Java file open, read, and write utilities

Java file utilities FAQ: Do you have any Java file utilities you can share?

As I was working on another Java/Swing application this weekend, I ran across my "Java file utilities" class, and I thought I'd share that class here today. It's nothing too major, but it does include Java methods that let you open, read, write, and copy files using Java.

My Java file utilities class

Without any further introduction, here's the source code for my Java file utilities class:

package com.devdaily.utils;

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

/**
 * Copyright 2010 Alvin J. Alexander, devdaily.com
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * A copy of the GNU Lesser General Public License is available at this URL:
 * http://www.gnu.org/licenses/lgpl.txt
 *
 */
public class FileUtils
{

  /**
   * Opens and reads a file, and returns the contents as one String.
   */
  public static String readFileAsString(String filename) 
  throws IOException
  {
    BufferedReader reader = new BufferedReader(new FileReader(filename));
    String line;
    StringBuilder sb = new StringBuilder();
    while ((line = reader.readLine()) != null)
    {
      sb.append(line + "\n");
    }
    reader.close();
    return sb.toString();
  }
  
  /**
   * Open and read a file, and return the lines in the file as a list of
   * Strings.
   */
  public static List<String> readFileAsListOfStrings(String filename) throws Exception
  {
    List<String> records = new ArrayList<String>();
    BufferedReader reader = new BufferedReader(new FileReader(filename));
    String line;
    while ((line = reader.readLine()) != null)
    {
      records.add(line);
    }
    reader.close();
    return records;
  }

  /**
   * Reads a "properties" file, and returns it as a Map (a collection of key/value pairs).
   * 
   * @param filename
   * @param delimiter
   * @return
   * @throws Exception
   */
  public static Map<String, String> readPropertiesFileAsMap(String filename, String delimiter)
  throws Exception
  {
    Map<String, String> map = new HashMap();
    BufferedReader reader = new BufferedReader(new FileReader(filename));
    String line;
    while ((line = reader.readLine()) != null)
    {
      if (line.trim().length()==0) continue;
      if (line.charAt(0)=='#') continue;
      // assumption here is that proper lines are like "String : http://xxx.yyy.zzz/foo/bar",
      // and the ":" is the delimiter
      int delimPosition = line.indexOf(delimiter);
      String key = line.substring(0, delimPosition-1).trim();
      String value = line.substring(delimPosition+1).trim();
      map.put(key, value);
    }
    reader.close();
    return map;
  }

  /**
   * Read a Java properties file and return it as a Properties object.
   */
  public static Properties readPropertiesFile(String canonicalFilename)
  throws IOException
  {
    Properties properties = new Properties();
    properties.load(new FileInputStream(canonicalFilename));
    return properties;
  }

  /**
   * Save the given text to the given filename.
   * @param canonicalFilename Like /Users/al/foo/bar.txt
   * @param text All the text you want to save to the file as one String.
   * @throws IOException
   */
  public static void writeFile(String canonicalFilename, String text) 
  throws IOException
  {
    File file = new File (canonicalFilename);
    BufferedWriter out = new BufferedWriter(new FileWriter(file)); 
    out.write(text);
    out.close();
  }

  /**
   * Write an array of bytes to a file. Presumably this is binary data; for plain text
   * use the writeFile method.
   */
  public static void writeFileAsBytes(String fullPath, byte[] bytes) throws IOException
  {
    OutputStream bufferedOutputStream = new BufferedOutputStream(new FileOutputStream(fullPath));
    InputStream inputStream = new ByteArrayInputStream(bytes);
    int token = -1;

    while((token = inputStream.read()) != -1)
    {
      bufferedOutputStream.write(token);
    }
    bufferedOutputStream.flush();
    bufferedOutputStream.close();
    inputStream.close();
  }

  public static void copyFile(File source, File destination) throws IOException
  {
    //if (!source.isFile() || !dest.isFile()) return false;

    byte[] buffer = new byte[100000];

    BufferedInputStream bufferedInputStream = null;
    BufferedOutputStream bufferedOutputStream = null;
    try
    {
      bufferedInputStream = new BufferedInputStream(new FileInputStream(source));
      bufferedOutputStream = new BufferedOutputStream(new FileOutputStream(destination));
      int size;
      while ((size = bufferedInputStream.read(buffer)) > -1)
      {
        bufferedOutputStream.write(buffer, 0, size);
      }
    }
    catch (IOException e)
    {
      // TODO may want to do something more here
      throw e;
    }
    finally
    {
      try
      {
        if (bufferedInputStream != null)
        {
          bufferedInputStream.close();
        }
        if (bufferedOutputStream != null)
        {
          bufferedOutputStream.flush();
          bufferedOutputStream.close();
        }
      }
      catch (IOException ioe)
      {
        // TODO may want to do something more here
        throw ioe;
      }
    }
  }
}

Java file utilities discussion

As you can see from that code, this class provides the following Java methods:

  • A Java file open and read method
  • A Java file write method to write text to a file
  • A Java file write method to write bytes to a file
  • A Java properties file reading method
  • A Java properties Map method
  • A Java file copy method

I hope this Java file utilities class helps in your own Java programming efforts, and it is shared freely here under the terms of the GNU LGPL.

Apache Commons IO library

A few people wrote to suggest that I mention the Apache Commons IO library, and that's a great suggestion. I didn't mean to slight them in any way, I was just trying to share some Java code here that demonstrated these file-related techniques. I'm a big fan of the Commons' libraries, and have used them in many projects.

I generally use my own library in Java/Swing/GUI projects where I'm giving users a collection of jar files, and I like to keep my filesize footprint down, but these days even adding the 1.2MB Commons IO library isn't a big deal.

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