Try to Converting TCP to UDP

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sorry i'm nob in java programming,
i'm high schooler who want learn java, but i have no teacher, please guide me , , ,

Here is the Client from book, then this code called using another script,,, connect -> openFile -> copyFile -> closeFile;

Code :

import java.io.*;
import java.net.*;
 
import com.deitel.ch17.AccountRecordSerializable;
 
public class ClientRemCopySeqFile{
	private ObjectInputStream input;
	private ObjectOutputStream socketOut;
	private Socket client;
	String host = "localhost";
 
	// enable user to select file to open
	public void openFile()
	{
		try // open file
		{
			input = new ObjectInputStream(
			new FileInputStream( "clients.ser" ) );
		} // end try
		catch ( IOException ioException )
		{
			System.err.println( "Error opening file." );
		} // end catch
	} // end method openFile
 
	public void connect() throws Exception
	{
		BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
		DatagramSocket clientSocket = new DatagramSocket();
		InetAddress IPAddress = InetAddress.getByName("localhost");
			byte[] sendData = new byte[1024];
			byte[] receiveData = new byte[1024];
		DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
		clientSocket.send(sendPacket);
	}
 
	public void copyFile()
	{
		AccountRecordSerializable record;
		try // input the values from the file
		{ 
			while ( true )
			{
			record = ( AccountRecordSerializable ) input.readObject();
 
			// display record contents
			System.out.printf( "%-10d%-12s%-12s%10.2f\n",
			record.getAccount(), record.getFirstName(),
			record.getLastName(), record.getBalance() );
			socketOut.writeObject( record ); // output record
			} // end while
		} // end try
		catch ( EOFException endOfFileException )
		{
			System.out.println("Records has been sent...");
			return; // end of file was reached
		} // end catch
		catch ( ClassNotFoundException cnf )
		{
			System.err.println( "Unable to create object."+cnf.toString() );
		} // end catch
		catch ( IOException ioException )
		{
			System.err.println( "Error during reading from file." );
		}  // end catch// end catch
 
	}
 
	// close file and terminate application
	public void closeFile()
	 {
		try // close file and exit
		{
		  if ( input != null )
			input.close();
		  if ( client != null )
			client.close();
		  if ( socketOut != null )
			socketOut.close();
		System.out.println("connection for "+host+" has been terminated ..."); 
		  System.exit( 0 );
		} // end try
		catch ( IOException ioException )
		{
		  System.err.println( "Error closing file." );
		  System.exit( 1 );
		} // end catch
	 } // end method closeFile
}
 
 
Here the Server from the book too, called using another program, connect -> openFile -> listen -> copyFile -> closeFile 
import java.io.*;
import java.net.*;
 
import com.deitel.ch17.AccountRecordSerializable;
 
public class ServerRemCopySeqFile{
	private ServerSocket server = null;
	private Socket client=null;
	private ObjectInputStream streamIn;
	private ObjectOutputStream output;
 
	public void connect() throws Exception
	{
		DatagramSocket serverSocket = new DatagramSocket(9876);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            while(true)
               {
                  DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                  serverSocket.receive(receivePacket);
                  String sentence = new String( receivePacket.getData());
                  InetAddress IPAddress = receivePacket.getAddress();
                  int port = receivePacket.getPort();
               }
 
	 }
 
	 public void listen(){
		try {
         client = server.accept();
		} catch (IOException e) {
         System.out.println("Did not accept connection: " + e);
         System.exit(1);
		}
 
		System.out.println("Client connection accepted. Moving to local port ...");
	 }
 
	// enable user to select file to open
	public void openFile(){
		try // open file
		{
		  output = new ObjectOutputStream(
		  new FileOutputStream( "remclients.ser" ) );
		} // end try
		catch ( IOException ioException )
		{
		  System.err.println( "Error opening file." );
		} // end catch
	} // end method openFile
 
	public void copyFile(){
		AccountRecordSerializable record;
		try // input the values from the file
		{ 
			streamIn = new ObjectInputStream(new BufferedInputStream(client.getInputStream()));
		  while ( true )
		  {
			record = ( AccountRecordSerializable ) streamIn.readObject();
 
			// display record contents
			System.out.printf( "%-10d%-12s%-12s%10.2f\n",
			record.getAccount(), record.getFirstName(),
			record.getLastName(), record.getBalance() );
			output.writeObject( record ); // output record
 
		  } // end while
		} // end try
		catch ( EOFException endOfFileException )
		{
			System.out.println("Records has been save...");
		  return; // end of file was reached
		} // end catch
		catch ( ClassNotFoundException classNotFoundException )
		{
		  System.err.println( "Unable to create object." );
		} // end catch
		catch ( IOException ioException )
		{
		  System.err.println( "Error during reading from file." );
		}  // end catch// end catch
	}
 
	// close file and terminate application
	public void closeFile()
	{
		try // close file and exit
		{
			if ( server != null )
				server.close();
			if ( client != null )
				client.close();
			if ( streamIn != null )
				streamIn.close();     
			System.out.println("All connections and file has been closed ..."); 
			System.exit( 0 );
		} // end try
		catch ( IOException ioException )
		{
			System.err.println( "Error closing file." );
			System.exit( 1 );
		} // end catch
 
	} // end method closeFile
}

Please help me, after run I got error message
Exception in thread "main" java.lang.NullPointerException
at ClientRemCopySeqFile.copyFile(ClientRemCopySeqFile .java:51)
at ClientRemCopySeqFileTest.main(ClientRemCopySeqFile Test.java:13)