Thread: using OOP in java

Hi,

I have 2 classes:
1) productType that have name and price of the productType and an empty consturcor.
2) superMarket that have name of supermarket and arraylist of productTypes (

private ArrayList<ProductType> products;

) and an empty constructor.

In each class I have function that get input from console and should store it into each class variables.

in productType i have function:

	public void getFromUser() throws IOException
	{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
 
		System.out.println("Enter product name:");
		name = br.readLine();
 
		System.out.println("Enter price:");
		price = Integer.parseInt(br.readLine());
	}

in supermarket i have function:

		public void getFromUser() throws IOException
		{
			int n=0;
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			ProductType p = new ProductType();
 
			System.out.println("Enter supermarket details: ");
			System.out.println("Enter name: ");
			this.name = br.readLine();
 
			System.out.println("How many products do you have: ");
			n = Integer.parseInt(br.readLine());
 
			for(int i=0;i<n;i++)
			{
				products.add(i,p);
				products.get(i).getFromUser();				
			}
		}

the main is something like:

	public static void main(String[] args) throws IOException{
 
		SuperMarket s1 = new SuperMarket();
		SuperMarket s2 = new SuperMarket();
 
		s1.getFromUser();
	}

The problem is when i get to line "products.add(i,p)" I get java.lang.NullPointerException
In the debug mode I can see that when I get to this line the "products" is null.

why do you think this happening, when I do "new SuperMarker()" in the main it should run the empty constructor and create new arraylist...
?

why not? the function is in class Supermarket that have an empty constructor:
public void SuperMarket()
{
this.name = null;
this.products = new ArrayList<ProductType>();
this.quentitys = new ArrayList<int[]>();
}

ok, this is the full source's

ProductType.class

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.FileReader;
import java.io.IOException;
 
public class ProductType {
 
	private String name;
	private double price;
 
	// constructors
	public void ProductType()
	{
		this.name = null;
		this.price = 0;
	}
 
	public void ProductType(String name,double price)
	{
		this.name = name;
		this.price = price;
	}
 
	// Getters and Setters
	public String getName()
	{
		return name;
	}
 
	public void setName(String name)
	{
		this.name = name;
	}
 
	public double getPrice()
	{
		return price;
	}
 
	public void setPrice(double price)
	{
		this.price = price;
	}
 
	public void printProd()
	{
		System.out.println("Product name: " + name + " Price is: " + price);
	}
 
	public void getFromUser() throws IOException
	{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
 
		System.out.println("Enter product name:");
		name = br.readLine();
 
		System.out.println("Enter price:");
		price = Integer.parseInt(br.readLine());
	}
}

SuperMarket.java

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
 
public class SuperMarket {
 
		private String name;
		private ArrayList<ProductType> products;
		private ArrayList<int[]> quentitys;
 
		// constructors
		public void SuperMarket()
		{
			this.name = null;
			this.products = new ArrayList<ProductType>();
			this.quentitys = new ArrayList<int[]>();
		}
 
		public void SuperMarket(String name, ArrayList<ProductType> products,ArrayList<int[]> quentitys)
		{
			this.name = name;
			this.products = products;
			this.quentitys = quentitys;
		}
 
		// Getters and Setters
		public String getName()
		{
			return name;
		}
 
		public void setName(String name)
		{
			this.name = name;
		}
 
		public ArrayList<ProductType> getProducts()
		{
			return this.products;
		}
 
		public void setProducts(ArrayList<ProductType> products)
		{
			this.products = products;
		}
 
		public ArrayList<int[]> getQuentitys()
		{
			return this.quentitys;
		}
 
		public void setQuentitys(ArrayList<int[]> quentitys)
		{
			this.quentitys = quentitys;
		}
 
		public void printSuper()
		{
			System.out.println("Super Market name: " + name);
			System.out.println("Products and quentity: ");
			for(int i=0;i<products.size();i++)
			{
				System.out.println("Product " + i+1 + " : " + " Name: " + products.get(i).getName() + 
						           " Price: " + products.get(i).getPrice() + " Quentity: " + quentitys.get(i));
			}
		}
 
		public void printUnderPrice(double price)
		{
			for(int i=0;i<products.size();i++)
			{
				if(products.get(i).getPrice()<price)
				{
					System.out.println("Product " + i+1 + " : " + " Name: " + products.get(i).getName() + 
					           " Price: " + products.get(i).getPrice() + " Quentity: " + quentitys.get(i));					
				}
			}
		}
 
		public void getFromUser() throws IOException
		{
			int n=0;
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			ProductType p = new ProductType();
 
			System.out.println("Enter supermarket details: ");
			System.out.println("Enter name: ");
			this.name = br.readLine();
 
			System.out.println("How many products do you have: ");
			n = Integer.parseInt(br.readLine());
 
			this.products = new ArrayList<ProductType>();
 
			for(int i=0;i<n;i++)
			{
				products.add(i,p);
				products.get(i).getFromUser();				
			}
		}
}

SuperMarket_demo.java

import java.io.IOException;
 
public class SuperMarket_demo {
 
	public static void main(String[] args) throws IOException{
 
		SuperMarket s1 = new SuperMarket();
		SuperMarket s2 = new SuperMarket();
 
		s1.getFromUser();
 
		s1.printSuper();
 
	}
 
}

from the console window:
"
Enter supermarket details:
Enter name:
my
How many products do you have:
2
Exception in thread "main" java.lang.NullPointerException
at SuperMarket.getFromUser(SuperMarket.java:97)
at SuperMarket_demo.main(SuperMarket_demo.java:10)
"
I think the empty(deafult) constructor of SuperMarker do not work and do not defint the products arrayList, but I dont understand why...

by post 3 do you mean - SuperMarket_demo.java? if is - so it do execute...

about the constructor, this is the empty constructor

public void SuperMarket()
{
this.name = null;
this.products = new ArrayList<ProductType>();
this.quentitys = new ArrayList<int[]>();
}

--- Update ---

I found my problem!!! I wrote "void" by mistake in the constructor prototype so it dosen't get into it.....

Thnk's Norm.

you are right, thnk's for your help.