes

Files

Reading assignment.

Java document on File class.

Introduction

File : you should all know what it is.
Directory/Folder : it contains a collection of files and directories.

File class

An abstract representation of file and directory pathnames.
It doesn't create a file when you create an instance of File! It is just an abstract representation. It won't do anything to your computer.
Files and directory are both represented by File class.

Files in Unix/Linux

File names are case sensitive.
Directories are seperated by '/'.
Example : pic10b/io/notes.html.

Files in Windows

File names are not case sensitive.
Directories are seperated by '\'.
Exampe : C:\java\bin\
You can use '/' as a seperator as well. So the following represent the same objects.
C:\java\bin\, C:/java/bin, C:\java/bin.
Notice that '\' is the escape character. (Like in "\n"). So if you want to create an instance of File with file name "java\hw1\hw.java". The you should use one of the follwoing code
new File("java\\hw1\\hw.java");
or
new File("java/hw1/hw.java");

Remark

You can use static char pathSeparatorChar to get the path seperator.

absolute or relative

An absolute pathname is complete in that no other information is required in order to locate the file that it denotes.
Example : C:\windows\Cookies
A relative pathname, in contrast, must be interpreted in terms of information taken from some other pathname. By default the classes in the java.io package always resolve relative pathnames against the current user directory.
Example: If you are working in C:\Submit\,then a relative path name can be hw1.java.

Constructors and methods

File

Constructors
Constructors/Methods	Description
File(String pathname), File(String parent, String filename), File(File directory, String filename)	Creates a new File instance by converting the given pathname string into an abstract pathname. Or creates a new File instance from a parent pathname string (or directory) and a child pathname string.

Test methods
Constructors/Methods	Description
exists()	Tests whether the file denoted by this abstract pathname exists.
isFile()	Tests whether the file denoted by this abstract pathname is a normal file. Returns `true` if and only if the file denoted by this abstract pathname exists and is a normal file; `false` otherwise.
isDirectory()	Tests whether the file denoted by this abstract pathname is a directory. Returns `true` if and only if the file denoted by this abstract pathname exists and is a directory; `false` otherwise.

Action Methods
Constructors/Methods	Description
boolean mkdir()	Creates the directory named by this abstract pathname. Returns true if and only if the directory was created; false otherwise.
boolean mkdirs()	Creates the directory named by this abstract pathname, including any necessary but nonexistent parent directories. Returns true if and only if the directory was created, along with all necessary parent directories; false otherwise
boolean delete()	Deletes the file or directory denoted by this abstract pathname. If this pathname denotes a directory, then the directory must be empty in order to be deleted. Returns true if and only if the file or directory is successfully deleted; false otherwise
boolean renameTo(File dest)	Renames the file denoted by this abstract pathname. Returns true if and only if the renaming succeeded; false otherwise

List Methods
Constructors/Methods	Description
String[] list()	Returns an array of strings naming the files and directories in the directory denoted by this abstract pathname. If this abstract pathname does not denote a directory, then this method returns null. Otherwise an array of strings is returned, one for each file or directory in the directory.
File[] listFiles()	Returns an array of abstract pathnames denoting the files in the directory denoted by this abstract pathname. If this abstract pathname does not denote a directory, then this method returns null. Otherwise an array of File objects is returned, one for each file or directory in the directory.

Other methods
Constructors/Methods	Description
long lastModified()	Returns the time that the file denoted by this abstract pathname was last modified.
long length()	Returns the length, in bytes, of the file denoted by this abstract pathname, or 0L if the file does not exist

Example

Question 1

Write a program TestExistence.java, such that
java TestExistance file tells you if the file exists or not.
Here are some outputs

C:\myjavaprograms> java TestExistance C:\windows\cookies
C:\windows\cookies exists.
C:\myjavaprograms> java TestExistance abcd\fgh\ijk
abcd\fgh\ijk doesn't exist.

Answer : TestExistence.java

import java.io.*;

public class TestExistence {
    public static void main(String[] args) {
        File file = new File(args[0]);
        if(file.exists()) {
            System.out.println(file + " exists.");
        } else {
            System.out.println(file + " does not exist.");
        }
    }
}

Remark
Try

java TestExistance C:/windows/cookies
java TestExistance C:/windows\cookies
Question 2
Write a method list all the files (and files in the subdirectories) in a given
directory. 
Answer
    public static void listAllFiles(File file) {
        if(file.isFile()) {
            System.out.println(file);
        } else if(file.isDirectory()) {
            File[] fileList = file.listFiles();
            for(int i = 0; i < fileList.length; i++) {
                listAllFiles(fileList[i]);
            }
        }
    }

Question 3
Write a program to create 3 directories pic10a, pic10b, pic20a
under the directory classes.
Answer : Question3.java
import java.io.File;

public class Question3 {
    public static void main(String[] args) {
        String[] dirNames = {"pic10a", "pic10b", "pic20a"};
        for(int i = 0; i < dirNames.length; i++) {
            new File("classes/" + dirNames[i]).mkdirs();
        }
    }
}

Question : do you know the difference between mkdir and mkdirs ?

Question 4
Write a program Question4.java such that 
java Question4 file shows the size(number of bytes) of the file. 
Answer: Question4.java
import java.io.File;

public class Question4 {
    public static void main(String[] args) {
        File file = new File(args[0]);
        if(file.isFile()) {
            System.out.println("The size of the file is " + file.length() + " bytes.");  
        } else {
            System.out.println(file + " is not a file.");
        }
    }
}