Generating Unique Random Number Upto A Limit In Java

Here we will try to figure out a way to generate unique random number within a upper limit .

The problem with general random number generation is there is a chance of collision but we want to return unique random numbers each time a client call us , this is one of the solution which is easy to understand .

import java.util.*;

public class GenerateRandom {
    public static int MAX = 100;

    Set<Integer> numberSet = new HashSet<>();




    public Integer generateRandomNumber(){
        Random random = new Random();
        if(numberSet.size() == 100){
            return -1;
        }
        else{
            int temp = random.nextInt(MAX) ;
            while(numberSet.contains(temp)){
                temp = random.nextInt(MAX);
            }
            numberSet.add(temp);
            return temp;
        }
    }
}

here , we are maintaining a hash set of the numbers we have generated and checking each time for the generated number if it is present in the set or not , and we keep on doing it till we get the unique number .

here , we have implemented with a upper limit of 100 .

P.S :- There is a drawback to this method as there will be less numbers left , there will be more collisions , hence high response time .

I will update with a better algorithm to do so.