Parse tree design

A context free grammar defines a language, e.g.

A parse tree shows how a sentence in the language is structured according to the grammar.

Terminology

In parsing lingo a symbol is a name in a grammar. Each symbol is either a terminal or a non-terminal. Non-terminals appear on the left hand side of a grammar production; terminals don't.

Class-Design

1st Design

That's the first Design that came up in class. It is a Composite Pattern.

• the addKid()-function in the Terminal class throws an exception
• symbol is of type enum, which has a representation for every symbol of the grammar.

Problem: Beware of Typeswitch

2nd Design

To overcome the Beware of Typeswitch problem an enum-hierarchy was added, so that a NonTerminal Symbol cannot have a Terminal Symbol.