尝试使用通过Iterator检索的对象时获取NullPointerException [英] Getting NullPointerException when trying to use an object retrieved using Iterator

问题描述

这是我的代码，它将元素添加到ArrayList

I this is my code which adds elements to ArrayList

if(countries==null){
  countries = new ArrayList();
  for(int x = 0; x < data.getRowCount(); x++)
  {
      String shortName = data.getString(x,0);
      String code = data.getString(x,1);
      countries.add(x, new Country(code, shortName));
  }
}

以下是迭代器代码

for( Iterator iter = countries.iterator(); iter.hasNext();)
{
   Country country = (Country)iter.next();
   Element optionselect = doc.createElement( "countryselect" );
   optionselect.setAttribute( "name", country.getShortName() );//Getting NULL Pointer Exception
   optionselect.setAttribute( "value", country.getCode() );
   countriesElement.appendChild( optionselect );
}

现在我在行上得到空指针异常：

Now I am getting null pointer exception at line:

optionselect.setAttribute( "name", country.getShortName() );

PS：由于它在生产环境中运行，因此无法对其进行调试，因此无法在本地服务器上进行复制

PS: I cannot debug it as it is running on production and i cannot replicate it on local server

乍一看，它似乎在ArrayList中有一些null，但是从代码中我无法确定这是怎么可能的。

From the first look it looks like there is some value null in the ArrayList, but from code I cannot make out how it is possible.

有人可以给它一些启发（也许是Java 5的一个错误）。

Can someone shed some light on it(maybe a bug of Java 5).

编辑：我也用此代码得到了空指针

I am also getting null pointer with this code

List countryCodes = new ArrayList();
for (int x = 0; x < countries.size(); x++)
{
    Country c = (Country) countries.get(x);
    countryCodes.add(x, c.getCode());// Throwing Exception
}

这确认了一件事，在国家/地区列表中有一个 null 对象，代码我看不到这怎么可能

This confirm one thing that there is somewhere null object in the countries List, but looking at the code I cannot see how is that possible

堆栈跟踪如下：

Stack Trace:java.lang.NullPointerException
    at com.ilrn.controller.modules.Users.appendCountryList(Users.java:985)
    at com.ilrn.controller.modules.Users.setupCountries(Users.java:1348)
    at com.ilrn.controller.modules.Users.gen_add(Users.java:1329)
    at com.ilrn.controller.modules.Users.generate(Users.java:190)
    at com.ilrn.controller.ShellModule.generateShell(ShellModule.java:1958)

Users.java:985是 optionselect.setAttribute（ name，country.getShortName（））;

Users.java:985 is optionselect.setAttribute( "name", country.getShortName() );

Stack Trace(number 2):java.lang.NullPointerException
    at com.ilrn.controller.dto.IlrnCountriesDTO.getCountryCodes(IlrnCountriesDTO.java:45)
    at sun.reflect.GeneratedMethodAccessor471.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:592)

其中IlrnCountriesDTO.java:45是 countryCodes.add（x，c.getCode（））;

Where IlrnCountriesDTO.java:45 is countryCodes.add(x, c.getCode());

也请注意该国家/地区列表仅加载一次（在首次请求且国家/地区为私有时）

Also please note that countries List is loaded only once(when requested first time and countries is private)

经过分析，我认为这是一个多线程问题。看完ArrayList代码后，我发现它不是同步的（因此对于相同的索引，大小可能增加了两次）

After analyzing I think this is a multi threading issue. After looking at the ArrayList code I see it is not synchronized(so maybe size was incremented two time for same index)

 public void add(int index, E element) {
    if (index > size || index < 0)
      throw new IndexOutOfBoundsException(
      "Index: "+index+", Size: "+size);
  ensureCapacity(size+1);  // Increments modCount!!
  System.arraycopy(elementData, index, elementData, index + 1,
         size - index);
  elementData[index] = element;
  size++;
 }

这是国家/地区类别（片段）

this is the country class(snipet)

private String code_;
private String shortName_;

/**
 * Constructor sets code and short name.
 * 
 * @param code Specifies the code.
 * @param shortName Specifies the short name.
 */
public Country(String code, String shortName)
{
    code_ = code;
    shortName_ = shortName;
}
public String getCode() {
        return code_;
}

public String getShortName() {
        return shortName_;
}

推荐答案

您添加的代码看起来有点off ...您有

Your adding code looks a little off... You have

if (countries != null){
    countries = new ArrayList();
    ...

但不应该这样：

if (countries == null){
    countries = new ArrayList();

通常，我们初始化是否为空，否则我们会吹走已经存在的任何东西

Normally, we initialise if something is null, otherwise we'd be blowing away anything that was already there.

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