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Thread: using OOP in java

  1. #1
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    Default 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...
    ?


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: using OOP in java

    why do you think this happening,
    The products variable that is being used has not been assigned a value.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: using OOP in java

    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[]>();
    }

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: using OOP in java

    Is the code in post#3 executed? It was not included in the original post so it was not found when a Search of the code for where the variable: products was used. You need to post all the relevant if you expect good answers.
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: using OOP in java

    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();
     
    	}
     
    }

  6. #6
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    Default Re: using OOP in java

    Can you also post the full contents of the error message?

    Did you see this question?
    Is the code in post#3 executed?

    Add a println() statement to make sure.
    If you don't understand my answer, don't ignore it, ask a question.

  7. #7
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    Default Re: using OOP in java

    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...

  8. #8
    Super Moderator Norm's Avatar
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    Default Re: using OOP in java

    Did you see this question?
    Is the code in post#3 executed?

    Add a println() statement to make sure.

    Do you know how to write a constructor? See the tutorial:
    http://docs.oracle.com/javase/tutori...structors.html
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
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    Default Re: using OOP in java

    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.

  10. #10
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    Default Re: using OOP in java

    it do execute
    But it was not being executed because it was not a constructor.
    It's so easy to hope that code is doing what you want it to do. Often adding a println() statement that shows what the code is actually doing(or not doing) helps you see the problems in code.
    If you don't understand my answer, don't ignore it, ask a question.

  11. #11
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    Default Re: using OOP in java

    you are right, thnk's for your help.

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