Question

i am using the below code to create a unique Id which is 8 character (including numeric and alphanumeric characters).

try {
            List<String> uuidList = new ArrayList<String>();
            int counter = 1;
            File file = new File("D://temp//temp1.txt");
            file.createNewFile();

            Writer writer = new FileWriter(file);
            BufferedWriter wr = new  BufferedWriter(writer);
            while(true) {
                int length = bitsArray.length;
                Random r = new Random();
                StringBuffer uuid = new StringBuffer();
                for(int i= 0; i < 8; i++) {
                    int nextRandomId = r.nextInt(length);
                    uuid.append(bitsArray[nextRandomId]);
                }
                String uuidString = uuid.toString();
                wr.write(uuidString);
                wr.newLine();
                if(counter != 1 && uuidList.contains(uuidString)) {
                    Thread.sleep(1000);
                    System.err.println(counter);
                    break;
                }
                //061e735145fc
                System.err.println(uuidString);
                uuidList.add(uuidString);
                counter++;
            }
        } catch (Exception e) {

        }

i need to know, if i use the above code.. then how many unique ids i can generate. Given

static String[] bitsArray = {"a","b","c","d","e","f","g","h","i",
                          "j","k","l","m","n","o","p","q","r",
                          "s","t","u","v","w","x","y","z",
                          "0","1","2","3","4","5","6","7","8","9"};

Please help..

Answer 1

In essence there is a 36⁸ total strings you can generate.

This theorem is explained by using Discrete Mathematics (using Bit String):

You have 8 character bit string and you need to fill choose 1 out of 36 characters:

__  __  __  __  __  __  __  __  
36  36  36  36  36  36  36  36  (characters a -- z, 0-- 9)

Therefore you have 36*36*36*36*36*36*36*36 = 36⁸= 2,821,109,907,456 total id’s.