如何在给定的时间内将随机生成的数字保持相同？

Question

What I want to do is generate a random number that determines a color. This color changes every time count is a multiple of 3. When count is 0, 1, 2, the color should stay the same. It should then change at 3.

count changes every time that the method printColor() is run.

I've tried doing this as you can see below, but I keep getting a NullPointerException. I cannot generate the randomNumber in the field or in a constructor as that is outside of the constraint of requirements.

import java.awt.*;
import java.util.*;
public class ColorGenerator {

private int count = 1;
private Random rand;
private int randomNumber;

public Color getColor(){
    Color lionColor = Color.RED;
    if (count % 3 == 0 || count == 1){
        randomNumber = rand.nextInt(3) + 1;     
    }
    if (randomNumber == 1){
        lionColor = Color.RED;
    } else if (randomNumber == 2){          
        lionColor = Color.BLUE;
    } else {
        lionColor = Color.GREEN;
    }
    return lionColor;
}

public void printColor(Color color){
     count++;
     System.out.println(color.toString());
}

}

Answer 1

You need to initialize rand . Change

private Random rand;

to

private Random rand = new Random();

Note that since you start at 1, there will only be 2 consecutive colors the first run. I recommend you to start count at 0 and drop the || count == 1 || count == 1 (since when count = 0, count % 3 == 0 will hold).