Java - The File Class

I/O Fundamentals

The Java language provides a simple model for input and output (I/O). All I/O is performed by writing to and reading from streams of data. The data may exist in a file or an array, be piped from another stream, or even come from a port on another computer. The flexibility of this model makes it a powerful abstraction of any required input and output.

The File Class

The File class is Java's representation of a file or directory path name. Because file and directory names have different formats on different platforms, a simple string is not adequate to name them. The File class contains several methods for working with the path name, deleting and renaming files, creating new directories, listing the contents of a directory, and determining several common attributes of files and directories.

Creating a File Object

You create a File object by passing in a String that represents the name of a file, and possibly a Stringor another Fileobject. For example,

File a = new File("/usr/local/bin/IPLAB");

defines an abstract file name for the smurf file in directory /usr/local/bin. This is an absolute abstract file name. It gives all path information necessary to find the file.

You could also create a file object as follows:

File b = new File("bin/IPLAB");

This is a relative abstract file name, because it leaves out some necessary path information, which will be filled in by the VM. By default, the VM will use the directory in which the application was executed as the "current path".

File Attribute Methods

The File object has several methods that provide information on the current state of the file.

boolean canRead()         Returns trueif the file is readable

Boolean canWrite()        Returns trueif the file is writeable

Boolean exists()   Returns trueif the file exists

boolean isAbsolute()      Returns trueif the file name is an absolute path name

boolean isDirectory()     Returns trueif the file name is a directory

boolean isFile()     Returns trueif the file name is a "normal" file (depends on OS)

boolean isHidden()         Returns trueif the file is marked "hidden"

long lastModified()        Returns a longindicating the last time the file was modified

long length()         Returns the length of the contents of the file

Text I/O Versus Binary I/O

Java's I/O classes are divided into two main groups, based on whether you want text or binary I/O. Readerand Writerclasses handle text I/O. InputStreamand OutputStream classes handle binary I/O.

Reader and InputStream

Java supplies Readers and InputStreams to read data; their use is similar. The following table shows the most commonly used methods in these classes. See the javadocs for the other methods available. Note that these two classes are abstract; you won't ever create an instance of either, but they provide the base implementation details for all other input classes.

Reader Class Methods:

Writer and OutputStream

Java supplies Writerand OutputStream to write data; their use is similar. The following table shows the methods provided in these classes. Note that these two classes are abstract; you won't ever create an instance of either, but they provide the base implementation details for all other output classes.

Reading and Writing Files

To read and write from files on a disk, use the following classes:

•         FileInputStream

•         FileOutputStream

•         FileReader

•         FileWriter

Each of these has a few constructors, where class is the name of one of the above classes:

•class(File) - create an input or output file based on the abstract path name passed in

•class(String)- create an input or output file based on the String path name

•class(FileDescriptor)- create an input or output file based on a FileDescriptor (you generally won't use this and this class will not discuss it)

•class(String, boolean)- [for output classes only] create an input or output file based on the path name passed in, and if the boolean parameter is true, append to the file rather than overwrite it

For example, we could copy one file to another by using: import java.io.*;

public class FileCopy {

public static void main(String args[]) { try {

// Create an input file

FileInputStream inFile = new FileInputStream(args[0]);

// Create an output file

FileOutputStream outFile = new FileOutputStream(args[1]);

// Copy each byte from the input to output int byteRead;

while((byteRead = inFile.read()) != -1) outFile.write(byteRead);

//   Close the files!!! inFile.close(); outFile.close();

}

//  If something went wrong, report it! catch(IOException e) { System.err.println("Could not copy "+ args[0] + " to " + args[1]); System.err.println("Reason:"); System.err.println(e);}