Interacting with a thread nullpointer exception

[s ravi chandran]

Hi,
This sample application is an improvement over my previous posted question Interacting-thread-class. I have created a StockAdder, StockBroker and StockBidder.

StockAdder and StockBroker are working fine. stock adder will add a stock with random price and stock broker will retrieve it. now i have added a logic where stock broker will initiate call to stock bidders to let highest bidder bid for the stock. if the bid value is successful, the bided stock is added to a list. the design is still incomplete.

now my issue is in running. I am getting null pointer error. I am giving the main classes here.

public class StockBroker  implements Runnable{
	private StockListRegistry stockRegister;
	private BiddingManager manager ;

	public StockBroker(StockListRegistry c) {
		stockRegister = c;
	}
	
	public void run() {
		Random random = new Random();
		
		while(true) {
			StockBusinessObject newStockReceived = stockRegister.getStock();
			// create bidders
			manager = new BiddingManager(random.nextInt(5));
			
			List<StockBidder> bidderList = manager.getBidders();
			// find max bidder;
			StockBidder maxBidder = null;
			int maxBiddingValue = 0;
			
			for(StockBidder bidder : bidderList)
			{
				int biddingValue = bidder.getBiddingPrice();
				
				if(biddingValue > maxBiddingValue)
				{
					maxBiddingValue = biddingValue;
					maxBidder = bidder;
				}
			}
			
			// only the highest bidder bids for the stock
			if(newStockReceived!= null)
			{
				// initiate bidding for the stock
				maxBidder.bidForStock(newStockReceived);            // error coming from this statement 
			}
			
			System.out.println("\n\n **** Transaction Complete ****  \n");
		}
	}
}

public class StockBidder {

	private String bidderName;
	private int biddingPrice;
	private List<StockBusinessObject> bidList;
	
	public StockBidder(String name, int expectedprice)
	{
		bidderName = name;
		biddingPrice = expectedprice;
		bidList = new ArrayList<StockBusinessObject>();
	}
	
	
	public String getBidderName() {
		return bidderName;
	}
	public void setBidderName(String bidderName) {
		this.bidderName = bidderName;
	}
	public int getBiddingPrice() {
		return biddingPrice;
	}
	public void setBiddingPrice(int biddingPrice) {
		this.biddingPrice = biddingPrice;
	}
	public List getBidList() {
		return bidList;
	}
	
	public void bidForStock( StockBusinessObject stock)
	{
		synchronized(this)
		{
			int stockListedBidValue = stock.getStockValue();
			
			if(this.biddingPrice >= stockListedBidValue)
			{
				bidList.add(stock);
				System.out.println("[Bid Success] from {"+ this.getBidderName()+"} for stock: "+stock);
			}
			else
			{
				System.out.println("[Bid Failure] from {"+ this.getBidderName()+"} for stock:"+stock);
			}
		}
		
	}
	
}

public class BiddingManager {

	int biddersCount =0;
	List<StockBidder> biddersList = null;

	public BiddingManager(int personCount)
	{
		biddersCount = personCount;
		biddersList = new ArrayList<StockBidder>(biddersCount);
		populateBidders(biddersList);
	}

	public void populateBidders( List<StockBidder> bList)
	{
		Random random = new Random();

		for(int i=0; i < biddersCount; i++)
		{
			StockBidder bidder = new StockBidder("Bidder-"+i, 100+ random.nextInt(500));
			bList.add(bidder);
		}

	}

	public List<StockBidder> getBidders()
	{
		return biddersList;
	}
}

this is the error I am getting.

Adding stock: Yahoo-[YAH]  501 | STOCKUP
Listing stock: Yahoo-[YAH]  501 | STOCKUP
[Bid Failure] from {Bidder-1} for stock:Yahoo-[YAH]  501 | STOCKUP


 **** Bidding transaction Complete ****

Adding stock: Yahoo-[YAH]  189 | STOCKDOWN
Listing stock: Yahoo-[YAH]  189 | STOCKDOWN
Exception in thread "Thread-1" java.lang.NullPointerException
        at exchange.contollers.StockBroker.run(StockBroker.java:48)
        at java.lang.Thread.run(Unknown Source)

I hope I am able to explain the issue clearly.. Please let me know what am i missing in the method call..

