Non-repeating random numbers inside array JAVA

You can generate numbers from, say, 1 to 6 (see below for another solution) then do a Collections.shuffle to shuffle your numbers.

    final List<Integer> l = new ArrayList<Integer>();
    for (int j = 1; j < 7; j++ ) {
        l.add( j );
    }
    Collections.shuffle( l );

By doing this you'll end up with a randomized list of numbers from 1 to 6 without having twice the same number.

If we decompose the solution, first you have this, which really just create a list of six numbers:

    final List<Integer> l = new ArrayList<Integer>();
    for (int j = 1; j < 7; j++ ) {
        l.add( j );
    }

So at this point you have the list 1-2-3-4-5-6 you mentioned in your question. You're guaranteed that these numbers are non-repeating.

Then you simply shuffle / randomize that list by swapping each element at least once with another element. This is what the Collections.shuffle method does.

The solutions that you suggested isn't going to be very efficient: depending on how big your list of numbers is and on your range, you may have a very high probability of having duplicate numbers. In that case constantly re-trying to generate a new list will be slow. Moreover any other solution suggesting to check if the list already contains a number to prevent duplicate or to use a set is going to be slow if you have a long list of consecutive number (say a list of 100 000 numbers from 1 to 100 000): you'd constantly be trying to randomly generate numbers which haven't been generated yet and you'd have more and more collisions as your list of numbers grows.

If you do not want to use Collections.shuffle (for example for learning purpose), you may still want to use the same idea: first create your list of numbers by making sure there aren't any duplicates and then do a for loop which randomly swap two elements of your list. You may want to look at the source code of the Collections.shuffle method which does shuffle in a correct manner.

EDIT It's not very clear what the properties of your "random numbers" have to be. If you don't want them incremental from 1 to 6, you could do something like this:

final Random r = new Random();
final List<Integer> l = new ArrayList<Integer>();
for (int j = 0; j < 6; j++ ) {
    final int prev = j == 0 ? 0 : l.get(l.size() - 1);
    l.add( prev + 1 + r.nextInt(42) );
}
Collections.shuffle( l );

Note that by changing r.nextInt(42) to r.nextInt(1) you'll effectively get non-repeating numbers from 1 to 6.

You have to check if the number already exist, you could easily do that by putting your numbers in a List, so you have access to the method contains. If you insist on using an array then you could make a loop which checks if the number is already in the array.

Using ArrayList:

ArrayList numbers = new ArrayList();

while(numbers.size() < 6) {
    int random = numGen(); //this is your method to return a random int

    if(!numbers.contains(random))
        numbers.add(random);
}

Using array:

    int[] numbers = new int[6];

    for (int i = 0; i < numbers.length; i++) {
        int random = 0;

        /*
         * This line executes an empty while until numGen returns a number
         * that is not in the array numbers yet, and assigns it to random
         */
        while (contains(numbers, random = numGen()))
            ;


        numbers[i] = random;
    }

And add this method somewhere as its used in the snippet above

private static boolean contains(int[] numbers, int num) {
    for (int i = 0; i < numbers.length; i++) {
        if (numbers[i] == num) {
            return true;
        }
    }
    return false;
}

Use List instead of array and List#contains to check if number is repeated.

you can use a boolean in a while loop to identify duplicates and regenerate

int[] array = new int[10]; // array of length 10

Random rand = new Random(); 

    for (int i = 0 ; i < array.length ; i ++ ) {

        array[i] = rand.nextInt(20)+1;  // random 1-20

        boolean found = true;

        while (found) { 

        found = false; 
  // if we do not find true throughout the loop it will break (no duplicates)

         int check = array[i]; // check for duplicate 

             for (int j = 0 ; j < i ; j ++) {

                 if ( array[j] == check ) {

                 found = true; // found duplicate                   
                 }
             }          

              if (found) {  
                    array[i] = rand.nextInt(20)+1 ;   // replace    
              }
        }
    }

System.out.println(Arrays.toString(array));

You may use java.util.Random. And please specify if you want any random number or just the number 1,2,3,4,5,6. If you wish random numbers then , this is a basic code:

import java.util.*;
public class randomnumber
{
    public static void main(String[] args)
    {
        Random abc = new Random();
        int[] a = new int[6];
        int limit = 100,c=0;
        int chk = 0;
        boolean y = true;
        for(;c < 6;)
        {
            int x = abc.nextInt(limit+1);
            for(int i = 0;i<a.length;i++)
            {
                if(x==a[i])
                {
                    y=false;
                    break;
                }
            }
            if(y)
            {
                if(c!=0)if(x == (a[c-1]+1))continue;
                a[c]=x;
                c++;
            }
        }

        for (Integer number : a) 
        {
            System.out.println(number);
        }
    }
}

if you don't understand the last for loop , please tell , i will update it.

Use List and .contains(Object obj) method. So you can verify if list has the random number add before.

update - based on time you can lost stuck in random loop.

    List<Integer> list = new ArrayList<Integer>();
    int x = 1;
    while(x < 7){

                list.add(x);
                x++;
    }
    Collections.shuffle(list);

    for (Integer number : list) {
        System.out.println(number);
    }

http://docs.oracle.com/javase/7/docs/api/java/util/List.html#contains(java.lang.Object)