Java IO Tutorial - Java File

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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;
// ww  w  .j  a v a2s  . c om
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;
// w  w  w.j  a  v  a2  s  .c  o  m
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.

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