Question

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

我认为这部分可能是我得到NullPointerException错误的原因，但我不知道我错过了什么。我已经为下面的程序提供了我的完整代码，以防万一有什么问题导致我发出错误信息。

//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。读完员工人数后，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。

Answer 2

我看到你们在@ mook的回答中喋喋不休，所以让我澄清一下，以便OP能够理解它而不是猜测。

class Example {

    int size;
    boolean[] array;

    void initArray() {

        size = 5;
        array = new boolean[size];
    }
}

在声明数组的大小及其类型后，您需要初始化数组。在上面的示例中，array包含5个布尔值。

如果你写了

class Example {

    int size;
    boolean[] array = new boolean[size];;

    void initArray() {

        size = 5;
    }
}

然后，由于默认情况下int设置为0，即使您在代码中稍后更改用于声明大小的变量，array的大小也将为0。这将给你一个错误，因为你将迭代到5。

@ mook的回答是正确的。

我收到了java.lang.NullPointerException：null错误，我不知道遗漏了什么

2 个答案: