Compute relative path from a directory to other file or directory. a derivative of https://gist.github.com/2940215

RelativePath.java

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

public class RelativePath {
    public static String relativePath(String origin, String target) {
        try {
            origin = (new File(origin)).getCanonicalPath();
            target = (new File(target)).getCanonicalPath();
        } catch (IOException e) {
            return null;
        }

        if (System.getProperty("os.name").indexOf("Windows") != -1) {
            if (origin.startsWith("\\\\") || target.startsWith("\\\\")) {
                // Windows UNC path not supported.
                return null;
            }
            char originLetter = origin.charAt(0);
            char targetLetter = target.charAt(0);
            if (Character.isLetter(originLetter)
                    && Character.isLetter(targetLetter)) {
                // Windows only
                if (originLetter != targetLetter) {
                    // Drive letters differ
                    return null;
                }
            }
        }

        final String[] originArray = splitBySeparator(origin);
        final String[] targetArray = splitBySeparator(target);
        final int maxCommonComponents = Math.min(originArray.length,
                targetArray.length);
        int commonComponents = 0;
        while (commonComponents < maxCommonComponents
                && originArray[commonComponents]
                        .equals(targetArray[commonComponents])) {
            commonComponents++;
        }

        final int ancestors = originArray.length - commonComponents;
        final int descendant = targetArray.length - commonComponents;
        final StringBuilder sb = new StringBuilder();
        for (int i = 0; i < ancestors; i++) {
            sb.append("..").append(File.separator);
        }
        for (int i = 0; i < descendant; i++) {
            sb.append(targetArray[commonComponents + i]).append(File.separator);
        }
        if (sb.length() != 0) {
            sb.setLength(sb.length() - File.separator.length());
        }
        if (sb.length() == 0) {
            return ".";
        }
        return sb.toString();
    }

    private static String[] splitBySeparator(String path) {
        final StringTokenizer tk = new StringTokenizer(path, File.separator,
                false);
        final List<String> components = new ArrayList<String>();
        while (tk.hasMoreTokens()) {
            components.add(tk.nextToken());
        }
        return components.toArray(new String[components.size()]);
    }
}