Objektorienteeritud programmeerimine (Narva Kolledž) 2017/18 kevad

Session 10

Input and output of primitive data types

Imagine an e-commerce application which saves the data about each item (item's name, quantity and price delimited by a space). Later if we want to read the file, we have to use different methods such as String.split, Integer.parseInt etc. The task is simple, but the data processing is complicated.

Another solution would be to set the format of that data at first using the classes DataOutputStream and DataInputStream.

Study how it is possible to write the objects into the file:

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

 class Item {
    private String name;
    private double quantity;
    private double price;

    public Item(String name, double quantity, double price) {
        this.name = name;
        this.quantity = quantity;
        this.price = price;
    }

    //save data about items
    public void mySave(DataOutputStream dos) throws Exception {
        //pay attention to the write methods and data types
        dos.writeUTF(name);
        dos.writeDouble(quantity);
        dos.writeDouble(price);
    }

    //read data about items
    public static Item myRead(DataInputStream dis) throws Exception {
        //pay attention to the read methods and data types
        String name = dis.readUTF();
        double quantity = dis.readDouble();
        double price = dis.readDouble();
        return new Item(name, quantity, price);
    }

     @Override
     public String toString() {
         return name + ": " + " quantity=" + quantity + " for " + price + '€';
     }
 }


 class Basket {
     private String client;
     private List<Item> items;

     public Basket(String client, List<Item> items) {
         this.client = client;
         this.items = items;
     }

     public void mySave(DataOutputStream dos) throws Exception {
         //pay attention to the write methods and data types
         dos.writeUTF(client);         //save data about client
         dos.writeInt(items.size());  //add the number of items in the basket to ease reading
         //save data about items in the client's basket
         for (Item i : items) {
             i.mySave(dos);
         }
     }

     public static Basket myRead(DataInputStream dis) throws Exception {
         //read data about client
         String client = dis.readUTF();
         //read data about items in the client's basket
         List<Item> items = new ArrayList<>();
         int rItems = dis.readInt(); // instantiate variable for the loop
         for (int i = 0; i < rItems; i++) {
             items.add(Item.myRead(dis));
         }
         return new Basket(client, items);
     }

     @Override
     public String toString() {
         return client + ": " + items;
     }
 }


class Test12 {
    public static void main(String[] args) throws Exception {
        Item chocolate = new Item("chocolate", 2.0, 1.29);
        Item orange = new Item("orange", 2.5, 0.89);
        Basket myBasket = new Basket("Mary", Arrays.asList(chocolate, orange));

        DataOutputStream dos = new DataOutputStream(new FileOutputStream("data.bin"));
        myBasket.mySave(dos);
        dos.close();

        DataInputStream dis = new DataInputStream(new FileInputStream("data.bin"));
        Basket dataFromFile = myBasket.myRead(dis); // the same as at the beginning
        dis.close();

        System.out.println(dataFromFile);

    }
}

Session 10