“ java.lang.NullPointerException”错误

Question

I'm getting an error "java.lang.NullPointerException" and BlueJ cannot seem to tell me what the error means. Below is the code in which the error is appearing:

public int getJobsWaiting()
{
    int count = 0;
    int i = 0;

    while (count < jobList.size())
    {
        Job temp = jobList.get(i);

        if(jobList.get(count).isCompleted() != true)
        {
            count = count + 1;
        }
        i = i + 1;
    }
    return count;
}

Answer 1

either jobList or jobList.get(count) is null . And if you try to invoke a method on null (in your case: size() or isCompleted() ), you get exactly that, a NullPointerException .

Answer 2

try this, it is less code and only jobList can be null

int count = 0;
for (Job tmpJob : jobList) {
  if (!tmpJob.isCompleted())
  count++;
}

return count;

Answer 3

Your code is difficult to understand. Variable temp is never used?

With this, if a job is completed, count+1 is not executed and the while is never ended?

if(jobList.get(count).isCompleted() != true)
{
   count = count + 1;
}

Check if this can help:

public int getJobsWaiting(List<Job> jobList) {
    int count = 0;
    for (int i = 0; i < jobList.size(); i++) {
        Job temp = jobList.get(i);
        if (temp != null && !temp.isCompleted()) {
            count++;
        }
    }
    return count;
}