Code compiles but get null pointer exeption

[jamesreno]

package assignment2;

public class AirLock {

private boolean lockState = false;

public AirLock(boolean newLock){
lockState = newLock;
}

public boolean isLockState() {
return lockState;
}

public void setLockState(boolean lockState) {
this.lockState = lockState;
}
__________________________________________________ __________________________________________________ ___

public class AirlockDoor implements Runnable{

private String name;
private AirLock airlock;
private String doorName;

public AirlockDoor(String name, AirLock airLock){
this.doorName = name;

}

public AirlockDoor() {
// TODO Auto-generated constructor stub
}


public void run() {
while(true){
synchronized(airlock){
System.out.println("" + name + "Open");
System.out.println("" + name + "Closed");
}
}
}

public synchronized void requestToOpen() {

System.out.println(doorName + " requests to be opened");



}

}
__________________________________________________ __________________________________________________ _________________

public class AstronautSimulator {

public static void main(String [] args){
simulate();
}

private static void simulate() {
// Create an AirLock object
// STEP 1: CREATE AN EMPTY VERSION OF THE AirlockDoor CLASS
AirlockDoor[] doors = new AirlockDoor[2];

// For this prototype this can be an empty (no method)
// class. It is a common object used in the Door class
// that contains the single lock-key that protects the
// imaginary airlock room from having two doors open at
// the same time
// STEP 2: CREATE THIS CLASS
AirLock MainAirLock = new AirLock(false);

// Give the doors a name (for debugging purposes) and
// a common airlock object used to store the "key" used
// by a critical section of code in the Door class


doors[0] = new AirlockDoor("Door 1", MainAirLock);
doors[1] = new AirlockDoor("Door 2", MainAirLock);

// AirlockDoor implements Runnable and so make sure
// we tie the objects to Thread objects

Runnable threadJob = new AirlockDoor();

// STEP 3: CREATE TWO THREADS FOR THE TWO DOORS AND START THEM
// ENTER CODE HERE

Thread doorController1 = new Thread(threadJob);

doorController1.setName("DoorLock");

doorController1.start();

// Loop infinitely trying to open airlock doors

while (true){
// random() returns a value in range 0.0..<1.0 multiplied by 10 and mod doors.length
// to return either 0 or 1 in order to randomly decide which door to request
int doorNum = (int)((Math.random() * 10) % doors.length);

// STEP 4: MAKE A REQUEST TO OPEN THE DOOR AND THEN ADD A DELAY BEFORE
// TRYING TO OPEN ANOTHER DOOR
// ENTER CODE HERE

doors[doorNum].requestToOpen();


try{
Thread.sleep(800);
}
catch(InterruptedException ex){
ex.printStackTrace();
}



}
}
}

[jim829]

airlock in AirLockDoor is never initialized.

Regards,
Jim

[jamesreno]

how do i initilise airlock

[jim829]

You have a constructor that intializes it when you create an instance of AirLockDoor(). Curious question though. Did you write this code yourself? Because if you did it should be obvious which constructor to use. And you need to assign the passed value of airlock to the instance field in the class.

Regards,
Jim

[jamesreno]

My friend helped me with some of his code as im a newbie at coding so i dont know what goes into this could you be of assistance

[jim829]

My friend helped me with some of his code as im a newbie at coding so i dont know what goes into this could you be of assistance

Well, if you are someone who is new to Java then I recommend that you start slowly. I can also tell you that the approach you are taking with the threads is not sound. If you want to grow your own threads then you need an understanding of wait, notify, and object locking. Creating and using your own threads is relatively advanced. I recommend you focus on the more basic stuff. You may want to check out the Java tutorials in my signature.

Regards,
Jim

[jamesreno]

i have to do it is for a assignment so i would really appricate teh help

[Norm]

Please edit your post and wrap your code with code tags:
[code]
YOUR CODE HERE
[/code]
to get highlighting and preserve formatting.

What specific questions do you have?

[jamesreno]

package assignment2;

public class AirLock {
    
    private boolean lockState = false;
   
    public AirLock(boolean newLock){
            lockState = newLock;
    }

    public boolean isLockState() {
            return lockState;
    }

    public void setLockState(boolean lockState) {
            this.lockState = lockState;
    }
______________________________________________________

public class AirlockDoor implements Runnable{
	
    private String name;
	private AirLock airlock;
	private String doorName;
	
	
	public AirlockDoor(String name, AirLock airlock){
            this.doorName = name;
            this.airlock = airlock;

    }

    public AirlockDoor() {
            // TODO Auto-generated constructor stub
    }

   
    public void run() {
        while(true){
            synchronized(airlock){
              System.out.println("" + name + "Open"); 
                          System.out.println("" + name + "Closed");
                    }
            }
    }

    public synchronized  void  requestToOpen() {

            System.out.println(doorName +  " requests to be opened");
           
           

    }

}

___________________________________________________________________________-

public class AstronautSimulator {
    
    public static void main(String [] args){
            simulate();
    }

    private static void simulate() {
            // Create an AirLock object
            // STEP 1: CREATE AN EMPTY VERSION OF THE AirlockDoor CLASS
    	class AirlockDoor{
    		
    	}
            AirlockDoor[] doors = new AirlockDoor[2];
           
            // For this prototype this can be an empty (no method)
            // class. It is a common object used in the Door class
            // that contains the single lock-key that protects the
            // imaginary airlock room from having two doors open at
            // the same time
            // STEP 2: CREATE THIS CLASS
            AirLock MainAirLock = new AirLock(false);
           
            // Give the doors a name (for debugging purposes) and
            // a common airlock object used to store the "key" used
            // by a critical section of code in the Door class
           
           
            doors[0] = new AirlockDoor("Door 1", MainAirLock);
            doors[1] = new AirlockDoor("Door 2", MainAirLock);
           
            // AirlockDoor implements Runnable and so make sure
            // we tie the objects to Thread objects
           
            Runnable threadJob = new AirlockDoor();
           
    // STEP 3: CREATE TWO THREADS FOR THE TWO DOORS AND START THEM
            // ENTER CODE HERE
           
            Thread doorController1 = new Thread(threadJob);
           
            doorController1.setName("airlock");
           
            doorController1.start();
           
            // Loop infinitely trying to open airlock doors
           
            while (true){
                    // random() returns a value in range 0.0..<1.0 multiplied by 10 and mod doors.length
                    // to return either 0 or 1 in order to randomly decide which door to request
                    int doorNum = (int)((Math.random() * 10) % doors.length);
                   
                    // STEP 4: MAKE A REQUEST TO OPEN THE DOOR AND THEN ADD A DELAY BEFORE
                    // TRYING TO OPEN ANOTHER DOOR
                    // ENTER CODE HERE
                   
                    doors[doorNum].requestToOpen();
                   
                   
                    try{
                            Thread.sleep(800);
                    }
                    catch(InterruptedException ex){
                            ex.printStackTrace();
                    }
                   
                   
                   
            }
    }
}

[jamesreno]

Exception in thread "main" java.lang.Error: Unresolved compilation problems:
The constructor AirlockDoor(String, AirLock) is undefined
The constructor AirlockDoor(String, AirLock) is undefined
Type mismatch: cannot convert from AirlockDoor to Runnable
The method requestToOpen() is undefined for the type AirlockDoor

at assignment2.AstronautSimulator.simulate(AstronautS imulator.java:30)
at assignment2.AstronautSimulator.main(AstronautSimul ator.java:6)

im just not sure what i have to do next i know i need to do what jim said but im not sure how to go about it

[jim829]

First, when is your assignment due? What methods and classes, if any, are you required to use?

Regards,
Jim

[jamesreno]

AirlockDoor class , e constructor that takes a String and an Airlock , an empty Airlock class , two thread objects

[jim829]

Okay, I am having a difficult time figuring out your design. You have a tight loop in the run method which doesn't do much. But here is one approach.

Have two threads of an AirLockDoor.
Each thread shares a boolean value which indicates the door is open.
When one thread opens a door, it sets the boolean to true.
When this value is true, the other door cannot be opened.
After a while, the open door closes and sets the value to false.
Then the other door may open.

Those methods must be synchronized so they can't open both doors at the same time.

The problem with this approach is that a single door may open and close several times before the
other door gets a chance. So you would need to add additional checks to ensure a door can't
be opened again until the other door has had a chance.

You won't need notify and wait. Just Thread.sleep and some boolean values.
That's the best I can offer at this time.

Regards,
Jim

[jamesreno]

Task 2: Look at the existing code and fill in the blanks
Examine the Door interface. Then take a look at the AstronautSimulator class. This is the one
you need to update. It will not compile because you will need to create an AirlockDoor class and
an Airlock Class. The following steps you need to follow are also listed in the
AstronautSimulator class:

Step 1: Create an empty AirlockDoor class. Note that you will also need to create an
appropriate constructor that takes a String and an Airlock.

