下面是一个从基本路径解析相对路径的方法，不管它们在相同或不同的根路径中:

public static String GetRelativePath(String path, String base){

    final String SEP = "/";

    // if base is not a directory -> return empty
    if (!base.endsWith(SEP)){
        return "";
    }

    // check if path is a file -> remove last "/" at the end of the method
    boolean isfile = !path.endsWith(SEP);

    // get URIs and split them by using the separator
    String a = "";
    String b = "";
    try {
        a = new File(base).getCanonicalFile().toURI().getPath();
        b = new File(path).getCanonicalFile().toURI().getPath();
    } catch (IOException e) {
        e.printStackTrace();
    }
    String[] basePaths = a.split(SEP);
    String[] otherPaths = b.split(SEP);

    // check common part
    int n = 0;
    for(; n < basePaths.length && n < otherPaths.length; n ++)
    {
        if( basePaths[n].equals(otherPaths[n]) == false )
            break;
    }

    // compose the new path
    StringBuffer tmp = new StringBuffer("");
    for(int m = n; m < basePaths.length; m ++)
        tmp.append(".."+SEP);
    for(int m = n; m < otherPaths.length; m ++)
    {
        tmp.append(otherPaths[m]);
        tmp.append(SEP);
    }

    // get path string
    String result = tmp.toString();

    // remove last "/" if path is a file
    if (isfile && result.endsWith(SEP)){
        result = result.substring(0,result.length()-1);
    }

    return result;
}

在另一个答案中提到的错误是由Apache HttpComponents中的URIUtils解决的

public static URI resolve(URI baseURI,
                          String reference)

对象的URI引用 基本URI。解决bug的方法 java.net.URI ()

递归产生一个较小的解决方案。如果结果不可能(例如不同的Windows磁盘)或不切实际(根目录只是普通目录)，则抛出异常。

/**
 * Computes the path for a file relative to a given base, or fails if the only shared 
 * directory is the root and the absolute form is better.
 * 
 * @param base File that is the base for the result
 * @param name File to be "relativized"
 * @return the relative name
 * @throws IOException if files have no common sub-directories, i.e. at best share the
 *                     root prefix "/" or "C:\"
 */

public static String getRelativePath(File base, File name) throws IOException  {
    File parent = base.getParentFile();

    if (parent == null) {
        throw new IOException("No common directory");
    }

    String bpath = base.getCanonicalPath();
    String fpath = name.getCanonicalPath();

    if (fpath.startsWith(bpath)) {
        return fpath.substring(bpath.length() + 1);
    } else {
        return (".." + File.separator + getRelativePath(parent, name));
    }
}

如果路径在JRE 1.5运行时或maven插件中不可用

package org.afc.util;

import java.io.File;
import java.util.LinkedList;
import java.util.List;

public class FileUtil {

    public static String getRelativePath(String basePath, String filePath)  {
        return getRelativePath(new File(basePath), new File(filePath));
    }

    public static String getRelativePath(File base, File file)  {

        List<String> bases = new LinkedList<String>();
        bases.add(0, base.getName());
        for (File parent = base.getParentFile(); parent != null; parent = parent.getParentFile()) {
            bases.add(0, parent.getName());
        }

        List<String> files = new LinkedList<String>();
        files.add(0, file.getName());
        for (File parent = file.getParentFile(); parent != null; parent = parent.getParentFile()) {
            files.add(0, parent.getName());
        }

        int overlapIndex = 0;
        while (overlapIndex < bases.size() && overlapIndex < files.size() && bases.get(overlapIndex).equals(files.get(overlapIndex))) {
            overlapIndex++;
        }

        StringBuilder relativePath = new StringBuilder();
        for (int i = overlapIndex; i < bases.size(); i++) {
            relativePath.append("..").append(File.separatorChar);
        }

        for (int i = overlapIndex; i < files.size(); i++) {
            relativePath.append(files.get(i)).append(File.separatorChar);
        }

        relativePath.deleteCharAt(relativePath.length() - 1);
        return relativePath.toString();
    }

}

如果你知道第二个字符串是第一个字符串的一部分:

String s1 = "/var/data/stuff/xyz.dat";
String s2 = "/var/data";
String s3 = s1.substring(s2.length());

或者，如果你真的想把句号放在开头，就像你的例子中那样:

String s3 = ".".concat(s1.substring(s2.length()));

Psuedo-code:

使用路径分隔符("/")拆分字符串 通过迭代拆分字符串的结果来找到最大的公共路径(因此在两个示例中，您最终会得到"/var/data"或"/a") 返回“。”+ whicheverPathIsLonger.substring (commonPath.length);