title: atoi in java using charAt
body: <p>I know that one solution to do this is the following:</p>
  
  <pre><code> String tmp = "12345";
        int result = 0;
        for (int i =0; i &lt; tmp.length(); i++){
            char digit = (char)(tmp.charAt(i) - '0');
            result += (digit * Math.pow(10, (tmp.length() - i - 1)));
  
        }
  
        System.out.println(result);
  </code></pre>
  
  <p>What I don't understand is why is:</p>
  
  <pre><code>char digit = (char)(tmp.charAt(i) - '0');
  </code></pre>
  
  <p>How can this convert into a digit?</p>