Step 2: Create an empty Airlock class. This one can remain empty for the reasons discussed in
the code comments.

Step 3: Create two thread objects using the Thread class: one for each of the two doors. Also
start the threads. You will need to do something to the AirlockDoor class, so that your code
compiles. Just do enough to get it to compile.

Step 4: Make a request to open a door (hint: see the interface) and then add an 800 millisecond
delay before trying to open another door. You will need to add a stubbed out method (empty
and will need implementing in Task 3) to the AirlockDoor class.

[jamesreno]

would you be able to help with some of the code just so i can get on the right track as i have very limited time

[Norm]

Most of us are here to help you design and write code and get it to work, not to write code.

Make a list of the problems you are trying to solve, start at the top of the list and work on that one.
When it compiles, executes and tests OK, move to the next one.

What's the next item on the list you are working on?

[jamesreno]

stuck on this part

Step 2: You will also need to implement the constructor. Simply store the parameter values in
instance variables. These instance variables will be shared by threads. Refer back to the slides
and lecture examples to see what implication this has on the way you should declare these
instance variables.



my lockoperator and doorName just shows null on my debugger

[jamesreno]

/**
 * Simulates astronauts trying to open spaceship airlock doors
 * @author cwl
 *
 */
public class AstronautSimulator {
	 
	public static void main(String [] args){
		simulate();
	}

	private static void simulate() {
		// Create an AirLock object
		// STEP 1: CREATE AN EMPTY VERSION OF THE AirlockDoor CLASS
		class AirlockDoor{
		  
		  private String name;
		  private AirLock airLock;
		  
		    
		  public AirlockDoor(String name ,AirLock airlock){
		     this.name = name;
		     airLock= new AirLock();
		  
		  }
		    
		  }
		AirlockDoor[] doors = new AirlockDoor[2];
		
		// For this prototype this can be an empty (no method)
		// class. It is a common object used in the Door class
		// that contains the single lock-key that protects the
		// imaginary airlock room from having two doors open at
		// the same time
		// STEP 2: CREATE THIS CLASS
		class Airlock{
		    
		  }
		AirLock airLock = new AirLock();
		
		// Give the doors a name (for debugging purposes) and
		// a common airlock object used to store the "key" used
		// by a critical section of code in the Door class
		doors[0] = new AirlockDoor("1234", airLock);
		doors[1] = new AirlockDoor("1235", airLock);
		
		// AirlockDoor implements Runnable and so make sure
		// we tie the objects to Thread objects
        // STEP 3: CREATE TWO THREADS FOR THE TWO DOORS AND START THEM
		// ENTER CODE HERE
        Thread doorController1 = new Thread();
		Thread doorController2 = new Thread();
		
		doorController1.start();
		doorController2.start();
		
		// Loop infinitely trying to open airlock doors
		while (true){
			// random() returns a value in range 0.0..<1.0 multiplied by 10 and mod doors.length
			// to return either 0 or 1 in order to randomly decide which door to request
			int door = (int)((Math.random() * 10) % doors.length);
			
			// STEP 4: MAKE A REQUEST TO OPEN THE DOOR AND THEN ADD A DELAY BEFORE
			// TRYING TO OPEN ANOTHER DOOR
			// ENTER CODE HERE
		}
	}
}
______________________________________________________________________________________________
/**
 * Write a description of class AirLock here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class AirLock {
    
    
    
    
}
_______________________________________________________________________
/**
 * Represents a door. Door are automatic and will open for a short time and then
 * close again automatically.
 * @author cwl
 *
 */
public abstract interface Door {
	/**
	 * Make a request to open the door. The door will in time open.
	 * How long this takes depends on the implementation
	 */
	public void requestToOpen();
}

ok so ive done
Step 1: Create an empty AirlockDoor class. Note that you will also need to create an
appropriate constructor that takes a String and an Airlock.

Step 2: Create an empty Airlock class. This one can remain empty for the reasons discussed in
the code comments.

Step 3: Create two thread objects using the Thread class: one for each of the two doors. Also
start the threads. You will need to do something to the AirlockDoor class, so that your code
compiles. Just do enough to get it to compile.

now im at this :

Step 4: Make a request to open a door (hint: see the interface) and then add an 800 millisecond
delay before trying to open another door. You will need to add a stubbed out method (empty
and will need implementing in Task 3) to the AirlockDoor class.

?????????????

what do i do here im worried