How can i write a string into a specified pos of a file?

Question

How can i write a string into a specified pos of a file without read all file into ram and write the whole file again?
for example:
the content of file is:
name=123
state=456
i want to modify the value of name with 789
(write to file without read all file into ram)
How can i do it? thank you

Answer 1

take this as an idea. it actually does what i decribed above. you sure need to make some modifications so it works for your special need. If you use it and add any valuable code to it or find any bugs, please let me know.
import java.io.*;
import java.util.*;
* Copyright (c) 2002 Frank Fischer <[email protected]>
* All rights reserved. See the LICENSE for usage conditions
* ObjectProperties.java
* version 1.0, 2002-09-12
* author Frank Fischer <[email protected]>
public class ObjectProperties
     // the seperator between the param-name and the value in the prooperties file
     private static final String separator = "=";
     // the vector where we put the arrays in
     private Vector PropertiesSet;
     // the array where we put the param/value pairs in
     private String propvaluepair[][];
     // the name of the object the properties file is for
     public String ObjectPropertiesFileName;
     // the path to the object'a properties file
     public String ObjectPropertiesDir;
     // reference to the properties file
     public File PropertiesFile;
     // sign for linebreak - depends on platforms
     public static final String newline = System.getProperty("line.separator");
     public ObjectProperties(String ObjectPropertiesFileName, String ObjectPropertiesDir, ObjectPropertiesManager ObjectPropertiesManager)
     //     System.out.println("Properties Objekt wird erzeugt: "+ObjectPropertiesFileName);
          this.ObjectPropertiesFileName = ObjectPropertiesFileName;
          this.ObjectPropertiesDir = ObjectPropertiesDir;
          // reference to the properties file
          PropertiesFile = new File(ObjectPropertiesDir+ObjectPropertiesFileName);
          // vector to put the param/value pair-array in
          PropertiesSet = new Vector();
     //     System.out.println("Properties File Backup wird erzeugt: "+name);
          backup();
     //     System.out.println("Properties File wird eingelesen: "+PropertiesFile);
          try
               //opening stream to file for read operations
               FileInputStream FileInput = new FileInputStream(PropertiesFile);
               DataInputStream DataInput = new DataInputStream(FileInput);
               String line = "";
               //reading line after line of the properties file
               while ((line = DataInput.readLine()) != null)
                    //just making sure there are no whitespaces at the beginng or end of the line
                    line = cutSpaces(line);
                    if (line.length() > 0)
                         //$ indicates a param-name
                         if (line.startsWith("$"))
                              // array to store a param/value pair in
                              propvaluepair = new String[1][2];
                              //get the param-name
                              String parameter = line.substring(1, line.indexOf(separator)-1);
                              //just making sure there are no whitespaces at the beginng or end of the variable
                              parameter = cutSpaces(parameter);
                              //get the value
                              String value = line.substring(line.indexOf(separator)+1, line.length());
                              //just making sure there are no whitespaces at the beginng or end of the variable
                              value = cutSpaces(value);
                              //put the param-name and the value into an array
                              propvaluepair[0][0] = parameter;
                              propvaluepair[0][1] = value;
                         //     System.out.println("["+ObjectPropertiesFileName+"] key/value gefunden:"+parameter+";"+value);
                              //and finaly put the array into the vector
                              PropertiesSet.addElement(propvaluepair);
          // error handlig
          catch (IOException e)
               System.out.println("ERROR occured while reading property file for: "+ObjectPropertiesFileName);
               System.out.println("ERROR CODE: "+e);
               // System.out.println("in ObjectProperties");
     // function to be called to get the value of a specific paramater 'param'
     // if the specific paramater is not found '-1' is returned to indicate that case
     public String getParam(String param)
          // the return value indicating that the param we are searching for is not found
          String v = "-1";
          // looking up the whole Vector
          for (int i=0; i<PropertiesSet.size(); i++)
               //the String i want to read the values in again
               String s[][] = new String[1][2];
               // trying to get out the array from the vector again
               s = (String[][]) PropertiesSet.elementAt(i);
               // comparing the param-name we're looking for with the param-name in the array we took out the vector at position i
               if (s[0][0].equals(param) == true)
                    //if the param-names are the same, we look up the value and write it in the return variable
                    v = s[0][1];
                    // making sure the for loop ends
                    i = PropertiesSet.size();
          // giving the value back to the calling procedure
          return v;
     // function to be called to set the value of a specific paramater 'param'
     public void setParam(String param, String value)
          // looking up the whole Vector for the specific param if existing or not
          for (int i=0; i<PropertiesSet.size(); i++)
               //the String i want to read the values in again
               String s[][] = (String[][]) PropertiesSet.elementAt(i);
               // comparing the param-name we're looking for with the param-name in the array we took out the vector at position i
               if (s[0][0].equals(param) == true)
                    //if the param-names are the same, we remove the param/value pair so we can add the new pair later in
                    PropertiesSet.removeElementAt(i);
                    // making sure the for loop ends
                    i = PropertiesSet.size();
          // if we land here, there is no such param in the Vector, either there was none form the beginng
          // or there was one but we took it out.
          // create a string array to place the param/value pair in
          String n[][] = new String[1][2];
          // add the param/value par
          n[0][0] = param;
          n[0][1] = value;
          // add the string array to the vector
          PropertiesSet.addElement(n);
     // function to save all data in the Vector to the properties file
     // must be done because properties might be changing while runtime
     // and changes are just hold in memory while runntime
     public void store()
          backup();
          String outtofile = "# file created/modified on "+createDate("-")+" "+createTime("-")+newline+newline;
          try
               //opening stream to file for write operations
               FileOutputStream PropertiesFileOuput = new FileOutputStream(PropertiesFile);
               DataOutputStream PropertiesDataOutput = new DataOutputStream(PropertiesFileOuput);
               // looping over all param/value pairs in the vector
               for (int i=0; i<PropertiesSet.size(); i++)
                    //the String i want to read the values in
                    String s[][] = new String[1][2];
                    // trying to get out the array from the vector again
                    s = (String[][]) PropertiesSet.elementAt(i);
                    String param = "$"+s[0][0];
                    String value = s[0][1];
                    outtofile += param+" = "+value+newline;
               outtofile += newline+"#end of file"+newline;
               try
                    PropertiesDataOutput.writeBytes(outtofile);
               catch (IOException e)
                    System.out.println("ERROR while writing to Properties File: "+e);
          catch (IOException e)
               System.out.println("ERROR occured while writing to the property file for: "+ObjectPropertiesFileName);
               System.out.println("ERROR CODE: "+e);
     // sometimes before overwritting old value it's a good idea to backup old values
     public void backup()
          try
               // reference to the original properties file
               File OriginalFile = new File(ObjectPropertiesDir+ObjectPropertiesFileName);
               File BackupFile = new File(ObjectPropertiesDir+"/backup/"+ObjectPropertiesFileName+".backup");
               //opening stream to original file for read operations
               FileInputStream OriginalFileInput = new FileInputStream(OriginalFile);
               DataInputStream OriginalFileDataInput = new DataInputStream(OriginalFileInput);
               //opening stream to backup file for write operations
               FileOutputStream BackupFileOutput = new FileOutputStream(BackupFile);
               DataOutputStream BackupFileDataOutput = new DataOutputStream(BackupFileOutput);
          //     String content = "";
               String line = "";
               // do till end of file
               while ((line = OriginalFileDataInput.readLine()) != null)
                    BackupFileDataOutput.writeBytes(line+newline);
          // error handlig
          catch (IOException e)
               System.out.println("ERROR occured while back up for property file: "+ObjectPropertiesFileName);
               System.out.println("ERROR CODE: "+e);
               System.out.println("this is a serious error - the server must be stopped");
     private String cutSpaces(String s)
          while (s.startsWith(" "))
               s = s.substring(1, s.length());
          while (s.endsWith(" "))
               s = s.substring(0, s.length()-1);
          return s;
     public String createDate(String seperator)
          Date datum = new Date();
          String currentdatum = new String();
          int year, month, date;
          year = datum.getYear()+1900;
          month = datum.getMonth()+1;
          date = datum.getDate();
          currentdatum = ""+year+seperator;
          if (month < 10)
               currentdatum = currentdatum+"0"+month+seperator;
          else
               currentdatum = currentdatum+month+seperator;
          if (date < 10)
               currentdatum = currentdatum+"0"+date;
          else
               currentdatum = currentdatum+date;
          return currentdatum;
     public String createTime(String seperator)
          Date time = new Date();
          String currenttime = new String();
          int hours, minutes, seconds;
          hours = time.getHours();
          minutes = time.getMinutes();
          seconds = time.getSeconds();
          if (hours < 10)
               currenttime = currenttime+"0"+hours+seperator;
          else
               currenttime = currenttime+hours+seperator;
          if (minutes < 10)
               currenttime = currenttime+"0"+minutes+seperator;
          else
               currenttime = currenttime+minutes+seperator;
          if (seconds < 10)
               currenttime = currenttime+"0"+seconds;
          else
               currenttime = currenttime+seconds;
          return currenttime;