＆＃34;显示java.lang.NullPointerException＃34;错误

Question

我收到错误“java.lang.NullPointerException”，BlueJ似乎无法告诉我错误的含义。下面是出现错误的代码：

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

jobList或jobList.get(count)为null。如果您尝试在null上调用某个方法（在您的情况下为size()或isCompleted()），那么您就会得到NullPointerException。

Answer 2

试试这个，代码更少，只有jobList可以为null

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

return count;

Answer 3

您的代码很难理解。永远不会使用变量温度？

这样，如果作业完成，则不执行count + 1而且while不会结束？

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

检查这是否有帮助：

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;
}