类测试期间发生NullPointerException

因此，我创建了一个应该创建模具的模具类，然后允许我滚动它们，拉出它们的值和边数。我现在的问题是，由于抛出NullPointerException，我无法在测试过程中掷骰子。

这是模具类的代码：

package com.catalyse.die;

import java.util.Random;

public class Die
{
    // instance variables 
    private static int dieNum = 0;
    private int myDieValue;
    private int myDieSides;
    private Random rand;

    // Dice Class Constructors


    public Die()
    {
        dieNum++;
        this.myDieValue = 1;
        this.myDieSides = 4;
    }


    public Die(int numSides)
    {
        if ((numSides < 4) || (numSides > 100)) {
            System.out.println("Error! You cannot have more than 100 sides or less than four!");
            System.exit(0);
        }
        else {
            myDieSides = numSides;
            myDieValue = 1;
        }
    }

    // getter methods


    public int getDieSides()
    {
        System.out.println(myDieSides);
        return 0;
    }


    public int getDieValue()
    {
        System.out.println(myDieValue);
        return 0;
    }

    // setter methods

    private void setDieSides(int newNumSides)
    {
        myDieSides = newNumSides;
    }

    public void rollDie()
    {
        myDieValue = (rand.nextInt(myDieSides) + 1);
    }

    // other methods

    public void printDie(int dieNum)
    {
        if (dieNum == 1) {
            System.out.println("Die Value: "+myDieValue);
        }
        else {
            System.out.println("Die "+dieNum+" Value: "+myDieValue);
        }
    }
}

这是测试课程。

package com.catalyse.die;

public class TestDieClass
{
    public static void main(String [] args)
    {
        Die One = new Die();
        Die Two = new Die(50);
        Die Three = new Die(99);

        One.getDieSides();
        One.getDieValue();

        Two.getDieSides();
        Two.getDieValue();

        Three.getDieSides();
        Three.getDieValue();

        One.rollDie();
        Two.rollDie();
        Three.rollDie();

        One.getDieValue();
        Two.getDieValue();
        Three.getDieValue();
    }
}

这是错误

4
1
50
1
99
1
Exception in thread "main" java.lang.NullPointerException
    at com.catalyse.die.Die.rollDie(Die.java:83)
    at com.catalyse.die.TestDieClass.main(TestDieClass.java:27)