Java Files

Java provides a number of different classes and methods for utilizing files and a computer’s file system. They include the File, FileReader, and FileWriter classes (all from the java.io package).

Accessing Files

Files and directories can be accessed with the File class.

Syntax

import java.io.File;

File myFile = new File(pathString);

The constructor of the File class accepts a pathString that specifies path/filename. Once declared, the new myFile object can be used to manipulate and gather information about the file or directory specified by pathString.

Methods

The File object includes the following methods to manipulate the specified file or directory:

• canRead(): Returns true if the file is readable.
• canWrite(): Returns true if the file is writable.
• createNewFile(): Creates an empty file. Returns true if successful.
• delete(): Deletes a file. Can delete a directory if it is empty.
• exists(): Returns true if the file/directory exists.
• getName(): Returns the name of the file/directory.
• getAbsolutePath(): Returns the full pathname of the file/directory.
• isDirectory(): Returns true if instance points to a directory.
• isFile(): Returns true if instance points to a file.
• length(): Returns the size of the file in bytes.
• list(): Returns a String[] array of the files in the directory.
• mkdir(): Creates a directory.

Example

The following example creates a File object, checks if its corresponding file exists, and if not, creates it (file access is placed within a try ... catch block in case the file system throws any errors):

import java.io.File;
import java.io.IOException;

public class FileExample {
  public static void main(String[] args) {
    try {
      File myFile = new File("test.txt");
      if (myFile.exists()) {
        System.out.println("File exists: " + myFile.getName());
      } else {
        if (myFile.createNewFile()) {
          System.out.println("File created: " + myFile.getName());
        } else {
          System.out.println("File was not created.");
        }
      }
    } catch (IOException e) {
      System.out.println("An error was thrown.");
      e.printStackTrace();
    }
  }
}

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Writing to a File

Data can be written to a file with the FileWriter class.

Syntax

import java.io.FileWriter;

FileWriter myWriter = new FileWriter(pathString, append);

The constructor of the FileWriter class takes a pathString that specifies a path/filename. Once declared, the new myWriter object can be used to write to the file specified by pathString. The optional append boolean specifies if writing will append to the file.

Methods

The following methods are provided by the FileWriter:

• close(): Closes the FileWriter. Should be done after all writes are complete.
• write(): Writes a string or char sequence to the file.

Example

The following example writes a string out to a file (again, within a try ... catch block):

import java.io.FileWriter;
import java.io.IOException;

public class FileWriterExample {
  public static void main(String[] args) {
    try {
      FileWriter myWriter = new FileWriter("test.txt");
      myWriter.write("Hello World!");
      myWriter.close();
    } catch (IOException e) {
      System.out.println("An error was thrown.");
      e.printStackTrace();
    }
  }
}

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Reading Files

Data can be read from a file with the FileReader class.

Syntax

import java.io.FileReader;

FileReader myReader = new FileReader(pathString);

The constructor of the FileReader class takes a pathString that specifies a path/filename. Once declared, the new myReader object can be used to read characters from the file specified by pathString.

Methods

The FileReader class offers the following methods to read from a file:

• close(): Closes the FileReader. Should be done after all reads are complete.
• read(): Reads a character from the file, or reads characters into a buffer.

Example

The following example reads all the characters from a file (again, within a try ... catch block):

import java.io.FileReader;
import java.io.IOException;

public class FileReaderExample {
  public static void main(String[] args) {
    try {
      FileReader myReader = new FileReader("test.txt");
      int i;
      while ((i = myReader.read()) != -1)
        System.out.print((char)i);
      myReader.close();
    } catch (IOException e) {
      System.out.println("An error was thrown.");
      e.printStackTrace();
    }
  }
}

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