How to create a java class using efficient way to work with strings
<p>A java class named dna  having 3 variables:</p>

<ul>
<li>a String to store sequence</li>
<li>an int to store number of invalid characters in the string</li>
<li>an int to store the number of valid characters in the string.</li>
</ul>

<p>One constructor String that takes dna sequence (a c g t) as an argument. 
How can I check the argument and count how many characters are invalid and how many are valid and store the count in proper variables.<strong>I can't use regular expression here</strong>, so I am lost. How can I USE String's <strong>charAt</strong> method and <strong>if</strong> statement here.</p>

<p>Please help....</p>
