正在获取NullPointerException，但找不到源

Question

我有这段MapReduce代码。它说我的cleanup()中有一个NullPointerException，但我似乎能弄清楚它在哪里。我甚至添加了一个while循环来避免null值，但这似乎不起作用。这是一项MapReduce工作，但基本原理保持不变。我会描述我的数据，但我不确定它是否与这个问题有关。

@SuppressWarnings("deprecation")
public class xvaluesMapper extends Mapper<LongWritable, Text, Text, Text> 
{
public Map<String, Long> snpLocations = new LinkedHashMap<String, Long>();
public Map<Long, String> nList = new LinkedHashMap<Long, String>(); 
public ArrayList<Long> l = new ArrayList<Long>();
public void setup(Context context) throws IOException, InterruptedException 
{
    super.setup(context);
    URI [] SNPLocation =context.getCacheFiles();
    if(SNPLocation.length == 0)
        throw new FileNotFoundException("Distributed Cache file not found.");
    File localFile = new File(SNPLocation[0]);
    FileInputStream f = new FileInputStream(localFile);

    int lineStream;
    while((lineStream = f.read()) != -1)
    {
        String strLine = String.valueOf(lineStream);// line;
        String[] tokens = strLine.toString().split("\t");
        String SNPID =  tokens[1];
        long location = Long.parseLong(tokens[3]);
        snpLocations.put(SNPID, location);  
    }
    f.close();
    l.addAll(snpLocations.values());


}
public void map(LongWritable key, Text values, Context context)
{
    String [] value = values.toString().split("\s");
    long position=0;
    for(String s: value)
    {
        ++position;
        nList.put(position, s);
    }
}
public void cleanup(Context context) throws IOException, InterruptedException
{
    while(l != null){
    for (long nPosition: l)
        {
        long actualPos = nPosition+7;
        long secActualPos = actualPos+1;
        //I used 7 because I am assuming the count for the nucleotide position starts from one instead of zero. Also There are 6 other variables like familyID, individual ID....etc
        //in the data set. The "secActualPos" is to account for the second pair.
        String twoSNPphenoTindID = nList.get(actualPos).toString() + "," + nList.get(secActualPos).toString()+  "," + nList.get(5).toString() + "," + nList.get(1);
        for(Entry<String, Long> entryPos: snpLocations.entrySet())
        {
            if(entryPos.getValue().equals(nPosition))

            context.write(new Text(String.valueOf(actualPos)), new Text(twoSNPphenoTindID));
        }
    }
};

}}

Answer 1

l是ArrayList<Long>。这意味着它可以容纳具有null值的对象。当您执行for (long nPosition: l)时，您正在对所有元素进行迭代，但它也在调用ArrayList中Long对象的方法longValue()。如果有一个对象的值为null，则会有一个NullPointerException。和这个是一样的。

Long element = null;
long a = element;

这抛出了一个NullPointerException，因为当你试图将Long赋给一个基元Long时，它实际上是在尝试执行这个

long a = element.longValue()

因为元素是null，所以不能引用null对象的方法。

使用for (Long nPosition: l)而不是for (long nPosition: l)，然后像这个一样在循环内检查nPosition的null值

for (Long nPosition: l)
    {
    if(nPosition == null)
        continue;
    long actualPos = nPosition+7;
    long secActualPos = actualPos+1;
    //I used 7 because I am assuming the count for the nucleotide position starts from one instead of zero. Also There are 6 other variables like familyID, individual ID....etc
    //in the data set. The "secActualPos" is to account for the second pair.
    String twoSNPphenoTindID = nList.get(actualPos).toString() + "," + nList.get(secActualPos).toString()+  "," + nList.get(5).toString() + "," + nList.get(1);
    for(Entry<String, Long> entryPos: snpLocations.entrySet())
    {
        if(entryPos.getValue().equals(nPosition))

        context.write(new Text(String.valueOf(actualPos)), new Text(twoSNPphenoTindID));
    }