how to prevent client to access public function of base class
<pre><code>//cls_a is defined by system framework, so we cannot change its definition
public class cls_a {
  public void method_a();
}

//I add a new cls_b implemenation and would like to 
//add a new method_a(int) function
//and prevent client side to access cls_a.method_a()
public class cls_b extends cls_a {
  public void method_a(int a) {
    //do something for a
    method_a();
  }
}

//client side
cls_b b = new cls_b();
b.method_a(2); //it's okay
b.method_a(); //should give something error when compiling code
</code></pre>

<p>I would like to design <code>cls_b</code> so that the Java compiler will give an error when client side calls <code>b.method_a()</code>. Anyone knows how to do it in Java?</p>
