Question

I got to what I thought was a finished program and now java is pulling this on me.

The error I get is as follows:

Exception in thread "main" java.lang.NullPointerException
    at hirecardemo.HireCar.isAvailable(HireCar.java:68)
    at hirecardemo.HireCarDemo.runSimulation(HireCarDemo.java:49)
    at hirecardemo.HireCarDemo.main(HireCarDemo.java:25)
Java Result: 1

Main Class:

package hirecardemo;
import java.util.Random;

public class HireCarDemo {

    public static void main(String[] args) {
        HireCar car0 = new HireCar("Toyota", "AV77 FGJ", 6000, 12300, 41500);
        HireCar car1 = new HireCar("Mercedes", "DI99 FTZ", 6700, 7000, 91800);
        HireCar car2 = new HireCar("Toyota", "FG82 FTP", 25000, 12000, 72000);
        HireCar car3 = new HireCar("Vauxhall", "TW56 LTS", 10000, 11000, 19001);
        HireCar car4 = new HireCar("Ford", "TD85 LTU", 13000, 12300, 12000);
        HireCar car5 = new HireCar("Susuki", "GU12 UTJ", 12000, 10000, 50000);

        HireCar[] fleet = {car0, car1, car2, car3, car4, car5};
        int minMileage = 1000;
        int maxMileage = 60000;
        int numberOFevents = 12;

        String [] results = HireCarDemo.runSimulation(fleet, numberOFevents,
                minMileage, maxMileage);

        for(int i = 0; i < numberOFevents; i++) {
            System.out.println(results[i]);
        }
    }

    /**
    * @param fleet the fleet of hire cars
    * @param numberOFevents the size of the events table to be generated
    * @param minMileage the assumed minimum mileage driven by any hired
    * car
    * @param maxMileage the assumed maximum mileage driven by any hired
    * car
    * @return table of events generated during the simulation
    */
    public static String[] runSimulation(HireCar [] fleet, int numberOFevents, 
            int minMileage, int maxMileage) {
        int n = fleet.length; // Number of cars in the fleet.
        Random carGenerator = new Random();
        String [] events = new String [numberOFevents];
        for(int i = 0; i < numberOFevents; i++) {
            int randomNumber = carGenerator.nextInt(n-1);
            if(fleet[randomNumber].isAvailable() == true)
            {
                fleet[randomNumber].hireOut();
                events[i] = fleet[randomNumber].getRegNumber() + " <HIRE OUT>";
            }

            else if(fleet[randomNumber].isOnHire() == true)
            {
                Random mileage = new Random();
                int randomMileage = mileage.nextInt(maxMileage - minMileage); 

                if(fleet[randomNumber].isBeingServiced() == true)
                {
                    events[i] = fleet[randomNumber].getRegNumber() + 
                            " <RETURN FROM HIRE>" + " <SEND FOR SERVICE>";
                } else {
                    events[i] = fleet[randomNumber].getRegNumber() + 
                            " <RETURN FROM HIRE>";                    
                }
            }
            else 
            {
                fleet[randomNumber].makeAvailable();
                events[i] = fleet[randomNumber].getRegNumber() + 
                        " <RETURN FROM SERVICE>";
            }
        }
        return events;  
    }
}

Here is my separate class that goes along with this:

//******************************************************************************
// HireCar.java                                             Author: Ryan Holder
// 
// Represents the car hire company's fleet of cars and the information on them.
//******************************************************************************
package hirecardemo;


public class HireCar {

    private String manufacturer, regNumber, carStatus;
    private int mileage, serviceInterval, lastService; // All in miles.    

    private boolean serviceDue() { 
        if((mileage- lastService) >= serviceInterval) {
            this.sendForService();  
            return true;
        } else { 
            return false;
        }
    }

   private void sendForService() {
       carStatus = "Servicing";      
   }

    //--------------------------------------------------------------------------
    // Default Constructor: Sets information for a new car.
    //--------------------------------------------------------------------------    
    public HireCar(String demoManufacturer, String demoRegNumber) {
        manufacturer = demoManufacturer;
        regNumber = demoRegNumber;
        carStatus = "Available";
        serviceInterval = 0;        
        lastService = 0;
        mileage = 0;                 
    }  

      public HireCar(String demoManufacturer, String demoRegNumber, 
             int demoMileage, int demoServiceInterval, int lastInterval) { 
         manufacturer = demoManufacturer;
         regNumber = demoRegNumber;
         mileage = demoMileage;     
     }

    public void setMileage(int demoMileage) {
        mileage = demoMileage;
    }    

    public void setServiceInterval(int demoServiceInterval) {
        serviceInterval = demoServiceInterval;
    }

    public void setLastService(int demoLastService) {
        lastService = demoLastService;    
    }

    public String getRegNumber() {
        return regNumber;
    }

    public void makeAvailable() {
        carStatus = "Available";
    }


    public boolean isAvailable() {
        if(carStatus.equals("Available") || carStatus.equals("Return from Service")) {
            return true;
        } else {
            return false;
        }
    }

    public boolean isOnHire() {
        if(carStatus.equals("On Hire")) {
            return true;
        } else {
            return false;
        }
    }


    public boolean isBeingServiced() {
        if(carStatus.equals("Being Serviced")) {
            return true;
        } else {
            return false;
        }
    }


 public void hireOut() {
        carStatus = "On Hire"; 
    }

    public void returnFromHire() {
        if(this.serviceDue() == true) {
            carStatus = "Being Serviced";            
        } else {
            carStatus = "Available For Hire";
        }        
    }

    public void returnFromService() {
        carStatus = "Return From Service";
    }

    public String ToString() {
        return("Manufacturer: <" + this.manufacturer + ">/n "
                + "Registration Number: <" + this.regNumber + ">/n" 
                + "Mileage: <" + this.mileage + ">/n" 
                + "Service Interval: <" + this.serviceInterval + "</n"
                + "Last Service: <" + this.lastService + "</n"
                + "Status: <" + this.carStatus + "</n");
    }
}

Answer 1

You are using 5-arg constructor to construct your HireCar instance: –

new HireCar("Toyota", "AV77 FGJ", 6000, 12300, 41500);

And in that constructor, you haven’t set the value for – "carStatus".

 public HireCar(String demoManufacturer, String demoRegNumber, 
         int demoMileage, int demoServiceInterval, int lastInterval) { 
     manufacturer = demoManufacturer;
     regNumber = demoRegNumber;
     mileage = demoMileage;     
 }

So, carStatus is still null. (You should set every field in this constructor. At least the references, because their default value is null)

So, when you invoke the isAvailable method for any of the instance you added in your array: –

fleet[randomNumber].isAvailable() 

It will result in a NPE, as in isAvailable method, you are invoking equals method on carStatus: –

if(carStatus.equals("Available") || carStatus.equals("Return from Service"))
     ^^^
 This is null here