So I am doing a program that works by having inputing a price and then using that price along with set values to do a calculation across two child classes that inherit from a superclass where a class is called there which is where the price is kept the issue I have is that whenever I call upon the payment calculation method I get a NullPointerException

package Lab1;
import java.util.Scanner;

public class main{
    public static void main(String[] args){
        Car c = new Car("model",1998, 123);
        ThirdPartyPolicy tp = new ThirdPartyPolicy("hello");
        ComprehensivePolicy comp = new ComprehensivePolicy(2,31);
        
        Scanner scan = new Scanner(System.in);
        System.out.println("Input price of car");
        c.price= scan.nextDouble(); 
       
        tp.printtp();
        comp.printcp();
         comp.calcPayment();
    }
}

 class Car{
     String model;
     int ManufacturingYear;
     double price;

     //constructor
     public Car(String model, int ManufacturingYear, int price){
     this.model=model;
     this.ManufacturingYear=ManufacturingYear;
     this.price=0;
     
     }
     enum CarType {
         SUV,
         SED,
         LUX,
         HATCH,
         ETC,
     }
    public double getprice(){
        return price;
    }
}

abstract class InsurancePolicies{
    String policyHolderName;
    int id;
    Car car;
   
    int numberofclaims = 1;
    double premium;
    
    int flatRate = 150;

    
    double calcPayment(){
    this.premium = ((car.price/100) (numberofclaims*200) flatRate);
    return premium;
    }
 
    public void print(){
    System.out.println(premium);
    }
   
}




class ThirdPartyPolicy extends InsurancePolicies{
    String comments;
    public ThirdPartyPolicy(String comments){
     this.comments=comments;
     }
    @Override
    public double calcPayment(){
    this.premium = ((car.price/100) (numberofclaims*200) flatRate);
    System.out.println( premium);
    return premium;
    }
    
    public void printtp(){
        super.print();
    
   
    
    
}
}
class ComprehensivePolicy extends InsurancePolicies{
int driverage;
int level;
public ComprehensivePolicy(int level, int driverage){
       
     this.level=level;
     this.driverage=driverage;
     
     }


    public Car getCar() {
        return car;
    }
    @Override
    double calcPayment(){
    this.premium = ((car.price/100) (numberofclaims*200) flatRate);
    if (driverage<30){
        this.premium  = ((30-driverage)*50);
        return premium;
    }
    return premium;
    }

    public void printcp(){
        super.print();

    }
}

If anyone can help me resolve this issue that would be appreciated

CodePudding user response：

InsurancePolicieshave a Car variable that is never initialized, so it's null by default. You should inform the car via polices' constructor or some kind of set method, before call de calculation.

CodePudding user response：

I pasted your code and ran it locally. When prompted, I entered "100". Here's the entire output:

Input price of car
100
0.0
0.0
Exception in thread "main" java.lang.NullPointerException: Cannot read field "price" because "this.car" is null
    at ComprehensivePolicy.calcPayment(main.java:102)
    at main.main(main.java:17)

Why did that fail? Because Car car; is declared in the abstract class, and code tries to use it (car.price) without first assigning a value or creating a Car object. So, car is null, and calling anything on it (like "car.price") will throw NullPointerException.

Here's a very simple fix to get it working: right after creating a new "ThirdPartyPolicy", assign the car you created a few lines earlier:

Car c = new Car("model", 1998, 123);
...
ThirdPartyPolicy tp = new ThirdPartyPolicy("hello");
tp.car = c;

After doing that, you'll see another exception which looks identical to the first. It's due to the shared code in the abstract class, again with null "car". Here's a similar simple fix to get it working.

ComprehensivePolicy comp = new ComprehensivePolicy(2, 31);
comp.car = c;

After adding both of those lines, the program runs without error.

Here's a cleaner way of making things work.

1. Modify the "ThirdPartyPolicy" constructor to require a Car parameter, and save the reference (this.car = car):

   public ThirdPartyPolicy(Car car, String comments) {
       this.comments = comments;
       this.car = car;
   }
   

2. Similar change for "ComprehensivePolicy":

   public ComprehensivePolicy(Car car, int level, int driverage) {
       this.car = car;
       this.level = level;
       this.driverage = driverage;
   }
   

3. Pass the "car" reference to each of the constructors:

   Car car = new Car("model", 1998, 123);
   ThirdPartyPolicy tp = new ThirdPartyPolicy(car, "hello");
   ComprehensivePolicy comp = new ComprehensivePolicy(car, 2, 31);