Problem with generate 3 random digit from 1-10 in method

Question

I have this code

 public int luckyNumbers() {
    for (int i = 1; i <= 3; i++) {
    }
    return (int) (Math.random() * 10);
}

And when i call it in my main class i only get one number. Where I made mistake? That is my method in class. And yeah, i want to generate 3 random numbers with for loop in range 1 - 10. I found some answers but not in Java and not with for loop. Sorry for asking

Answer 1

To explain why your code doesn't return 3 values, whenever you call your method, the for loop runs 3 times but does nothing as there is no code inside the loop. And the return statement calculates a random value and returns that value. So in effect, when you call your luckyNumbers() method once, only one value is returned.

To generate 3 random numbers according to your idea by using a for loop, you can do something like:

public void luckyNumbers() {
    for (int i = 1; i <= 3; i++) {
    //Print random number here
    System.out.print((Math.random() * 10));
    }
}

Another simple way would be to call this luckyNumbers() method n number of times to return n random number from another method.

If your goal is to return 3 lucky numbers at a time, then add the generated random number to an integer array or ArrayList basing on your requirement and return that.

Answer 2

LuckyNumbers is ok so far, but it gives numbers [0..9], so + 1:;

class Test {

  public int luckyNumbers() {
   return (int) (Math.random() * 10) + 1;
  }

..and in the main method typically (with System.out.println()):

  public static void main(String... args) {
    Test testObj = new Test();
    for (int i = 1; i <= 3; i++) {
      System.out.println(testObj.luckyNumbers());
    }
  }
}

EDIT:

Voila (not in main method, but then (why not!?) void):

public void luckyNumbers() {
  for (int i = 1; i <= 3; i++) {
    System.out.println((int) (Math.random() * 10) + 1);
  }
}
public static void main(String... args) {
   new Test().luckyNumbers();
}

EDIT2:

Voila, 3 numbers [1-10]:

public int[] luckyNumbers() {
 return new int[] {
   Math.random() * 10) + 1,
   Math.random() * 10) + 1, 
   Math.random() * 10) + 1
 };
}

EDIT3 (drawLuckyUsers method):

• Input: a List of all users, with individual lucky number [1-10]
• Output: a List of winning users

---

public java.util.List<User> drawLuckyUsers(java.util.List<User> allUsers) {
   //helper structure to map all users by luckyNumber
   java.util.Map<Integer, List<User>> helper = new java.util.TreeMap<>();
   for (User u : allUsers) {
      if(helper.conatins(u.getLuckyNumber())) {
         helper.get(u.getLuckyNumber()).add(u);
      } else {
         List<User> tmp = new ArrayList<>(2);
         tmp.add(u);
         helper.put(u.getLuckyNumber(), tmp);
      }
   }
   java.util.List<User> winner = new java.util.ArrayList<>();
   for(int i = 0; i < 3; i++) {
       winner.addAll(helper.get(luckyNumbers()));
   }
   return winner;
}

with.. luckNumbers() v1:

public int luckyNumbers() {
  return (int) (Math.random() * 10) + 1;
}