Applied Design Patterns with Java

Behavioral :: Interpreter (243) {C ch 18}

Example - Java :: Patterns\Behavioral\Interpreter

Pattern Concept: to define a grammar for a language, and use that grammar to interpret statements in the language. The Interpreter pattern drives three use cases:

1. When a command Interpreter is needed to parse user commands that allow a query facility that returns different results, based on the parsed input.
2. When a program must parse an algebraic string, a common occurrence in math and graphics packages.
3. When a program must produce varied output different from how the input is stored or represented, i.e.: like a Report Generator, or Traffic Analyzer, etc.

Interpreting a language takes three steps:

1. parsing language symbols into tokens
2. reducing the tokens into actions
3. executing the actions

Examples - UML : SimpleParse and InterpDemo

Here is a sample parse output, with a diagram showing the parse reduction, and the representive classes to accomplish this in UML:

Here is Cooper's Class Diagram for the application using this logic, followed by the Rose equivalent:

The example Java program is called 'InterpDemo'.

The UML diagram is above, and the list of Java files is below:

Issues and consequences of the Interpreter pattern include:

• When an Interpreter is used, an easy mechanism for the user to enter commands in the language must be furnished;
• Generating a language and its grammar are non-trivial tasks, requiring care and extensive testing, and the issue of what happens when a user enters a string that violates the grammar must be handled robustly and in a user-friendly manner;
• Even a GUI has an implied grammar behind it (proper values and sequencing of user actions in a given panel);
• The Interpreter pattern has the advantage of being readily extensible, and a grammar can be revised, once the mechanism of parsing and reducing are fully understood and implemented;
• For grammars of any complexity (greater than six cases), use a class for each case. A large grammar may have many classes associated with it;
• Adding more complexity to a grammar is self-defeating; at some point, the grammar may become too complex to readily maintain.

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