Math.random在代码中无法正常工作

Question

package arrays;
import java.util.Scanner;

public class Raffle {
    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        System.out.println("Enter the number of partcipants ");
        int no=scan.nextInt();
        String participants[]=new String[no];
        System.out.println("Please enter their names and check out the random winner");
        for(int i=0;i<no;i++) {
            participants[i]=scan.next();
        }
        int rand=(int)Math.random()*no;
        System.out.println(rand);
        System.out.println("The winner of the raffle is "+participants[rand]);
    }
}

I used Math.random() but I see during run that always the 0 index position is giving the result and not the random value. Any suggestions? I am on eclipse oxygen.

Answer 1

You are casting the result of Math.random() to int prior to multiplying it by no , which results in 0 , since Math.random() < 1.0 .

Change

int rand=(int)Math.random()*no;

to

int rand=(int)(Math.random()*no);

Answer 2

Casting a double between 0.0 (inclusive) and 1.0 (exclusive) will always round down to zero. A Math.random() value should first be multiplied with some factor before casting it to an int to get a value between zero (inclusive) and the multiplication factor (exclusive). Another possible solution would be to use the nextInt() method of java.util.Random.