Question

我有这个类创建寄存器和客户端的模拟，并询问完成它们的总时间和平均等待时间。我创建了一个FIFO数组，它由我所拥有的IntQueueImpl类列出的队列组成。现在代码编译好了，但是当我运行它时，我得到一个运行时。

这是班级：

import java.util.Scanner;



public class QueueSimulation

{

public static void main(String[] args)
{

int a, tail, head;
int i = 0;

String s;

Scanner input = new Scanner(System.in);



System.out.println("This programs simulates a queue of customers at registers.");   
System.out.println("We assume the every customer take 4 seconds to leave the register.");
System.out.println("Enter the number of registers you want to simulate:"); 

a = input.nextInt();


while(a==0 || a <0){
System.out.println("0 registers or no registers is invalid. Enter again: ");   
 a = input.nextInt();
}

IntQueueImpl fifo[] = new IntQueueImpl[a];

  System.out.println("Enter how many customers enter per second.For example: 0 0 1 1 2 2 2 3 3.         Enter: ");     
  s = input.nextLine(); 
  s = input.nextLine();   
  String[] parts = s.split(" ");      

  int[] A = new int[parts.length];


  for(int j = 0; j < parts.length; j++){
  A[j]= Integer.parseInt(parts[j]);
  while(A[j] < 0){
  System.out.println(A[j]+" is invalid.Enter positive integer: ");
  A[j]= input.nextInt();
  }
  }

  input.close();



 head = tail = 0;

  int T[] = new int[A.length];

  int temp= 0;

 for(int j = 0 ; j< A.length ; j++){

if(fifo[tail].isEmpty()) fifo[tail].put(A[j] +4);
else{
temp = fifo[tail].size();
fifo[tail].put(A[j] + 4*temp);
}
 if(tail == a-1){
tail=0;
}else tail++;
if(j>=4){

temp = fifo[head].get();
T[j]= j-temp;
if(head == a-1){
head=0;
}else head++;


}

}
int sum = 0;
for(int j= 0; j<T.length; j++){
sum =+ T[j];
}
System.out.println("All customers leaved at: " + sum + " seconds.");



}

}

我得到的运行时错误是：

   Exception in thread "main" java.lang.NullPointerException
    at QueueSimulation.main(QueueSimulation.java:58)

这是命令：

 if(fifo[tail].isEmpty()) fifo[tail].put(A[j] +4);

我已经尝试了很多东西来避免它，但没有任何反应......

Answer 1

有很多地方可以在您的代码中抛出NPE。其中几个是： -

 a = input.nextInt();// input can be null
 String[] parts = s.split(" ");// s can be null
 fifo[tail].isEmpty()) fifo[tail].put(A[j] +4)//fifo[tail] can be null

运行时PointerNullException求解决方案

1 个答案: