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Tip of the Day
Language: Java
Expertise: Intermediate
Feb 3, 2000

Generate a Relative Path

Sometimes in an application you will have a "home" directory associated with the application, and then some files associated with that home directory. To have the files in an independent position relative to the root of the file system, you may want to use relative paths to the files rather than absolute or canonical paths. If your input specifies files in absolute form, you can use the following code to generate the relative path from a 'home' directory to a file, whether it resides under that directory or in some other place in the file system. Examples:
 
home = "/a/b/c"
file = "/a/b/c/d/e.txt"
relative path = "d/e.txt"

home = "/a/b/c"
file = "/a/d/f/g.txt"
relative path = "../../d/f/g.txt"

--------------------------------------------------------------------------
import java.io.*;
import java.util.*;

/**
 * this class provides functions used to generate a relative path
 * from two absolute paths
 * @author David M. Howard
 */
public class RelativePath {
	/**
	 * break a path down into individual elements and add to a list.
	 * example : if a path is /a/b/c/d.txt, the breakdown will be [d.txt,c,b,a]
	 * @param f input file
	 * @return a List collection with the individual elements of the path in
reverse order
	 */
	private static List getPathList(File f) {
		List l = new ArrayList();
		File r;
		try {
			r = f.getCanonicalFile();
			while(r != null) {
				l.add(r.getName());
				r = r.getParentFile();
			}
		}
		catch (IOException e) {
			e.printStackTrace();
			l = null;
		}
		return l;
	}

	/**
	 * figure out a string representing the relative path of
	 * 'f' with respect to 'r'
	 * @param r home path
	 * @param f path of file
	 */
	private static String matchPathLists(List r,List f) {
		int i;
		int j;
		String s;
		// start at the beginning of the lists
		// iterate while both lists are equal
		s = "";
		i = r.size()-1;
		j = f.size()-1;

		// first eliminate common root
		while((i >= 0)&&(j >= 0)&&(r.get(i).equals(f.get(j)))) {
			i--;
			j--;
		}

		// for each remaining level in the home path, add a ..
		for(;i>=0;i--) {
			s += ".." + File.separator;
		}

		// for each level in the file path, add the path
		for(;j>=1;j--) {
			s += f.get(j) + File.separator;
		}

		// file name
		s += f.get(j);
		return s;
	}

	/**
	 * get relative path of File 'f' with respect to 'home' directory
	 * example : home = /a/b/c
	 *           f    = /a/d/e/x.txt
	 *           s = getRelativePath(home,f) = ../../d/e/x.txt
	 * @param home base path, should be a directory, not a file, or it doesn't
make sense
	 * @param f file to generate path for
	 * @return path from home to f as a string
	 */
	public static String getRelativePath(File home,File f){
		File r;
		List homelist;
		List filelist;
		String s;

		homelist = getPathList(home);
		filelist = getPathList(f);
		s = matchPathLists(homelist,filelist);

		return s;
	}

	/**
       * test the function
       */
	public static void main(String args[]) {
		if (args.length != 2) {
			System.out.println("RelativePath <home> <file>");
			return;
		}
		System.out.println("home = " + args[0]);
		System.out.println("file = " + args[1]);
		System.out.println("path = " + getRelativePath(new File(args[0]),new
File(args[1])));
	}
}
David M.
 
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