Thanks
ravi

[Ulf Dittmer]

Hard to say because the StockBroker class you posted isn't exactly what you used. Specifically, here it doesn't even have a line 48 - which is where the exception states the error occurred. You should always post the exact code you use, not some edited version, at least if you want exception line numbers to be meaningful.

So the exercise for you is: look at line 48 of your StockBroker class and find out which object used in that line is null. That's very easy to do. My guess is that's "maxBidder", because that is not set through all code paths, and you're not checking it for null values.

[s ravi chandran]

Ulf Dittmer wrote:Hard to say because the StockBroker class you posted isn't exactly what you used. Specifically, here it doesn't even have a line 48 - which is where the exception states the error occurred. You should always post the exact code you use, not some edited version, at least if you want exception line numbers to be meaningful.

So the exercise for you is: look at line 48 of your StockBroker class and find out which object used in that line is null. That's very easy to do.

Sorry, I removed few lines from the top which were import statements and package declaration.. adding the full class now..

package exchange.contollers;

import java.util.List;
import java.util.Random;

import exchange.bidders.BiddingManager;
import exchange.bidders.StockBidder;
import exchange.datafeed.StockBusinessObject;
import exchange.datafeed.StockListRegistry;


public class StockBroker  implements Runnable{
	private StockListRegistry stockRegister;
	private BiddingManager manager ;

	public StockBroker(StockListRegistry c) {
		stockRegister = c;
	}
	
	public void run() {
		Random random = new Random();
		
		while(true) {
			StockBusinessObject newStockReceived = stockRegister.getStock();
			// create bidders
			manager = new BiddingManager(random.nextInt(5));
			
			List<StockBidder> bidderList = manager.getBidders();
			// find max bidder;
			StockBidder maxBidder = null;
			int maxBiddingValue = 0;
			
			for(StockBidder bidder : bidderList)
			{
				int biddingValue = bidder.getBiddingPrice();
				
				if(biddingValue > maxBiddingValue)
				{
					maxBiddingValue = biddingValue;
					maxBidder = bidder;
				}
			}
			
			// only the highest bidder bids for the stock
			if(newStockReceived!= null)
			{
				// initiate bidding for the stock
				maxBidder.bidForStock(newStockReceived);
			}
			
			System.out.println("\n\n **** Bidding transaction Complete ****  \n");
		}
	}
}

[Ulf Dittmer]

I had actually edited my previous post - so the answer for you is already in there. Not sure if you saw that.

[s ravi chandran]

Ulf Dittmer wrote:I had actually edited my previous post - so the answer for you is already in there. Not sure if you saw that.

Ok.. I shd have refreshed the page.. Will check the maxbidder object. Thanks!

[s ravi chandran]

Ulf Dittmer wrote:I had actually edited my previous post - so the answer for you is already in there. Not sure if you saw that.

Thanks for the help. I found out the issue. I was generating bidders randomly. Sometimes the random count was coming as 0. hence no bidders were created and max bidder was coming as null.. now one more thing is required here. in place of max bidder logic, I want to use countdown latch or cyclic barrier to let all the bidders bid seperately for the same stock and at the end, decide the successful bid.

is there any possible design to achieve this correctly?

[s ravi chandran]

Hi,
I am working on the second part of this design. triggering parallel bidding for the stocks from stock broker. the updated classes are given below:

public class StockBidder  implements Callable<StockBidder>{

	private String bidderName;
	private int biddingPrice;
	private StockBusinessObject stockToBid;

	public StockBidder(String name, int expectedprice)
	{
		bidderName = name;
		biddingPrice = expectedprice;
	}

	public String getBidderName() {
		return bidderName;
	}
	public void setBidderName(String bidderName) {
		this.bidderName = bidderName;
	}
	public int getBiddingPrice() {
		return biddingPrice;
	}
	public void setBiddingPrice(int biddingPrice) {
		this.biddingPrice = biddingPrice;
	}

	public void setStockForBidding(StockBusinessObject stock)
	{
		stockToBid = stock;
	}

	@Override
	public StockBidder call() throws Exception {

		System.out.println("[Bidder: "+this.getBidderName()+"] bidding for ["+stockToBid.getStockName()+" --> "+stockToBid.getStockValue()+"] for price: "+this.getBiddingPrice() );
		return  bidForStock(stockToBid);
	}


	public StockBidder bidForStock( StockBusinessObject stock)
	{
		StockBidder successfullBidder = null ;

		synchronized(stockToBid)
		{
			int stockListedBidValue = stock.getStockValue();

			if(this.getBiddingPrice() >= stockListedBidValue)
			{
				successfullBidder= this;
//				System.out.println("[Bid Success] from {"+ this.getBidderName()+"} for stock: "+stock);
			}
			else
			{
//				System.out.println("[Bid Failure] from {"+ this.getBidderName()+"} for stock:"+stock);
			}

		}

		return successfullBidder;
	}
}

public class StockBroker  implements Runnable{
	private StockListRegistry stockRegister;
	private BiddingManager manager ;
	private final Object lock = new Object();

	public StockBroker(StockListRegistry c) {
		stockRegister = c;
	}

	public void run() {
		Random random = new Random();

		while(true) {
			StockBusinessObject newStockReceived = stockRegister.getStock();

			// create bidders
			manager = new BiddingManager(random.nextInt(10) + 1);
			List<StockBidder> bidderList = manager.getBidders();

			synchronized(lock)
			{
				ExecutorService pool = Executors.newFixedThreadPool(bidderList.size());
				Set<Future<StockBidder>> successfulBidders = new HashSet<Future<StockBidder>>();

				for(StockBidder bidder : bidderList)
				{
					Callable<StockBidder> individualBidding = bidder;
					bidder.setStockForBidding(newStockReceived);
					Future<StockBidder> future = pool.submit(individualBidding);
					successfulBidders.add(future);
				}
				
				pool.shutdown();
				System.out.println("\n SUCCESSFULL BIDS FOR : ["+newStockReceived.getStockName()+"]");

				for(Future<StockBidder> futureBid: successfulBidders)
				{
					try {
						StockBidder bidder = futureBid.get();
						if(bidder!=null)
							System.out.println("[Bidder: "+bidder.getBidderName()+"] got stock for price: "+bidder.getBiddingPrice()); 	

					} catch (InterruptedException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					} catch (ExecutionException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
				}
			}

			System.out.println();
			System.out.println("\n\n **** Bidding transaction Complete ****  \n");
		}
	}
}

now my problem is with the sequence of events in stock broker. first the bidding is to be executed using executor service. once the bidding has been submitted, i should be getting list of successful bidders. but the problem is the sequence is running parallel... is this is a design problem or am assuming the sequence incorrectly.

this is what I am getting in result

Adding stock: Google-[GOOG]  119 | STOCKDOWN
Listing stock: Google-[GOOG]  119 | STOCKDOWN

 SUCCESSFULL BIDS FOR : [Google]
[Bidder: Bidder-0] bidding for [Google --> 119] for price: 182
[Bidder: Bidder-1] bidding for [Google --> 119] for price: 242
[Bidder: Bidder-0] got stock for price: 182
[Bidder: Bidder-1] got stock for price: 242



 **** Bidding transaction Complete ****

Adding stock: Google-[GOOG]  577 | STOCKUP
Listing stock: Google-[GOOG]  577 | STOCKUP

 SUCCESSFULL BIDS FOR : [Google]
[Bidder: Bidder-2] bidding for [Google --> 577] for price: 515
[Bidder: Bidder-1] bidding for [Google --> 577] for price: 134
[Bidder: Bidder-0] bidding for [Google --> 577] for price: 424

this is how it should always come like:

Adding stock: Yahoo-[YAH]  302 | STOCKDOWN
Listing stock: Yahoo-[YAH]  302 | STOCKDOWN
[Bidder: Bidder-0] bidding for [Yahoo --> 302] for price: 435
[Bidder: Bidder-2] bidding for [Yahoo --> 302] for price: 297
[Bidder: Bidder-1] bidding for [Yahoo --> 302] for price: 359
[Bidder: Bidder-4] bidding for [Yahoo --> 302] for price: 266
[Bidder: Bidder-3] bidding for [Yahoo --> 302] for price: 514
[Bidder: Bidder-5] bidding for [Yahoo --> 302] for price: 240

 SUCCESSFULL BIDS FOR : [Yahoo]
[Bidder: Bidder-3] got stock for price: 514
[Bidder: Bidder-1] got stock for price: 359
[Bidder: Bidder-0] got stock for price: 435

Thanks
Ravi