Chapter: Java The Complete Reference - The Java Library - Input/Output: Exploring java.io

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File - java.io

Although most of the classes defined by java.io operate on streams, the File class does not. It deals directly with files and the file system.

File

 

Although most of the classes defined by java.io operate on streams, the File class does not. It deals directly with files and the file system. That is, the File class does not specify how information is retrieved from or stored in files; it describes the properties of a file itself. A File object is used to obtain or manipulate the information associated with a disk file, such as the permissions, time, date, and directory path, and to navigate subdirectory hierarchies.

Files are a primary source and destination for data within many programs. Although there are severe restrictions on their use within applets for security reasons, files are still a central resource for storing persistent and shared information. A directory in Java is treated simply as a File with one additional property—a list of filenames that can be examined by the list( ) method.

 

The following constructors can be used to create File objects:

 

File(String directoryPath)

 

File(String directoryPath, String filename) File(File dirObj, String filename) File(URI uriObj)

 

Here, directoryPath is the path name of the file; filename is the name of the file or subdirectory; dirObj is a File object that specifies a directory; and uriObj is a URI object that describes a file.

 

The following example creates three files: f1, f2, and f3. The first File object is constructed with a directory path as the only argument. The second includes two arguments—the path and the filename. The third includes the file path assigned to f1 and a filename; f3 refers to the same file as f2.

File f1     = new File("/");

File         f2          =       new File("/","autoexec.bat");

File         f3          =       new File(f1,"autoexec.bat");

File defines many methods that obtain the standard properties of a File object. For example, getName( ) returns the name of the file; getParent( ) returns the name of the parent directory; and exists( ) returns true if the file exists, false if it does not. The following example demonstrates several of the File methods. It assumes that a directory called java exists off the root directory and that it contains a file called COPYRIGHT.

 

// Demonstrate File.

import java.io.File;

 

class FileDemo {

 

static void p(String s) { System.out.println(s);

 

}

 

public static void main(String args[]) { File f1 = new File("/java/COPYRIGHT");

 

p("File Name: " + f1.getName()); p("Path: " + f1.getPath());

 

 

p("Abs Path: " + f1.getAbsolutePath()); p("Parent: " + f1.getParent()); p(f1.exists() ? "exists" : "does not exist");

 

p(f1.canWrite() ? "is writeable" : "is not writeable"); p(f1.canRead() ? "is readable" : "is not readable");

 

p("is " + (f1.isDirectory() ? "" : "not" + " a directory"));

p(f1.isFile() ? "is normal file" : "might be a named pipe");

p(f1.isAbsolute() ? "is absolute" : "is not absolute");

p("File last modified: " + f1.lastModified());

p("File size: " + f1.length() + " Bytes");

 

}

 

}

 

This program will produce output similar to this:

 

File Name: COPYRIGHT Path: \java\COPYRIGHT

 

Abs Path: C:\java\COPYRIGHT Parent: \java

 

exists

 

is writeable is readable

 

is not a directory is normal file

 

is not absolute

 

File last modified: 1282832030047 File size: 695 Bytes

 

Most of the File methods are self-explanatory. isFile( ) and isAbsolute( ) are not. isFile( ) returns true if called on a file and false if called on a directory. Also, isFile( ) returns false for some special files, such as device drivers and named pipes, so this method can be used to make sure the file will behave as a file. The isAbsolute( ) method returns true if the file has an absolute path and false if its path is relative.

File includes two useful utility methods of special interest. The first is renameTo( ), shown here:

 

boolean renameTo(File newName)

 

Here, the filename specified by newName becomes the new name of the invoking File object. It will return true upon success and false if the file cannot be renamed (if you attempt to rename a file so that it uses an existing filename, for example).

The second utility method is delete( ), which deletes the disk file represented by the path of the invoking File object. It is shown here:

 

boolean delete( )

 

You can also use delete( ) to delete a directory if the directory is empty. delete( ) returns true if it deletes the file and false if the file cannot be removed.

 

Here are some other File methods that you will find helpful:

 

Method : Description

 :

void deleteOnExit( ) : Removes the file associated with the invoking object when the Java Virtual Machine terminates.

 

long getFreeSpace( ) : Returns the number of free bytes of storage available on the partition associated with the invoking object.

 

long getTotalSpace( ) : Returns the storage capacity of the partition associated with the invoking object.

 

long getUsableSpace( ) : Returns the number of usable free bytes of storage available on the partition associated with the invoking object.

 

boolean isHidden( ) : Returns true if the invoking file is hidden. Returns false otherwise.

 

boolean setLastModified(long millisec) : Sets the time stamp on the invoking file to that specified by millisec, which is the number of milliseconds from January 1, 1970, Coordinated Universal Time (UTC).

