import java.util.ArrayList;
import javax.swing.JOptionPane;


public class ClientList
{
    private ArrayList<Client> clientData;
    private final String    NEW_LINE = "\n";

    private int cursor;

    //manager mode
    public ClientList ( )
    {
        clientData = new ArrayList<Client> ( );
        buildClientList ( );

        if (clientData.size ( )  != 0)
            cursor = 0; //not empty, place cursor at the start of the vector
        else
            cursor = -1;    //empty

    }

    //manager mode
    public void buildClientList ( )
    {

        String textStream = new FileOperations ( ).readFile ( );
        JOptionPane.showMessageDialog (null, "client file loaded");

        //build the client list
        parseTextStream (textStream);
    }

    //build a vector from data read from a xml text file
    public void parseTextStream (String textStream)
    {
        int len = textStream.length ( );
        Client newClient = null;

        while (len != 0)
        {
            //get a line that ends with "\n"

            int pos = textStream.indexOf (NEW_LINE);
            if (pos == -1)
                break;

            String aLine = textStream.substring (0,pos);

            //the rest of the text stream
            textStream = textStream.substring (pos+1);
            len = textStream.length ( );

            aLine = aLine.trim ( );

            pos = aLine.indexOf ("<client>");
            if (pos != -1)
            {
                newClient = new Client ( ); //start of a client
                continue;   //skip the rest of the loop
            }

            pos = aLine.indexOf ("</client>");
            if (pos != -1)      //end of a client
            {
                clientData.add(newClient);  //append to the vector
                continue;
            }

            pos = aLine.indexOf ("<name>");
            if (pos != -1)
            {
                //extract what's between the start and end tags
                String newName = extractData ("<name>",aLine,"</name>");
                newClient.setName (newName);
                continue;
            }

            pos = aLine.indexOf ("<id>");
            if (pos != -1)
            {
                String newId = extractData ("<id>",aLine, "</id>");
                newClient.setId (newId);
                continue;
            }


            pos = aLine.indexOf ("<phone>");
            if (pos != -1)
            {
                String newPhone = extractData ("<phone>",aLine, "</phone>");
                newClient.setPhone (newPhone);
                continue;
            }

        } //end while


    } //end parseTextStream



    public String extractData (String startTag, String aLine, String endTag)
    {
        //remove start tag
        aLine = aLine.substring (startTag.length ( ));

        //look for end tag
        int pos = aLine.indexOf (endTag);
        //found
        if (pos != -1)
            return (aLine.substring (0,pos)).trim ( );
        else // end tag not found, return the rest of the line
            return aLine.trim ( );

    }


    public int getSize ( )
    {
        return clientData.size ( );
    }

    public int getCursor ( )
    {
            return cursor;
    }

    public Client getCurrent ( )
    {
        return clientData.get (cursor);
    }

    public Client getNext (  )
    {
        if (cursor != -1)
        {
            cursor = (cursor + 1) % clientData.size ( ); //wrap from rear to front
            return clientData.get (cursor);
        }
        else
            return null;
    }

    public Client getBack ( )
    {
        if (cursor != -1)
        {
            if (cursor == 0)
                cursor = clientData.size ( ) -1;  //wrap from front to rear
            else
                cursor--;
            return clientData.get (cursor);
        }
        else
            return null;
    }

    public void setCurrent (Client aClient)
    {
        clientData.set(aClient,cursor); //overwrite the current client
    }

    public void resetCursor ( )
    {
            if (cursor != -1)
                cursor = 0;
    }

    public String toString ( )
    {
        String outString = "<clients>" + NEW_LINE;

        int count = clientData.size ( );    //count of clients

        for (int i = 0; i < count; i++)
                outString += clientData.get (i);

        outString += ("</clients>" + NEW_LINE);

        return outString;
    }


}

getting following error
.\ClientList.java:164: error: no suitable method found for set(Client,int)
        clientData.set(aClient,cursor); //overwrite the current client
                  ^
    method List.set(int,Client) is not applicable
      (argument mismatch; Client cannot be converted to int)
    method AbstractList.set(int,Client) is not applicable
      (argument mismatch; Client cannot be converted to int)
    method ArrayList.set(int,Client) is not applicable
      (argument mismatch; Client cannot be converted to int)
1 error

Tool completed with exit code 1

The first parameter of ArrayList.set() is an integer. Client is clearly not an integer and so you are getting that error.

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