IO / Serialization

[wuan luen hong]

I attempted to insert a data into an ArrayList and write it into a file and then later on pull it out then read and print it out. However, each time I run the program, when I insert the data using scanner class, instead of adding it into the list, it rewrites the entire file instead, displaying only the inserted input. I don't understand what went wrong, as it is supposed to update phoneList and then called it out as another list called newPhoneList and print, but it now only prints whatever is provided in the input. The constructor is as below:

import java.io.*;

class phone implements Serializable
{
	private String name,phoneNum;
	
	public phone(String na, String num)
      {
		name=na;
		phoneNum=num;
	}
	public String getName()
      {
		return name;
	}
	public String getphoneNum()
      {
		return phoneNum;
	}	
		
	   public String toString()
   {
   	   String out = "NAME: " + name + "\t PHONE: " +  phoneNum ;
   	   return out;
	} 

}

And this part is the main method:

import java.io.*;

import java.util.*;

class phoneMain 
{
   public static void main (String[] args) throws IOException, ClassNotFoundException
  {
  	  Scanner scanner = new Scanner(System.in);
      List<phone> phoneList = new ArrayList<phone>();
	
	  System.out.print("Please enter name: ");
	  String name = scanner.nextLine();
	  System.out.print("Please enter phone number: ");
	  String phoneNum = scanner.nextLine();
      phoneList.add(new phone(name,phoneNum));
      
      for (int i = 0; i<phoneList.size(); i++)
 			 System.out.println(phoneList.get(i).toString() );
      System.out.println("phoneList set up");

        ObjectOutputStream out = null;
        try 
        {
            out = new ObjectOutputStream(new
                    BufferedOutputStream(new FileOutputStream("phoneFile")));
            out.writeObject(phoneList);
        } 
        finally 
        {
            out.close();
        }
            
        ObjectInputStream in = null;
        try 
        {
            in = new ObjectInputStream(new
                    BufferedInputStream(new FileInputStream("phoneFile")));
             List<phone> newPhoneList = (List<phone>) in.readObject();
             
             for (int i = 0; i<newPhoneList.size(); i++)
 			    System.out.println(newPhoneList.get(i).toString() );
        } 
        finally
        { 
            in.close();
            out.close();
        }
   }//end main
}//end class CarMain
 

[Rob Spoor]

wuan luen hong wrote:

      List<phone> phoneList = new ArrayList<phone>(); 

There's your List - a newly created one. You then add one single element to it, and write it to the file. Of course this file will have a List with only have one element.
What you need to do is read the List from the file instead of creating it each time again. Only if the file does not exist should you create a new List:

List<phone> phoneList;
File file = new File("phoneFile");
if (file.exists())
{
    // read from the file
}
else
{
    phoneList = new ArrayList<phone>();
}

[Peter Taucher]

That's what you're doing in your main method:
1) Let the user enter a 'phone' and print it out
2) Serialize a list of 'phone' objects that always contains only one element
3) Recreate a list of 'phone' objects that always contains only one element from serialized file and print it out

That's what you probably want to do:
1) Read previously serialized contents
2) Let user enter new 'phone' object
3) Serialize the list

Don't work with 'phoneList' and 'newPhoneList' ... but always with the same list.

[Shanky Sohar]

It is because everytime you are creating a new file..not checking for existing file and data in it..

[Shanky Sohar]

beaten 3 minutes by Peter Taucher & Rob Prime

[wuan luen hong]

Hi Rob, I can see now what the logical error is. Instead of only creating a new list for the first time, I've repeatedly create a list, replacing the old one each time I run the program. That would explain why it is only displaying one input and not adding on to the list.

Hi,Peter, what you're implying is that, when I insert data into a list, I should also read from the same list? Doesn't that mean that the 2nd part of the code where the newPhoneList takes place is redundant? I've always had the impression that when I perform Serialization, after I insert the data into a prior list, i have to create another new list to read from the first.

[wuan luen hong]

Hi again Rob,
Regarding your reply earlier, if the file should exist, the logic would be to immediately grab the list in the existing file instead of creating a new list. I don't however know how to read it as a list again. Is it done using the readObject() method?

[Rob Spoor]

You're already doing it, with newPhoneList.

[wuan luen hong]

I tinkered with the code a little and came out with this:

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

public class testing{
	public static void main(String args[]) throws IOException, ClassNotFoundException{
		Scanner input = new Scanner(System.in);
		//List<phone> phoneList;  
		File file = new File("phoneFile.txt");  
			if (!file.exists()){  
      			List<phone> phoneList = new ArrayList<phone>();
				System.out.println("Please enter name :");
   				String name = input.nextLine();
   				System.out.println("Please enter number :");
   				String number = input.nextLine();
   				phoneList.add(new phone(name,number));
   				System.out.println(phoneList.toString());
   				
   				ObjectOutputStream out = null;  
        		try   
        		{  
            		out = new ObjectOutputStream(new  
                    BufferedOutputStream(new FileOutputStream("phoneFile.txt")));  
            		out.writeObject(phoneList);  
        		}   
        		finally   
        		{  
            		out.close();  
        		}  

        	} 
        		
			else  
			{  
		   		ObjectInputStream in = null;
				ObjectInputStream out = null;
        			try 
        			{
            			in = new ObjectInputStream(new
                    	BufferedInputStream(new FileInputStream("phoneFile.txt")));
             			List<phone> newPhoneList = (List<phone>) in.readObject();
						
						boolean check = true;
						while(check){
					
						System.out.println("Please enter name :");
   						String name = input.nextLine();
   						System.out.println("Please enter number :");
   						String number = input.nextLine();

							if(!name.equals("END") || !number.equals("END")){
								check = true;
								newPhoneList.add(new phone(name,number)); 
							}
							else{
								check = false;
							}
						}
             			for (int i = 0; i<newPhoneList.size(); i++)
 			    		System.out.println(newPhoneList.get(i).toString() );
                        //This part here is not working how I thought it should be
            		/*out = new ObjectOutputStream(new  
                    	BufferedOutputStream(new FileOutputStream("phoneFile.txt")));   
                    	out.writeObject(newPhoneList);*/
       			
        			} 
        			finally
        			{ 
            			in.close();

        			}  
			}  
	}
}
   

Although now I can input and keep the first information there, I still can't append the next few information into that text file (phoneFile.txt) that I'll be entering in the next few attempts. It gives me an error when i use the method writeObject() (in the commented part towards the end of the code) to update the text file, saying incompatible type or something. Any idea?

[Rob Spoor]

Not without seeing the error.

But you are now duplicating much code. Think the following through: what is the only difference between having an existing file and having to create a new List? Is the entire reading of user input part of it? Is the writing of the List at the end part of it?

You should separate your code into three parts:
- the creation / reading of the list
- handling user input
- writing the results back to the file