NullPointerException异常在while循环试图添加新的Class实例ArrayList的时候 [英] NullPointerException in while loop when trying to add new Class instances to ArrayList

问题描述

我越google一下，我发现了更多的困惑。

我把在从CSV其他一些细节，然后我需要转成Person对象并存储在一个名为名单的人是该类俱乐部实例变量，一个未知长度的名称列表其成员基本上是列表。

这是一个更复杂的东西，我需要做的，我需要通过文件创建的每一行对象，然后我需要添加到列表集合while循环非常简化的版本。

当我运行我的code，虽然我很为难如何避免它，我不断收到NullPointerException错误。我猜，我的变量p当我创建新的对象都需要更改每个循环，但我不认为这是有可能改变的变量动态是什么呢？

没想到我怎么能每次都提交对象集合了有效的非空引用。将是任何帮助非常感激。我试图削减了所有不必要的东西在下面的code。

感谢您

 类俱乐部//类ArrayList实例变量
   私人的ArrayList＆LT;＆人GT;人;   //类构造函数俱乐部
   市民俱乐部（）
   {列表与LT;人＆GT;人=新的ArrayList＆LT;＆GT;（）;}   公共无效readInClubMembers（）
   {
      //有关文件输入一些变量
      串currentLine;
      字符串名称;
      串ageGroup;
      而（bufferedScanner.hasNextLine（））
      {
         //一些线将在扫描器输入从文件
         名称= lineScanner.next（）;
         ageGroup =年轻;
         人p =新的Person（）; //我想事情会错在这里
         people.add（P）;
         p.setName（名）;
         p.setAgeGroup（ageGroup）;
      }
   }
 

解决方案

删除列表＆LT;人＆GT; 在人= ... 在构造函数中，否则，您声明一个新的局部变量人构造阴影中的字段的里面的人（这是后不再使用）。这使得类字段未初始化（空），然后导致NPE。

您想取而代之的是初始化领域是什么人：

 公开俱乐部（）{
    //你也可以使用this.people = ...要明确
    人=新的ArrayList＆LT;＆GT;（）;
}
 

以示区别：

 类示例{
    私人诠释mynumber的;    公共示例（）{
        为mynumber = 42; //设置类领域
        INT mynumber的= 1337; //声明了一个新的局部变量阴影类领域
        为mynumber = -13; //访问在最接近范围的对象，它是局部变量
        this.myNumber = 0; //明确地访问类的字段
    }
}
 

The more I google this the more confused I'm getting.

I'm bringing in a list of names of unknown length with some other details in from a CSV which I then need to turn into Person objects and store in a list called people which is the instance variable for the class Club, a list of its members basically.

This is a very simplified version of something more complex I need to do in which I need to while loop through a file creating objects for each line which I then need to add to a list collection.

I keep getting a nullPointerException error when I run my code though and I'm stumped how to avoid it. I'm guessing that my variable p when I create the new object would need to change on each loop but I don't think it's possible to change variables dynamically is it?

Can't think how I can commit the object to the collection with a valid non null reference each time. Would be very grateful for any help. I've tried to cut out all the unnecessary stuff in the code below.

Thank you

   //class arraylist instance variable of class "Club"
   private ArrayList<Person> people;

   //class constructor for Club
   public Club()
   {List<Person> people = new ArrayList<>();}

   public void readInClubMembers()
   {
      //some variables concerning the file input
      String currentLine;
      String name;
      String ageGroup;
      while (bufferedScanner.hasNextLine())
      {
         //some lines bringing in the scanner input from the file
         name = lineScanner.next();
         ageGroup = "young";
         Person p = new Person(); // i guess things are going wrong around here
         people.add(p);
         p.setName(name);
         p.setAgeGroup(ageGroup);
      }
   }

解决方案

Remove the List<Person> before people = … inside the constructor, otherwise you are declaring a new local variable people inside the constructor shadowing the field people (which is then never used). This leaves the class field uninitialized (null) and then causes the NPE.

What you want instead is initializing the field people:

public Club() {
    // you can also use "this.people = …" to be explicit
    people = new ArrayList<>();
}

To show the difference:

class Example {
    private int myNumber;

    public Example() {
        myNumber = 42; // sets the field of the class
        int myNumber = 1337; // declares a new local variable shadowing the class field
        myNumber = -13; // accesses the object in the closest scope which is the local variable
        this.myNumber = 0; // explicitly accesses the class field
    }
}

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