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Thread: Agent-Smoker output error help needed

  1. #1
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    Default Agent-Smoker output error help needed

    I am trying to implement the agent-smoker problem in Java: There are 3 smokers, and 1 agent. Each smoker has 1 of the 3 ingredients (matches, paper, tobacco). The agent has an infinite amount of all ingredients, and randomly puts 2 on the table. The smoker that is missing those 2 ingredients grabs it, rolls a cigarette, and this process repeats. Here is what I have so far:



    import java.util.ArrayList;
    import java.util.Random;
    class CigaretteSmokersProblem {
     
        public static void main(String[] args) {
            table smokingtable = new table();
     
            agent controlAgent = new agent(smokingtable);
     
            controlAgent.start();
     
            for (int i = 0; i < 3; i++)
            {
                // each smoker-thread gets the controlAgent, so the thread can wake up the agent and we don't have to do notifyAll();
                smoker smokerThread = new smoker(smokingtable, i, "Smoker " + Integer.toString(i+1), controlAgent);
                smokerThread.start();
            }
        }
    }
     
    class smoker extends Thread {
     
        private table smokingtable = new table();
        private String ingredient;
        private int ingredientNumber;
        private agent controlAgent;
     
        public smoker(table pSmokingtable, int pIngredientNumber, String pName, agent pAgent)
        {
            // only save the number of the ingredient, we'll get the ingredient's name while the thread is running
            ingredientNumber = pIngredientNumber;
            this.smokingtable = pSmokingtable;
            setName(pName);
            controlAgent = pAgent;
        }
     
        @Override
        public void run()
        {
            while(true)
            {
                ingredient = smokingtable.getSmokerIngredient(ingredientNumber);
     
                if (!smokingtable.hasIngredient(ingredient) && !smokingtable.isEmpty())
                {
                    System.out.println(getName() + " has " + ingredient + ".");
                    try {
                        doSmoke();
                        System.out.println(getName() + " tells the agent to start the next round.");
                        // the thread tells the agent to continue
                        controlAgent.wake();
                    } catch (Exception e) {}
                }
            }
        }
     
        public synchronized void doSmoke() throws Exception
        {
            System.out.println(getName() + " rolls the cigarette.");
            Thread.sleep(2500);
            System.out.println(getName() + " smokes.");
            Thread.sleep(2500);
            System.out.println(getName() + " has finished.");
        }
     
      }
     
    class agent extends Thread {
     
        private table smokingtable;
     
        public agent(table pSmokingtable)
        {
            smokingtable = pSmokingtable;
        }
     
        @Override
        public void run()
        {
            while(true)
            {
                try {
                    Thread.sleep(5000);
                } catch (Exception e) {}
                smokingtable.setAgentIngredients();
                // this triggers the smoker-threads to look at the table
                System.out.println("\n");
                System.out.println("The agents puts " + smokingtable.getAgentIngredients() + " on the table.");
                // pause the agent while one smoker thread is running
                pause();
            }
        }
     
        public synchronized void wake()
        {
            try
            {
                notify();
            } catch(Exception e){}
        }
     
     
        public synchronized void pause()
        {
            try
            {
                this.wait();
            } catch (Exception e) {}
        }
     
     
    }
     
    /*Class for creating table*/
    class table {
        //Variables for storing ingredients(tabacco,paper,matches)
        private ArrayList allIngredients  = new ArrayList();
        private ArrayList agentIngredients = new ArrayList();
     
        public table()
        {
            allIngredients .add("tabacco");
            allIngredients .add("paper");
            allIngredients .add("matches");
        }
        /*Function for setting two ingredients randomly*/
        public void setAgentIngredients()
        {
            Random random = new Random();
     
            agentIngredients.clear();
     
            ArrayList copyAllElements = (ArrayList) allIngredients .clone();
     
            int ingredient1 = random.nextInt(copyAllElements.size());
            agentIngredients.add(copyAllElements.get(ingredient1));
     
            copyAllElements.remove(ingredient1);
            int ingredient2 = random.nextInt(copyAllElements.size());
            agentIngredients.add(copyAllElements.get(ingredient2));
        }
        /*Function for checking weather the table is empty*/
        public boolean isEmpty()
        {
            return (agentIngredients.size() == 0);
        }
        /*Function for getting two ingredients that 
        are set previously by the function setAgentIngredients()*/
        public synchronized String getAgentIngredients()
        {
            notifyAll();
            return agentIngredients.toString();
        }
        /*Function for getting the ingredient 
        with the corresponding smoker*/
        public synchronized String getSmokerIngredient(int pIngredient)
        {
            try {
                this.wait();
            } catch (Exception e) {}
            return (String) allIngredients .get(pIngredient);
        }
        /*Function for Checking the smoker has the same ingredient 
        as in the table if so smoking is is not possible*/
        public boolean hasIngredient(String ingredientName)
        {
            return (agentIngredients.contains(ingredientName));
        }
     
        public synchronized void pause()
        {
            try {
                this.wait();
            } catch (Exception e) {}
        }
    }



    The output is usually correct,


    The agents puts [matches, tabacco] on the table.
    Smoker 2 has paper.
    Smoker 2 rolls the cigarette.
    Smoker 2 smokes.
    Smoker 2 has finished.
    Smoker 2 tells the agent to start the next round.


    But sometimes, the order gets messed up for some reason:

    Smoker 3 has matches.
    Smoker 3 rolls the cigarette.
    The agents puts [paper, tabacco] on the table.
    Smoker 3 smokes.
    Smoker 3 has finished.
    Smoker 3 tells the agent to start the next round.

    Smoker 3 has matches and rolls the cigarette before the agent puts the resources on the table.
    Can anybody figure out what part of my code is wrong?
    Thanks!


  2. #2
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    Default Re: Agent-Smoker output error help needed

    There is probably nothing "wrong" with your code. From "Introduction to Programming Using Java, Sixth Edition," by David J. Eck, Chapter 12:
    In a normal, single-threaded program, things happen in a definite, predictable order from beginning to end. In a multi-threaded program, there is a fundamental indeterminacy. You can't be sure what order things will happen in. This indeterminacy is what makes parallel programming so difficult!

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