What is the difference between absolute, relative and canonical path of file or directory?

• Absolute path is the full directory such as C:\abcd\test.txt. The definition of absolute pathname is system dependent. On UNIX systems, a pathname is absolute if its prefix is "/". On Win32 systems, a pathname is absolute if its prefix is a drive specifier followed by "\\", or if its prefix is "\\".
  For example, We have two directories: abcd and abcd1 and test.txt file is in abcd directory.

  C:\abcd

  C:\abcd1

  In Java under Windows, you may have the following possible absolute paths that refer to the same file test.txt.

  c:\abcd\test.txt
  C:\abcd\test.txt
  c:\abcd\TEST.TXT
  c:\abcd\.\test.txt
  c:\abcd\..\abcd\test.txt

• Relative path is relative to the directory you are in, so if you are in the above directory, then if you reference file test.txt as relative, it assumes the same directory you are in. When you do ../ then it goes back one directory.
• Canonical paths are a bit harder. For starters, all canonical paths are absolute (but not all absolute paths are canonical). A single file existing on a system can have many different paths that refer to it, but only one canonical path. Canonical gives a unique absolute path for a given file. The details of how this is achieved are probably system-dependent.
  For the above example, we have one and only one canonical path:

  c:\abcd\test.txt

The following example shows how there can be many different paths (and absolute paths) to the same file, which all have the exact same canonical path. Thus canonical path is useful if you want to know if two different paths point to the same file or not.

import java.io.*;

public class Test {
    public static void showPaths(String pathName) throws IOException {
        File file = new File(pathName);
        System.out.println("path: " + file.getPath());
        System.out.println("absolute path: " + file.getAbsolutePath());
        System.out.println("canonical path: " + file.getCanonicalPath());
        System.out.println();
    }
            
    public static void main(String[] s) throws IOException {
        File file = new File(new File("test.txt").getAbsolutePath());
        String parent = file.getParent();
        File parentFile = new File(parent);
        String parentName = parentFile.getName();
        String grandparent = parentFile.getParent();
        file.createNewFile();

        showPaths("test.txt");
        showPaths("TEST.TXT");
        showPaths("." + File.separator + "TEST.TXT");
        
        showPaths(parent
            + File.separator + "."
            + File.separator + "test.txt");
            
        showPaths(grandparent
            + File.separator + parentName
            + File.separator + ".."
            + File.separator + parentName
            + File.separator + "test.txt");
    }
}

On window XP and Java 5.0, the results are:

path: test.txt
absolute path: C:\abcd\test.txt
canonical path: C:\abcd\test.txt

path: TEST.TXT
absolute path: C:\abcd\TEST.TXT
canonical path: C:\abcd\test.txt

path: .\TEST.TXT
absolute path: C:\abcd\.\TEST.TXT
canonical path: C:\abcd\test.txt

path: C:\abcd\.\test.txt
absolute path: C:\abcd\.\test.txt
canonical path: C:\abcd\test.txt

path: C:\abcd\..\abcd\test.txt
absolute path: C:\abcd\..\abcd\test.txt
canonical path: C:\abcd\test.txt

More about "Canonical"

Canonical path is validated with the File System while Absolute path is still abstract and may not be able to represent any physical path. An IOException is thrown if the path name is not possible (Z:/x:/test.txt). For example,

import java.io.*; 
public class Test {
    public static void main(String[] s) throws IOException {
        File file = new File ( "Z:/a,b,b", "a*b*c" );
        System.out.print ( "Absolute Path:" );
        System.out.println ( file.getAbsolutePath () );
        System.out.print ( "Canonical Path:" );
        System.out.println ( file.getCanonicalPath () );
    }
}

Prints:

Absolute Path:Z:\a,b,b\a*b*c
Canonical Path:java.io.IOException: Invalid argument
at java.io.Win32FileSystem.canonicalize(Native Method)
at java.io.File.getCanonicalPath(File.java:437)
at Test.main(Test.java:9)

Since file systems differ over platforms, FileSystem is an abstract class, and canonicalize() is an abstract method. A canonical path is an absolute path, but how it is expressed is up to the concrete implementation of java.io.FileSystem. You'd need to run the code on two separate systems to see the difference between the two methods, assuming there is one.