java - 我收到 java.lang.NullPointerException : null error and I don't know what is missing

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

 public void FillArray()
{
    for (int i = 1; i < numEmp; i++)
    {
        employees[i] = true;
    }
}

我认为这部分可能是我收到 NullPointerException 错误的原因，但我不知道我错过了什么。我已将程序的完整代码放在下面，以防出现错误而导致错误消息。

//客户端类

public class Downsize
{
    public static void main (String [] args)
    {
        System.out.print("Do you want to downsize the company? (Y/N): ");
        String dummy = APIO.getString().toUpperCase();        
        while (dummy.equals("Y"))
        {
            Employee employee = new Employee();
            System.out.print("Do you want to downsize the company? (Y/N): 
            dummy = APIO.getString().toUpperCase();
        }
    }
}  

//对象类

 public class Employee
    {
        int numEmp;
        int sprayer;
        int winner;
        boolean [] employees;

        public Employee()
        {
            System.out.print("How many employees? (0 to end): ");
            int numEmp = APIO.getInt();
            System.out.print("Who gets the spray can first?: ");
            int sprayer = APIO.getInt();
            FillArray();
            Selection();
            Winner();
        }

        public void FillArray()
        {
            for (int i = 1; i < numEmp; i++)
            {
                employees[i] = true;
            }
        }

        public void Selection()
        {        
            System.out.println("EM="); //debugging method
            for (boolean em: employees)
            {
                System.out.println(em);
            }

            int complete = numEmp;
            while (complete > 1)
            {
                System.out.print("spraycan passed to #" + sprayer);
                if ((sprayer + 1) > numEmp)
                {
                    sprayer = 0;
                }
                while (employees[sprayer + 1] == false)
                {
                    sprayer++;
                    if (sprayer >= numEmp)
                    {
                        sprayer = 0;
                    }
                }
                employees[sprayer + 1] = false;
                System.out.print("  -  sprays #" + (sprayer + 1) + "'s hair");
                complete--;
                sprayer++;
                while (employees[sprayer] == false)
                {
                    sprayer++;
                    if (sprayer > numEmp)
                    {
                        sprayer = 1;
                    }
                }
            }
        }

        public void Winner()
        {
            if (sprayer == 0)
            {
                sprayer = 1;
                System.out.print("\nThe Winner is #" + sprayer);
                System.out.print("\n");
            }
        }
    }

//错误消息的堆栈跟踪

java.lang.NullPointerException
    at Employee.FillArray(Employee.java:22)
    at Employee.<init>(Employee.java:14)
    at Downsize.main(Downsize.java:9)

Answer 1

您必须初始化employees 数组。读入员 worker 数后，执行 employees = new boolean[numEmp];(在 int numEmp = APIO.getInt(); 行之后)。否则，它为 null，因此尝试访问 employees[i] 会抛出 NullPointerException。