Question

我已经关注了nullpointerexception的相关链接，例如：

How to solve java.lang.NullPointerException error?

What is a NullPointerException, and how do I fix it?

我从代码中排除了大部分的nullpointer异常，但我无法理解跟随nullpointer异常的原因。

 Exception in thread "main" java.lang.NullPointerException
      at Calculate.main(Calculate.java:9)

我在调试模式下运行并看到＆＃34; sc.items.add（cd）＆＃34; （Calculate.java第9行）;一片空白！任何人都可以解释原因。

我正在实施访客模式。我正在计算不同物品的成本。一件是CD。以下是代码：

ShoppingCart.java

   import java.util.ArrayList;

   public class ShoppingCart {

     ArrayList<Visitable> items;

     public double calculatePostage(){
        //Create a visitor

        PostageVisitor visitor = new PostageVisitor();

     // iterate through all items
          for( Visitable item: items){

             item.accept(visitor);
         }

        double postage = visitor.getTotalPostage();

        return postage;

     }
 }

Calculate.java

 public class Calculate {

      public static void main(String[] args) {
          // TODO Auto-generated method stub
        ShoppingCart sc = new ShoppingCart();

        CD cd = new CD(100);
        sc.items.add(cd);   
       double postageInMain =  sc.calculatePostage(); 
       System.out.println(postageInMain);

      }

}

CD.java

    public class CD implements Visitable {

         private double price;

         @Override
         public void accept(Visitor visitor) {
            // TODO Auto-generated method stub

     }
      public CD(double price){
            this.price = price;
     }    
     double getPrice(){
          return price;
     }

 }

PostageVisitor.java

    public class PostageVisitor implements Visitor {
      private double totalPostageForCart;



      public void visit (CD cd){

            totalPostageForCart += cd.getPrice(); 
       }

   public void visit (DVD dvd){

       totalPostageForCart += dvd.getPrice();
  }
    // return the internal state
    public double getTotalPostage(){

         return totalPostageForCart;

    }

 }

Visitable.java

  public interface Visitable {
    public void accept(Visitor visitor);
 }

Visitor.java

 public interface Visitor {
     public  void visit(Book book);


    public void visit(CD cd);


}

Answer 1

类items中的字段ShoppingCart未使用ArrayList对象初始化，这就是您获得NPE的原因。

import java.util.ArrayList;

   public class ShoppingCart {

     ArrayList<Visitable> items = new ArrayList<>(); // <---- this will solve the problem

     public double calculatePostage(){
        //Create a visitor

        PostageVisitor visitor = new PostageVisitor();

     // iterate through all items
          for( Visitable item: items){

             item.accept(visitor);
         }

        double postage = visitor.getTotalPostage();

        return postage;

     }
 }

另外，我想提一下，在接口方法中你真的不需要public修饰符，它只是多余的。

Answer 2

将ShoppingCart.java文件ArrayList<Visitable> items;代码更改为

ArrayList<Visitable> items;
items = new ArrayList<>();

Java中的空指针异常（实现访问者模式）

2 个答案: