无法弄清楚为什么我得到一个nullpointerexception

Question

Q1：运行此命令时，我得到了nullpointerexception。 当我在Hamburger中评论AdditionalItems（）并将其放在HealthyBurger中时，它运行良好，所以我真的不知道我该怎么做。

Q2：有什么更好的方法让extraItems（）不再重复4次？

public class Hamburger {




    public Hamburger(String breadType, String meatType, int price){

        this.breadType = breadType;
        this.meat = meatType;
        this.price = price;

      additionalItems();
    }




    public void additionalItems(){

        System.out.println("Would you like lettuce/carrot/cheese/tomato? (y/n)x4");
        String s = scanner.next();
        if(s.equalsIgnoreCase("y")){
            this.setLettuce(true);
            this.setPrice(price+2);
            System.out.println("Lettuce added! Current Price: " + this.price);
        } else {            System.out.println("did not add ");
        }


           ......

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package com.company;
import java.util.Scanner;

public class HealthyBurger extends Hamburger{

    private boolean salad;
    private boolean corn;



    Scanner scanner2 = new Scanner(System.in);

    public HealthyBurger(){
        super("brown roll", "red", 7);

     // this.additionalItems();
    }

    public void additionalItems(){

        System.out.println("would you like salad to your healthyburger? y/n");
        String x2 = scanner2.next();
        if(x2.equalsIgnoreCase("y")){
            this.setSalad(true);
            System.out.println("Added salad to your burger!");
            this.setPrice(this.getPrice() + 2);
        }else {
            System.out.println("did not add salad");
        }
             ........

    }