a list of 20 numbers,each number generate 1-100,how to make no repitition?

Question

I have looked at a lot of others,i still don't get how to input the non-repeat code into my code below.Newbie here,require someone to explain.

import java.util.Random;

class random{
public static void main(String[] args) {
    Random dice = new Random();
    int number;
    for (int counter = 1; counter <= 20; counter++) {
        number = 1 + dice.nextInt(100);
        System.out.println(number + " ");
    }
}

Answer 1

First add the random number to a set, this will stop duplicate entries. Then loop through and print out the set. There are a couple different ways you could do this but here is a way with the least amount of change to your code:

Set<Integer> numbers = new HashSet<Integer>();
Random dice = new Random();
int number;
for (int counter = 1; counter <= 20;) {
    number = 1 + dice.nextInt(100);
    if(numbers.add(number)) { //Only increment the counter if the number wasn't already in the set
        counter++;
    }
}
for(Integer num : numbers) {
    System.out.println(num + " ");
}

Answer 2

A little bit simpler approach than suggested by MrMadsen is to control the set size:

Set<Integer> numbers = new LinkedHashSet<>();
Random dice = new Random();
while(numbers.size() < 20)
    numbers.add(dice.nextInt(100)+1);
for(Integer num : numbers) {
    System.out.println(num + " ");
}

Just for completeness here's Java-8 solution:

new Random().ints(1, 101).distinct().limit(20).forEach(System.out::println);

Answer 3

instead of just printing the random number have a set(which allows only unique elements) to capture the number and have a if condition to check the length of the set (set.size() == 20) come out of the loop until then you run the for loop to find the random numbers. I think this will solve your problem

Answer 4

Keep track of what is generated previously. Try this

public static void main(String...args) {
        Random dice = new Random();
        Set<Integer> previouslyGenerated = new  HashSet<Integer>();
        int number = 0,counter = 0;      
        while (counter <20){
            number = 1+dice.nextInt(100);
            if (!previouslyGenerated.contains(number)){
                previouslyGenerated.add(number);
                System.out.println(number + " ");
                counter++;
            }       

        }       

}