Write.out to data.dat file

Question

Am new to all this an pretty sure this stuff is basic but god its doing my head in....
Basically i`ve got to write a program in java that can write data to a data.dat file. Thats fine. I`ve done that.
Thing is when you enter the data it replaces the top line of the data.dat file with the new data rather than simply adding the new data to the file.
Please help guys

Answer 1

ok...the method for actually entering the new data (its for an address/contacts database) is here...
public static void insertRecord(String temp2, String temp1, String temp3, String temp4, int number_of_records)
     number_of_records = number_of_records + 1;
               // increment record count as we are adding another one      
               for (int i=0; i < number_of_records - 1; i++)
                    // here we see if the surname of the new record input is lower than the ones already in the arrays and also less than the next one after that
                    if(temp2.compareToIgnoreCase(lastName) > 0 && temp2.compareToIgnoreCase(lastName[i+1]) < 0)
                    int insertPoint = i;
               // we then assign the new record data into the arrays "in the correct place"
                    lastName[i] = temp2;
                    firstName[i] = temp1;
                    telNumber[i] = temp3;
                    emailAddress[i] = temp4;     
and the method to actually write it to the data.dat file (or in this case a new file of my choice so i can compare the original with the new one) is...
public static void write_out_file(int number_of_records, String filename)
//public static void write_out_file(int no_of_records, String filename)
          PrintStream write_out;
          int counter;
          try
               // Create new file
               write_out = new PrintStream(new File(filename));
               // Output all records
               for(counter = 0; counter < number_of_records; counter++)
                    // Output a record
                    write_out.print(lastName[counter]);
                    write_out.print(",,");
                    write_out.print(firstName[counter]);
                    write_out.print(",,");
                    write_out.print(telNumber[counter]);
                    write_out.print(",,");
                    write_out.println(emailAddress[counter]);
               // Close file
               write_out.close();
          catch (FileNotFoundException e)
               e.printStackTrace();