RandomAccessFile: How do I Clear the txt file and write multiple lines of..

Question

Hello all,
I am a 6th grade teacher and am taking a not so "Advanced Java Programming" class. Could someone please help me with the following problem.
I am having trouble with RandomAccessFile.
What I want to do is:
1. Write multiple lines of text to a file
2. Be able to delete previous entries in the file
3. It would also be nice to be able to go to a certian line of text but not manditory.
import java.io.*;
public class Logger
RandomAccessFile raf;
public Logger()
     try
          raf=new RandomAccessFile("default.txt","rw");
          raf.seek(0);
          raf.writeBytes("");
     catch(Exception e)
          e.printStackTrace();
public Logger(String fileName)
     try
          raf=new RandomAccessFile(fileName,"rw");
          raf.seek(0);
          raf.writeBytes("");
     catch(Exception e)
          e.printStackTrace();
public void writeLine(String line)
     try
          long index=0;
          raf.seek(raf.length());          
          raf.writeBytes(index+" "+line);
     catch(Exception e)
          e.printStackTrace();
public void closeFile()
     try
          raf.close();
     catch(Exception e)
          e.printStackTrace();
     }

Answer 1

Enjoy! The length of the code is highly attributable to the test harness/shell thingy at the end. But anyway seems to work nicely.
import java.io.*;
/** File structure is as follows. 1st four bytes (int) with number of live records. Followed by records.
<p>Records are structured as follows<ul>
<li>Alive or dead - int
<li>Length of data - int
<li>Data
</ul>*/
public class SequentialAccessStringFile{
  private static int ALIVE = 1;
  private static int DEAD = 0;
  private int numRecords, currentRecord;
  private RandomAccessFile raf;
  /** Creates a SequentialAccessStringFile from a previously created file. */
  public SequentialAccessStringFile(String filename)throws IOException{
    this(filename,false);
  /** Creates a SequentialAccessStringFile. If createnew is true then a new file is created or if it
      already exists the old one is blown away. You must call this constructor with true if you do
      not have an existing file. */
  public SequentialAccessStringFile(String filename, boolean createnew)throws IOException{
    this.raf = new RandomAccessFile(filename,"rw");
    if(createnew){
      truncate();
    this.currentRecord = 0;
    this.raf.seek(0);
    this.numRecords = raf.readInt();
  /** Truncates the file deleting all existing records. */
  public void truncate()throws IOException{
    this.numRecords = 0;
    this.currentRecord = 0;
    this.raf.setLength(0);
    this.raf.writeInt(this.numRecords);
  /** Adds the given String to the end of this file.*/
  public void addRecord(String toAdd)throws IOException{
    this.raf.seek(this.raf.length());//jump to end of file
    byte[] buff = toAdd.getBytes();// uses default encoding you may want to change this
    this.raf.writeInt(ALIVE);
    this.raf.writeInt(buff.length);
    this.raf.write(buff);
    numRecords++;
    this.raf.seek(0);
    this.raf.writeInt(this.numRecords);
    this.currentRecord = 0;   
  /** Returns the record at given index. Indexing starts at zero. */
  public String getRecord(int index)throws IOException{
    seekToRecord(index);
    int buffLength = this.raf.readInt();
    byte[] buff = new byte[buffLength];
    this.raf.readFully(buff);
    this.currentRecord++;
    return new String(buff); // again with the default charset
  /** Returns the number of records in this file. */
  public int recordCount(){
    return this.numRecords;
  /** Deletes the record at given index. This does not physically delete the file but simply marks the record as "dead" */
  public void deleteRecord(int index)throws IOException{
    seekToRecord(index);
    this.raf.seek(this.raf.getFilePointer()-4);
    this.raf.writeInt(DEAD);
    this.numRecords--;
    this.raf.seek(0);
    this.raf.writeInt(this.numRecords);
    this.currentRecord = 0;
  /** Removes dead space from file.*/
  public void optimizeFile()throws IOException{
    // excercise left for reader
  public void close()throws IOException{
