Not able to generate a unique user-number <p>I have a problem when I'm trying to generate a unique customer-id in my application. I want the numbers to start from 1 and go up. I have a register-class using tree-map that generates the next customer-number using this code:</p> <pre><code>public String generateNumber() { int number = 1; for(Map.Entry&lt;String, Forsikringkunde&gt; entry : this.entrySet()) { if(entry.getValue().getNumber().equals(String.valueOf(number))) { number++; } }return String.valueOf(number); } </code></pre> <p>When I generate customers in my application I get duplicates of the numbers even though I iterate through the map. When creating a customer I create the object, run this method, use a set-method for the ID and adds it to the register, but it doesn't work. Anyone have a solution?</p>