How can i Update a File in Java, which class Should i use?

Question

my program has the following code,

BufferedWriter bufWriter = new BufferedWriter(new FileWriter(new File("xyz.dat"),true));
                        bufWriter.write(nameField+"/"+ageField+"/"+genderField+"\n");
                        bufWriter.flush();

Which creates a file.. Sample Format of data stored in file..

Name1/Age1/Gender1                            // for user1
Name2/Age2/Gender2                            // for user2
.
.
.
NameN/AgeN/GenderN                            //for userN

Suppose I need to change the age of the user5 then how can i do it? I can navigate to the 5th user and I can get the data through the split("/",3);method but how to make changes for that particular user ? I am really very confused here.

Answer 1

Youhave to make the serilizable object of each record and then acess it and update it as below,

import java.io.*;

public class phoneBook {
private ObjectOutputStream output;
private ObjectInputStream input;
File fileName = new File("d:\\java\\data\\phone.dat");

public static void main(String[] args) {
   phoneBook pb = new phoneBook();
   pb.writeFile(); // open, write and close the file
   pb.readFile(); // open, read and close the file
}

public void writeFile() {

   // I could have put this into an array which would have told me how many
   // records i have, it could then have be used in the readFile method below
   // but lets keep things very simple
   Record r1 = new Record("Paul Valle", "0207-568-789");
   Record r2 = new Record("Lorraine Valle", "0207-345-356");
   Record r3 = new Record("Dominic Valle", "0207-765-693");
   Record r4 = new Record("Jessica Valle", "0207-789-876");

   try {
      // Open a file handle for writing
      output = new ObjectOutputStream( new FileOutputStream( fileName));

      // Write some data to the file it could throw
      // InvalidClassException or NotSerializableException exceptions
      output.writeObject( r1 );
      output.writeObject( r2 );
      output.writeObject( r3 );
      output.writeObject( r4 );

      // Flush the ObjectOutputStream. This will write any buffered
      // output bytes and flush through to the FileOutputStream
      output.flush();

      // Close the file
      output.close();
   } catch (InvalidClassException icex) {
      System.out.println("Invalid Class");
   } catch (NotSerializableException nsex) {
      System.out.println("Object is not serializable");
   } catch (IOException e) {
      System.out.println("Problems either flushing or closing file");
   }
}

public void readFile() {
   Record r; // this object will hold the records when retrieved from the file

   try {
      // Open the file handle for reading
      input = new ObjectInputStream( new FileInputStream(fileName));

      // I know i have 4 records so lets read them, this is where i could have used the array
      // by using the length of the array i would have know how many records i have.
      for (int i = 0; i < 4; i++ ) {
         // Here we implicity cast the retrieved Object
         r = ( Record ) input.readObject();
         if (r.getName() == 'YOURMATCHINGNAME')
          {
               r.setName("NEWNAME");
               r.setPhone("NEWPHONENUMBER");  
               try {
                      // Open a file handle for writing
                      output = new ObjectOutputStream( new FileOutputStream( fileName));

                     // Write same data to the file it could throw
                     // InvalidClassException or NotSerializableException exceptions
                     output.writeObject( r );

                     // Flush the ObjectOutputStream. This will write any buffered
                    // output bytes and flush through to the FileOutputStream
                    output.flush();

                    // Close the file
                   output.close();
                   } catch (InvalidClassException icex) {
                       System.out.println("Invalid Class");
                   } catch (NotSerializableException nsex) {
                      System.out.println("Object is not serializable");
                   } catch (IOException e) {
                      System.out.println("Problems either flushing or closing file");
                  }  
                 finally{
                    break;
                  }  
          }  

      }

      // Close the file
      input.close();

   } catch (EOFException eofex) {
      System.out.println("No more records to read");
   } catch (ClassNotFoundException cnfex) {
      System.out.println("Unable to create object - class not found");
   } catch (IOException e ) {
      System.out.println("Unable to close file");
   }
 }
}

 // Serialization involves saving the current state of an object to a stream,
 // and restoring an equivalent object from that stream.
class Record implements Serializable {

private String name;
private String phone;

// Constructor
public Record() { this ("", ""); }

// Overloaded Constructor
public Record(String n, String p) {
   name = n;
   phone = p;
}

// The get and set methods
 public void setName(String n) { name = n; }

 public void setPhone(String p) { phone = p; }

 public String getName() { return name; }

 public String getPhone() { return phone; }
}

This will do.Let me know if any problem.

Answer 2

1) Solution for a short file (which fits in memory) with Java 7

List<String> lines = Files.readAllLines(path, StandardCharsets.UTF_8);
String line5 = replaceAge(lines.get(4), newAge);
lines.set(4, line5);
Path tmp = Files.createTempFile(prefix, suffix, attrs);
Files.write(path, lines)
Files.move(tmp, path, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);

2) For a big file

    Path tmp = Files.createTempFile(prefix, suffix, attrs);
    try (BufferedWriter bw = Files.newBufferedWriter(path, StandardCharsets.UTF_8);
            BufferedReader br = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
        String line;
        for (int i = 1; (line = br.readLine()) != null; i++) {
            if (i == 5) {
                line = replaceAge(line, newAge);
            }
            bw.write(line);
            bw.newLine();
        }
    }
    Files.move(tmp, path, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);