Force inherited classes to define method
<p>The superclass is Account, and I have two subclasses - CurrentAccount and SavingsAccount.</p>

<p>The superclass will have a method applyInterest(), which will calculate the interest using the rate specified by an inherited class. I don't know how to force a class to define this though.</p>

<p>The only option I can think of is to force the subclasses to implement applyInterest(), and just set the rate in there.</p>
