在java混乱中创建对象对象

Question

hello everyone I am newto programming and I am just a little bit confused. I have three files in java

Product.java this file defines the product attributes like product name, quantity, ID in this file i dont have a main

ProductCollection.java this file create a vector to hold data temporarily when accessing database

Database.java database connection

EServer communicate with client and retrieve access database

rs = query.executeQuery(queryString);
pColl = new ProductCollection();<-------------------confuse
product extractedProduct; <------------------------ confuse
while(rs.next())
{

    extractedProduct = (Product).addProduct(rs);
    // Object [ ] extractedProduct  = ProductCollection.addProduct(rs);
}

EClient.java

can someone explain this two line please

[ pColl = new ProductCollection();] [ product extractedProduct;]

also I am struggling to add an object to store the result temporary. Can someone help me please I want extract out the contents of a row and set up a Product object extractedProduct

Answer 1

pColl = new ProductCollection();

This is as simple assignment. On the left hand side you have a variable, pColl , and on the right hand side you have an expression that creates a new object of type ProductCollection and returns a reference to it.

product extractedProduct;

This is a variable declaration which says, "from this line on, I want to be able to use a variable which I call extractedProduct and it refers to a product " (or, put another way, you declare a variable of type product ).

---

The Java convention says that class names should start with a capital letter, so you probably want to change the name of the class so it reads

Product extractedProduct;

---

Useful links:

• Official trail on Java Variables

Answer 2

ProductCollection is a class. Typically in any OO language the program operates on instances of objects. Think of the class as a template for creating new instances of the class.

so the

pColl = new ProductCollection();

line creates a new ProductCollection instance and gives your a reference to that instance, pColl. On that particular line the type of pColl is not defined, so it must be somewhere else in the code before that line, or there is an error. You could do

ProductCollection pColl = new ProductCollection();

if that is the case. The next line

product extractedProduct;

means that extranctedProduct is a reference to an instance of product. As a note, if product is a class it should be Product with a capital P. Since you don't assign a value, extractedProduct is uninitialized.

Answer 3

import java.sql.*;
import java.net.*;
import java.io.*;
import java.util.Enumeration;

public class EServer
{

public static void main(String[] args)
{
   // Net and IO objects
   ServerSocket ss= null;
   Socket s = null;
   BufferedReader bf = null;
   PrintWriter pw = null;

   // Queries

   // Query which retireves all products
   String allQuery = "Select * from StoreProducts";
   // Query which retrieves products which are out of stock
   String outOfStockQuery = "Select * from StoreProducts where Quantity = 0";
   String queryString = "";
   String messageBack = "";

   // Database objects
   Connection cn = null;
   Statement query = null;
   ResultSet rs = null;

   String lineRead= "";
   ProductCollection pColl = new ProductCollection();
   Enumeration pEnumeration;

   try
   {
      // Server socket set up on port 2000
      ss = new ServerSocket(2000);

      System.out.println("...Server socket set up");
      // Set up a database
      Database db = new Database
        ("darrel", "", "sun.jdbc.odbc.JdbcOdbcDriver", "jdbc:odbc:", "products");
      System.out.println("...Database set up");
      // Get a connection to the database
      Database.establishConnection();
      cn = Database.getConnection();
      System.out.println("...Database connection set up");
      //Set up a query object
      query = cn.createStatement();
      // Loop waiting for connections
      int count = 1;
      while(true)
      {
         System.out.println("...Waiting for connection "+count);
         count++;
         s = ss.accept();
                 pw = new PrintWriter(s.getOutputStream(), true);
         bf = new BufferedReader(new InputStreamReader(s.getInputStream()));
                 boolean looper = true;
         while(true)
         {

             lineRead = bf.readLine();
             switch(lineRead.charAt(0))//
             {
                // Out of Stock command
                case 'O':{
                           queryString = outOfStockQuery;
                           break;
                         }
                // All products
                case 'A':{
                           queryString = allQuery;
                           break;
                         }
                // Client has terminated
                case 'E':{
                           looper = false;
                           break;
                          }
                 }
              //Check if client has terminated
              if(!looper) break;
              // Client has not terminated
              // Execute the required query and create result set
              rs = query.executeQuery(queryString);
              // Create collection of products
              pColl = new ProductCollection();
              // Process the rows that have been extracted
              // Place them in pColl
              Product extractedProduct;
              while(rs.next())
              {

*extractedProduct = (Product) pColl.addProduct(rs);*


              }
               // Form the collection of products, each terminated by asterisk
               pEnumeration = pColl.elements();
               // messageBack is to be concatenated to so initialise it to the empty string
               messageBack= "";
               while(pEnumeration.hasMoreElements())
               {
                  messageBack+=(Product)pEnumeration.nextElement()+"*";
               }
               // Now send back the collection string to the client for display
               pw.println(messageBack);
             }
          }
        }
        catch(Exception e)
           {System.out.println("Trouble setting up the database "+e);}
        // Close database connection
        try
        {
           cn.close();
        }
        catch(Exception e)
            {System.out.println("Problem closing connection");}
    }
}