Calculates the relative path between a specified root directory and a target path. : Path « File Input Output « Java

Calculates the relative path between a specified root directory and a target path.

  Copyright 2009 Tomer Gabel <>

  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  See the License for the specific language governing permissions and
  limitations under the License.

  ant-intellij-tasks project (

  $Id: 106 2009-09-30 02:07:29Z tomergabel $

//package com.tomergabel.util;

import java.util.Deque;
import java.util.ArrayDeque;

public class PathUtils {
     * Calculates the relative path between a specified root directory and a target path.
     * @param root   The absolute path of the root directory.
     * @param target The path to the target file or directory.
     * @return The relative path between the specified root directory and the target path.
     * @throws IllegalArgumentException <ul><li>The root file cannot be null.</li><li>The target cannot be
     *                                  null.</li><li>The root file must be a directory.</li><li>The root file must be
     *                                  absolute.</li></ul>
    public static String relativize( final File root, final File target ) throws IllegalArgumentException {
        if ( root == null )
            throw new IllegalArgumentException( "The root file cannot be null." );
        if ( target == null )
            throw new IllegalArgumentException( "The target cannot be null." );
        if ( !root.isDirectory() )
            throw new IllegalArgumentException( "The root file must be a directory." );
        if ( !root.isAbsolute() )
            throw new IllegalArgumentException( "The root file must be absolute." );
        if ( !target.isAbsolute() )
            return target.toString();

        if ( root.equals( target ) )
                return ".";
        // Deconstruct hierarchies
        final Deque<File> rootHierarchy = new ArrayDeque<File>();
        for ( File f = root; f != null; f = f.getParentFile() )
            rootHierarchy.push( f );
        final Deque<File> targetHierarchy = new ArrayDeque<File>();
        for ( File f = target; f != null; f = f.getParentFile() )
            targetHierarchy.push( f );

        // Trace common root
        while ( rootHierarchy.size() > 0 && targetHierarchy.size() > 0 &&
                rootHierarchy.peek().equals( targetHierarchy.peek() ) ) {
        // Create relative path
        final StringBuilder sb = new StringBuilder( rootHierarchy.size() * 3 + targetHierarchy.size() * 32 );
        while ( rootHierarchy.size() > 0 ) {
            sb.append( ".." );
            if ( rootHierarchy.size() > 0 || targetHierarchy.size() > 0 )
                sb.append( File.separator );
        while ( targetHierarchy.size() > 0 ) {
            sb.append( targetHierarchy.pop().getName() );
            if ( targetHierarchy.size() > 0 )
                sb.append( File.separator );
        return sb.toString();


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