Question

Single Lane Bridge Problem : Java Threads

Given :

//Use ReentrantLock for mutual exclusion

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class bridge  {
    private final ReentrantLock myLock = new ReentrantLock();

    public bridge(){

    }

    public String cross(){
        myLock.lock();
        try {

            return "crossing the bridge";

        } finally {
            myLock.unlock();
        }
    }
}

public class lane extends Thread
{
    int cars;
    bridge Bridge;
    public lane(int Cars1, bridge Bridge1)
    {
        cars = Cars1;
        Bridge = Bridge1;
    }

    public void run(){
        for(int x=0;x

//Important clue :

<Thread-ID> waiting to cross

This means that this thread is now competing for the lock and will have to wait its turn.

- Once a thread has gained access to the critical section it should output:

<Thread-ID> crossing the bridge

- It takes different cars a different amount of time to cross the bridge, so your code should simulate this by sleeping for a random amount of time when accessing the critical section.

- Once a car has left the bridge, your program should output:

<Thread-ID> exiting

- Threads can enter the critical section in any order.

Create main such that the output is as follows :

Thread-0 waiting to cross

Thread-0 crossing the bridge

Thread-0 exiting

Thread-1 waiting to cross

Thread-2 waiting to cross

Thread-1 crossing the bridge

Thread-1 exiting

Thread-2 crossing the bridge

Thread-2 exiting

.....

.....

.....

.....

Answer 1

import java.util.concurrent.*;

// Defines a class SingleLaneBridgeDriver

public class SingleLaneBridgeDriver

{

// main method definition

public static void main(String[] ss)

{

// Creates an object of the class Bridge

final Bridge bridge = new Bridge();

  

// Defines an anonymous object for the class Thread

// for right to left lane

Thread laneRightToLeft = new Thread( new Runnable()

{

// Overrides the run() method

@Override

public void run()

{

// Loops infinitely

while(true)

{

// Creates an object of the class Car

Car currentCar = new Car(bridge);

  

// Creates a Thread for Car class object

Thread threadRL = new Thread(currentCar);

// Sets the name of the car to thread id

currentCar.setCarName("Thread: " + threadRL.getId());

// Calls the start() method to start the thread

threadRL.start();

  

// Try block for pause time

try

{

TimeUnit.SECONDS.sleep((long)(Math.random()*10));

}// End of try block

  

// Catch block to handle InterruptedException for sleep() method

catch(InterruptedException exp)

{

exp.printStackTrace();

}// End of catch

}// End of while loop

}// End of method

});// End of anonymous class

// Defines an anonymous object for the class Thread

// for right to left lane

Thread laneLeftToRight = new Thread( new Runnable()

{

// Overrides the run() method

@Override

public void run()

{

// Loops infinitely

while(true)

{

// Creates an object of the class Car

Car currentCar = new Car(bridge);

// Creates a Thread for Car class object

Thread threadLR = new Thread(currentCar);

// Sets the name of the car to thread id

currentCar.setCarName("Thread: " + threadLR.getId());

// Calls the start() method to start the thread

threadLR.start();

  

// Try block for pause time

try

{

TimeUnit.SECONDS.sleep((long)(Math.random()*10));

}// End of try block

  

// Catch block to handle InterruptedException for sleep() method

catch(InterruptedException exp)

{

exp.printStackTrace();

}// End of catch

}// End of while loop

}// End of method

});// End of anonymous class

  

// Calls the start method for both laneRightToLeft and

// laneLeftToRight

laneRightToLeft.start();

laneLeftToRight.start();

}// End of main method

}// End of driver class

// Defines class Bridge

class Bridge

{

// Declares an object of class Semaphore

private final Semaphore semaphore;

// Default constructor definition

public Bridge()

{

semaphore = new Semaphore(1);

}// End of constructor

  

// Method to implement mechanism of crossing bridge

public void crossBridge(Car newCar)

{

// try block begins

try

{

// Displays the name of the waiting car

System.out.printf("Car %s is waiting to cross the bridge.\n",

newCar.getCarName());

  

// Used to wait until a permit is not available

semaphore.acquire();

  

// Displays the name of the crossing car

System.out.printf("Car %s is crossing the bridge.\n",

newCar.getCarName());

// Calculates the time duration

long stopTime = (long)(Math.random() * 10);

// Sleeps for the duration of stopTime

TimeUnit.SECONDS.sleep(stopTime);

}// End of try block

  

// Catch block to handle InterruptedException for sleep() method

catch(InterruptedException exp)

{

exp.printStackTrace();

}// End of catch block

  

// finally block to display the crossed bridge car name

finally

{

System.out.printf("Car %s has crossed the bridge.\n",

newCar.getCarName());

semaphore.release();

}// End of finally block

}// End of method

}// End of class

// Defines a class Car which implements Runnable interface

class Car implements Runnable

{

// To store name of the car

private String carName;

// Declares an object of class bridge

private Bridge bridge;

  

// Parameterized constructor to assign parameter Bridge object to

// instance object

public Car(Bridge bri)

{

bridge = bri;

}// End of constructor

  

// Overrides the run() method of Runnable interface

public void run()

{

// Calls the method to cross the bridge

bridge.crossBridge(this);

}// End of method

// Method to return car name

public String getCarName()

{

return carName;

}// End of method

// Method to set the car name

public void setCarName(String name)

{

carName = name;

}// End of method

}// End of class Car

Sample Output:

Car Thread: 17 is waiting to cross the bridge.
Car Thread: 16 is waiting to cross the bridge.
Car Thread: 17 is crossing the bridge.
Car Thread: 18 is waiting to cross the bridge.
Car Thread: 17 has crossed the bridge.
Car Thread: 16 is crossing the bridge.
Car Thread: 16 has crossed the bridge.
Car Thread: 18 is crossing the bridge.
Car Thread: 19 is waiting to cross the bridge.
Car Thread: 20 is waiting to cross the bridge.
Car Thread: 18 has crossed the bridge.