Concrete base class

Ordinary design

In Object-Oriented-Design, base class is designed as abstract and generic one, which does not have any derived class specific methods.

For example, neither javax.swing.JComponent, its base class java.awt.Container, or java.awt.Component have getUnitIncrement(int) method, whih is specific to javax.swing.JScrollBar.

The way of thinking described above makes base classes robust against for changes and re-useful.

Decision in PageMixer

At first, Token class is designed as generic one in PageMixer framework. It had only getName() and isNamedAs(Symbol) at that time.

But it causes belows:

• many instanceof invocations to examine what given token is
• many narrowing(down cast) to access methods of derived classes

Invocation of instanceof and casting cost so much because almost all Filters(*Watchers, in fact) want to know whether given Token is of start/end tag or not, and has specified attribute or not.

So, for performance reason, it is natural to define in Token class below examination methods, which depends on HTML tag Token specific characteristics.

• isStart()
• isEnd()
• isEmpty()
• findAttribute(Symbol name)
• findAttributes(Symbol name)
• getAttributes()