 

boolean setReadOnly( ) : Sets the invoking file to read-only.

 

Methods also exist to mark files as readable, writable, and executable. Because File implements the Comparable interface, the method compareTo( ) is also supported.

JDK 7 added a method to File called toPath( ), which is shown here: Path toPath( )

 

toPath( ) returns a Path object that represents the file encapsulated by the invoking File object. (In other words, toPath( ) converts a File into a Path.) Path is packaged in java.nio.file and is part of NIO. Thus, toPath( ) forms a bridge between the older File class and the newer Path interface. (See Chapter 21 for a discussion of Path.)

 

Directories

 

A directory is a File that contains a list of other files and directories. When you create a File object that is a directory, the isDirectory( ) method will return true. In this case, you can call list( ) on that object to extract the list of other files and directories inside. It has two forms. The first is shown here:

 

String[ ] list( )

 

The list of files is returned in an array of String objects.

 

The program shown here illustrates how to use list( ) to examine the contents of a directory:

 

// Using directories.

import java.io.File;

 

class DirList {

 

public static void main(String args[]) {

String dirname = "/java";

File f1 = new File(dirname);

if (f1.isDirectory()) {

System.out.println("Directory of " + dirname); String s[] = f1.list();

for (int i=0; i < s.length; i++) {   

File f = new File(dirname   +    "/" + s[i]);

if (f.isDirectory()) {          

System.out.println(s[i] +    " is a directory");

} else {

 

System.out.println(s[i] + " is a file");

 

}

 

}

 

} else {

 

System.out.println(dirname + " is not a directory");

 

}

 

}

 

}

 

Here is sample output from the program. (Of course, the output you see will be different, based on what is in the directory.)

 

Directory of /java bin is a directory lib is a directory demo is a directory COPYRIGHT is a file README is a file index.html is a file include is a directory src.zip is a file

 

src is a directory

 

Using FilenameFilter

 

You will often want to limit the number of files returned by the list( ) method to include only those files that match a certain filename pattern, or filter. To do this, you must use a second form of list( ), shown here:

 

String[ ] list(FilenameFilter FFObj)

 

In this form, FFObj is an object of a class that implements the FilenameFilter interface. FilenameFilter defines only a single method, accept( ), which is called once for each file in a list. Its general form is given here:

 

boolean accept(File directory, String filename)

 

The accept( ) method returns true for files in the directory specified by directory that should be included in the list (that is, those that match the filename argument) and returns false for those files that should be excluded.

 

The OnlyExt class, shown next, implements FilenameFilter. It will be used to modify the preceding program to restrict the visibility of the filenames returned by list( ) to files with names that end in the file extension specified when the object is constructed.

 

import java.io.*;

 

public class OnlyExt implements FilenameFilter { String ext;

 

public OnlyExt(String ext) { this.ext = "." + ext;

 

}

 

public boolean accept(File dir, String name) {

return name.endsWith(ext);

 

}

 

}

 

The modified directory listing program is shown here. Now it will only display files that use the .html extension.

 

// Directory of .HTML files.

import java.io.*;

 

class DirListOnly {

 

public static void main(String args[]) {

String dirname = "/java";

 

File f1 = new File(dirname);

FilenameFilter only = new OnlyExt("html");

String s[] = f1.list(only);

 

for (int i=0; i < s.length; i++) { System.out.println(s[i]);

 

}

 

}

 

}

 

The listFiles( ) Alternative

There is a variation to the list( ) method, called listFiles( ), which you might find useful. The signatures for listFiles( ) are shown here:

File[ ] listFiles( )

 

File[ ] listFiles(FilenameFilter FFObj)

File[ ] listFiles(FileFilter FObj)

These methods return the file list as an array of File objects instead of strings. The first method returns all files, and the second returns those files that satisfy the specified FilenameFilter. Aside from returning an array of File objects, these two versions of listFiles( ) work like their equivalent list( ) methods.

 

 

The third version of listFiles( ) returns those files with path names that satisfy the specified FileFilter. FileFilter defines only a single method, accept( ), which is called once for each file in a list. Its general form is given here:

 

boolean accept(File path)

 

The accept( ) method returns true for files that should be included in the list (that is, those that match the path argument) and false for those that should be excluded.

 

Creating Directories

 

Another two useful File utility methods are mkdir( ) and mkdirs( ). The mkdir( ) method creates a directory, returning true on success and false on failure. Failure can occur for various reasons, such as the path specified in the File object already exists, or the directory cannot be created because the entire path does not exist yet. To create a directory for which no path exists, use the mkdirs( ) method. It creates both a directory and all the parents of the directory.

 


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