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Thread: Complies but shows null expection on console

  1. #1
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    Default Complies but shows null expection on console

    okay i'm trying to stop the console displaying a null expection but i'm not sure why its doing it i was wondering if you guys could help me


    package  miniass3;
    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;
            }
     
    }

    package miniass3;
    public class AirlockDoor implements Runnable {
     
            private AirLock lockOperator;
            private String doorName;
     
            public AirlockDoor(String name, AirLock airLock){
                    doorName = name;
     
            }
     
            public AirlockDoor() { }
     
            public void run() {    
                    while (true){
                            if(lockOperator.isLockState() == false){
                                    System.out.println("False");
                            }
                    }
            }
     
            public synchronized  void  requestToOpen() {
     
                    System.out.println(doorName +  " requests to be opened");
            }
         }

    package uk.ac.aber.dcs.airlock;
     
     
    import miniass3.AirLock;
    import miniass3.AirlockDoor;
     
    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);
                    Thread doorController2 = new Thread(threadJob);
     
                    doorController1.setName("DoorLock");
                   doorController2.setName("DoorLock");
     
                    doorController1.start();
                   doorController2.start();
     
                    // Loop infinitely which try's to open the 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();
                            }
     
     
     
                    }
            }
    }


  2. #2
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    Default Re: Complies but shows null expection on console

    NullPointer is a runtime exception. Runtime exceptions are not caught by the compiler, because they only occur during program runtime (the compiler doesn't run the code to make sure it works, it just checks your syntax).
    NullPointer indicates you forgot to set a variable, but you are attempting to use that variable to call a method. Provide the full stack-trace message of the exception. The stack-trace should indicate which line the exception is occurring on. When we look at the line of code, we should be able to figure out which variable has a null value.
    NOTE TO NEW PEOPLE LOOKING FOR HELP ON FORUM:

    When asking for help, please follow these guidelines to receive better and more prompt help:
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