Relative, absolute paths and other file methods in Java

The Java File class encapsulates the filesystem in a simplified manner. Its constructor accepts absolute and relative (to the working directory of the program) paths. For instance:

new File(".") //--> program's current directory

But using relative paths can cause trouble in some situations, since the working directory could be modified. Besides that, if there are input fields for file paths you should avoid relative paths unless in special cases like if the selected file is part of your program.

Checking if a path is relative

The File.isAbsolute() method gives us a hand in this task, telling if the path is absolute. Example:

File f1 = new File("..");
System.out.println(f1.isAbsolute()); //prints false

File f2 = new File("c:\\temp");
System.out.println(f2.isAbsolute()); //prints true

How to get the absolute path

Another useful method is getAbsolutePath(). It returns the absolute path to an instance of File.

Another example:

File f1 = new File("\\directory\\file.xml");
System.out.println(f1.getAbsolutePath()); //prints C:\directory\file.xml

File f2 = new File("c:\\directory\\file.xml");
System.out.println(f2.getAbsolutePath()); //prints c:\directory\file.xml

Other nice features of `File`

File contains various interesting features for specific use cases, such as:

getParentFile: returns a File pointing to the directory that contains the current file or directory.
getAbsoluteFile: returns another instance of File with an absolute path.
toURI: returns a URI (Universal Resource Identifier) that begins with file:. It’s useful to network operations.
isFile e idDirectory: tells if File points to a file or directory, respectively.
exists: tells if the file or directory really exists in filesystem.
canRead e canWrite: tells if you can read or write to the file, respectively.
createNewFile: creates a new blank file.
delete: removes the file or directory (if empty).
length: retuns the file size in bytes.
list e listFiles: lists files and directories, if File is a directory.
mkdir e mkdirs: creates a new directory, if File is a directory. The latter also creates parent directories if needed.
getFreeSpace: returns the available space in the device to where File is pointing to.
createTempFile: static method that returns an unique temporary file to be used by the application. The method deleteOnExit will delete the file at the termination of the program.

The class File also contains some important static attributes useful to read and write files in different platforms:

File.separator: path-name separator character. On Unix and Linux it is /, while on Windows it is \.
File.pathSeparator: path-separator character, in order to create a path list with various directories, like PATH system variable. On Unix and Linux it is :, while on Windows it is ;.