This is a simple program created for get combinations for any given number set. Note: efficience wise this is not hundred persent good.

Combination.java

import java.util.ArrayList;
import java.util.List;

public class Combinations {

	public static void main(String[] args) {
		List<String> list = new ArrayList<>();
		int[] a = {0,1,2};//change values as many you want
		for(int i: a){
			list.add(i+"");//add first numbers in array- 1,2,3
		}
		for(int times = 0; times < a.length; times++){
			String first = a[times]+"";
		
			for(int i = 0; i< a.length; i++){
				String temp = first + i;
				list.add(temp);//add to arraylist
			}
		}
		for(String s: list){
			System.out.print(s + " ");
		}
	}

}
/*---------------Output---------------------------

output: 0 1 2 00 01 02 10 11 12 20 21 22 

------------------------------------------------*/