Java IO Tutorial - Java File

An object of the File class is an abstract representation of a pathname of a file or a directory.

Create File

We can create a File object from

A pathname
A parent pathname and a child pathname
A URI (uniform resource identifier)

We can use one of the following constructors of the File class to create a file:

File(String pathname)
File(File parent, String child)
File(String parent, String child)
File(URI uri)

If we have a file pathname string of test.txt, we can create an abstract pathname as the following code.

File dummyFile = new File("test.txt");

A file named test.txt does not have to exist to create a File object using this statement.

The dummyFile object represents an abstract pathname, which may or may not point to a real file in a file-system.

The File class has several methods to work with files and directories.

Using a File object, we can create a new file, delete an existing file, rename a file, change permissions on a file, and so on.

isFile() and isDirectory() from the File class tells whether a File object represents a file or a directory.

Current Working Directory

The current working directory for a JVM is set depending on how we run the java command.

We can get the current working directory for the JVM by reading the user.dir system property as follows:

String workingDir = System.getProperty("user.dir");

Use the System.setProperty() method to change the current working directory.

System.setProperty("user.dir", "C:\\myDir");

To specify C:\test as the user.dir system property value on Windows, we run our program like so:

java -Duser.dir=C:\test your-java-class

File's Existence

We can check if the abstract pathname of a File object exists using the exists() method of the File class.

boolean fileExists = dummyFile.exists();

Full Source Code

import java.io.File;
//www.java2s.com
public class Main {
 public static void main(String[] argv) {
 // Create a File object
 File dummyFile = new File("dummy.txt");
 // Check for the file's existence
 boolean fileExists = dummyFile.exists();
 if (fileExists) {
 System.out.println("The dummy.txt file exists.");
 } else {
 System.out.println("The dummy.txt file does not exist.");
 }
 }
}

The code above generates the following result.

Path

The absolute path identifies the file uniquely on a file system. A canonical path is the simplest path that uniquely identifies the file on a file system.

We can use the getAbsolutePath() and getCanonicalPath() methods to get the absolute and canonical paths represented by a File object, respectively.

import java.io.File;
import java.io.IOException;
//www.java2s.com
public class Main {
 public static void main(String[] args) {
 printFilePath("dummy.txt");
 printFilePath(".." + File.separator + "notes.txt");
 }
 public static void printFilePath(String pathname) {
 File f = new File(pathname);
 System.out.println("File Name: " + f.getName());
 System.out.println("File exists: " + f.exists());
 System.out.println("Absolute Path: " + f.getAbsolutePath());
 try {
 System.out.println("Canonical Path: " + f.getCanonicalPath());
 }
 catch (IOException e) {
 e.printStackTrace();
 }
 }
}

The code above generates the following result.

File separator

Different operating systems use a different character to separate two parts in a pathname.

For example, Windows uses a backslash (\) as a name separator in a pathname, whereas UNIX uses a forward slash (/).

The File class defines a constant named separator Char, which is the system-dependent name separator character.

We can use the File.separator Char constant to get the name separator as a character.

The File.separator constant gives we the name separator as a String.

Using the name separator in your program will make your Java code work on different platforms.