private string member what to initialize with
<p>In Java what is the best practice of initializing the private string members</p>

<pre><code>class A {
       private String mem = ??//
       private int num;
       A (int a) {
          this.num = a;
       }
       A (int a, String s) {
           this.num = a;
           this.mem  = s; //What if s is null (Is this a good practice)
       }

}
</code></pre>