    this.raf.close();
  /** Positions the file pointer just before the size attribute for the record we want to read*/
  private void seekToRecord(int index)throws IOException{
    if(index>=this.numRecords){
      throw new IOException("Record "+index+" out of range.");           
    if(index<this.currentRecord){
      this.raf.seek(4);
      currentRecord = 0;     
    int isAlive, toSkip;
    while(this.currentRecord<index){
      //skip a record
      isAlive = this.raf.readInt();
      toSkip = this.raf.readInt();
      this.raf.skipBytes(toSkip);
      if(isAlive==ALIVE){
           this.currentRecord++;
    // the next live record is the record we want
    isAlive = this.raf.readInt();
    while(isAlive==DEAD){
      toSkip = this.raf.readInt();
      this.raf.skipBytes(toSkip);
      isAlive = this.raf.readInt();     
  public static void main(String args[])throws Exception{
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    System.out.println("Create a new file? y/n");
    System.out.println("(No assumes file exists)");
    System.out.print("> ");
    String command = br.readLine();
    SequentialAccessStringFile test = null;
    if(command.equalsIgnoreCase("y")){
      System.out.println("Name of file");
      System.out.print("> ");
      command = br.readLine();
      test = new SequentialAccessStringFile(command,true);     
    }else{
      System.out.println("Name of file");
      System.out.print("> ");
      command = br.readLine();
      test = new SequentialAccessStringFile(command);     
    System.out.println("File loaded. Type ? for help");
    boolean alive = true;
    while(alive){
      System.out.print("> ");
      command = br.readLine();
      boolean understood = false;
      String[] commandArgs = command.split("\\s");
      if(commandArgs.length<1){
           continue;
      if(commandArgs[0].equalsIgnoreCase("quit")){
           test.close();           
           alive = false;
           understood = true;           
      if(commandArgs[0].equalsIgnoreCase("list")){
           System.out.println("#\tValue");
           for(int i=0;i<test.recordCount();i++){
             System.out.println(i+"\t"+test.getRecord(i));
           understood = true;
      if(commandArgs[0].equalsIgnoreCase("truncate")){
           test.truncate();
           understood = true;
           System.out.println("File truncated");
      if(commandArgs[0].equalsIgnoreCase("add")){
            test.addRecord(commandArgs[1]);
            understood = true;
            System.out.println("Record added");
      if(commandArgs[0].equalsIgnoreCase("delete")){
            int toDelete = Integer.parseInt(commandArgs[1]);
            if((toDelete<0)||(toDelete>=test.recordCount())){
              System.out.println("Record "+toDelete+" does not exist");
            }else{
              test.deleteRecord(toDelete);
              System.out.println("Record deleted");
            understood = true;
      if(commandArgs[0].equals("?")){
           understood = true;
      if(!understood){
           System.out.println("'"+command+"' unrecognized");
           commandArgs[0] = "?";
      if(commandArgs[0].equals("?")){
           System.out.println("list - prints current file contents");
           System.out.println("add [data] - adds data to file");
           System.out.println("delete [record index] - deletes record from file");
           System.out.println("truncate - truncates file (deletes all record)");
           System.out.println("quit - quit this program");
           System.out.println("? - displays this help");
    System.out.println("Bye!");
}Sample output with test program
C:\>java SequentialAccessStringFile
Create a new file? y/n
(No assumes file exists)
yName of file
mystringsFile loaded. Type ? for help
add appleRecord added
add orangeRecord added
add cherryRecord added
add pineappleRecord added
list#       Value
0       apple
1       orange
2       cherry
3       pineapple
delete 5Record 5 does not exist
delete 1Record deleted
list#       Value
0       apple
1       cherry
2       pineapple
add kiwiRecord added
list#       Value
0       apple
1       cherry
2       pineapple
3       kiwi
quitBye