我该怎么做，以便如果用户输入0，输出线说您不能在Java中除以零

我试图没有用户输入为" a" 0，因为这会导致方程除以零，输出错误，我将能够拥有println说，如果用户输入0的" a"是我的代码，我不确定我在做什么，我仍然是新手，只需要帮助：

import java.util.Scanner;
class a3main{
    public static void main(String[] args){
    Scanner keyboard = new Scanner(System.in);
    System.out.println("Enter the value of a");
    int a = keyboard.nextInt();
    System.out.println("Enter the value of b");
    int b = keyboard.nextInt();
    System.out.println("Enter the value of c");
    int c = keyboard.nextInt();
    double R1, R2, dis;
    dis = b * b - 4 * a * c; // This is the discriminant formula
    if(dis > 0 )
    {
        System.out.println("The roots are both real numbers and unequal");
        R1 = (-b + Math.sqrt(dis))/(2*a);
        System.out.println("The first root is: " + R1);
        R2 = (-b - Math.sqrt(dis))/(2*a);
        System.out.println("The second root is: " + R2);
        }
        else if(dis == 0)
        {   
         System.out.println("The roots are both equal and are equal");
         R1 = (-b + Math.sqrt(dis))/(2*a);
         System.out.println("The root is: " + R1);
                }    
         else if (a == 0)
    {
         System.out.println("You cannot divide by 0");
    }   
             else 
             {
       System.out.println("The roots are imaginary");
                     }
    }
}