java - Arraylist 中的空指针异常

Question

即使我已经放置了空检查，我仍然收到空指针异常:

@Override 
    public ArrayList<DataCache> getData()   
    { 
        if(contentOf != null)
        {
            StoreData data = new StoreData(this);
            if(data!=null)
            {
                ArrayList<DataCache> cacheOf = null;
                System.out.println("Size of ContentOf"+contentOf.size());
                for (int i=0;i<contentOf.size();i++)
                {
                    System.out.println("Value of ContentOf"+contentOf.get(i).mFeed);
                    ArrayList<DataCache> cache = contentOf.get(i).mFeed.getData();
                    if (cache != null)
                        cacheOf.add(cache.get(i));
                }
                return cacheOf;
            }
        }
}

异常(exception):

02-03 10:19:18.770: E/AndroidRuntime(8680): FATAL EXCEPTION: main
02-03 10:19:18.770: E/AndroidRuntime(8680): java.lang.NullPointerException
02-03 10:19:18.770: E/AndroidRuntime(8680): at 
com.activity.MainFragmentActivity.getData(MainFragmentActivity.java:198)

Answer 1

在添加元素之前还需要初始化cacheOf ArrayList:

ArrayList<DataCache> cacheOf = new ArrayList<DataCache>(); //initialize here
System.out.println("Size of ContentOf"+contentOf.size());  //This will be zero.
for (int i=0;i<contentOf.size();i++) {
      //..your code here...
    if (cache != null){
       if(i<=cache.size())
         cacheOf.add(cache.get(i));
     }
}