Today's pattern is the Interpreter pattern, which defines a grammatical representation for a language and provides an interpreter to deal with this grammar.

Interpreter in the Real World 

The first example of interpreter that comes to mind, is a translator, allowing people to understand a foreign language. Perhaps musicians are a better example: musical notation is our grammar here, with musicians acting as interpreters, playing the music.

Design Patterns Refcard
For a great overview of the most popular design patterns, DZone's Design Patterns Refcard is the best place to start. 

The Interpreter Pattern

The Interpreter pattern is known as abehavioural pattern, as it's used to manage algorithms, relationships and responsibilities between objects.. Thedefinition of Interpreter as provided in the original Gang of Four book on DesignPatterns states: 

Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

The following diagram shows how the interpreter pattern is modelled.

Context contains information that is global to the interpreter. The AbstractExpression provides an interface for executing an operation. TerminalExpression implements the interpret interface associated with any terminal expressions in the defined grammar. 

The Client either builds the Abstract Syntax Tree, or the AST is passed through to the client. An AST is composed of both TerminalExpressions and NonTerminalExpressions. The client will kick off the interpret operation. Note that the syntax tree is usually implemented using the Composite pattern

The pattern allows you to decouple the underlying expressions from the grammar. 

When Would I Use This Pattern?

The Interpreter pattern should be used when you have a simple grammar that can be represented as an Abstract Syntax Tree. This is the more obvious use of the pattern. A more interesting and useful application of Interpreter is when you need a program to produce different types of output, such as a report generator. 

So How Does It Work In Java?

I'll use a simple example to illustrate this pattern. We're going to create our own DSL for searching Amazon. To do this, we'll need to have a context that uses an Amazon web service to run our queries.

//Context public class InterpreterContext{//assume web service is setupprivate AmazonWebService webService;   public InterpreterContext(String endpoint){//create the web service.}   public ArrayList<Movie> getAllMovies(){   return webService.getAllMovies();}public ArrayList<Book> getAllBooks(){  return webService.getAllBooks();}}

Next, we'll need to create an abstract expression: 

//Abstract Expression public abstract class AbstractExpression{   public abstract String interpret( InterpreterContext context);}

We'll have many different expressions to interpret our queries. For illustration,let's create just one: 

//Concrete Expression public class BookAuthorExpression extends AbstractExpression{private String searchString;   public BookAuthorExpression(String searchString)   {this.searchString = searchString;   }   public String interpret(InterpreterContext context)   {ArrayList<Book> books = context.getAllBooks();StringBuffer result = new StringBuffer();for(Book book: books){ if(book.getAuthor().equalsIgnoreCase(searchString)) {result.append(book.toString()); }}return result;   }}

Finally, we need a client to drive all of this. Let's assume that our language is of the following type of syntax: 

books by author 'author name'

The client will determine which expression to use to get our results: 

//client public class AmazonClient {  private InterpreterContext context;  public AmazonClient(InterpreterContext context) {this.context = context; }  /**  * Interprets a string input of the form   *   movies | books by title | year | name '<string>'  */ public String interpret(String expression) {//we need to parse the string to determine which expression to use AbstractExpression exp = null; String[] stringParts = expression.split(" ");String main = stringParts[0];String sub = stringParts[2];//get the query partString query = expression.substring(expression.firstIndexOf("'"), expression.lastIndexOf("'"));if(main.equals("books")){if(sub.equals("title"){  exp = new BookTitleExpression(query);}if(sub.equals("year"){   exp = new BookYearExpression(query);}}else  if(main.equals("movie"))  {//similar statements to create movie expressions  }    if(exp != null){exp.interpret(context);} }   public static void main(String[] args)  {InterpreterContext context  = new InterpreterContext("http://aws.amazon.com/");AmazonClient client = new AmazonClient();//run a queryString result = client.interpret("books by author 'John Connolly'");System.out.println(result); }}

I admit the example is a bit simple, and you would probably have a more intelligent context, but that should give you the idea of how this pattern works.

Watch Out for the Downsides

Efficiency is a big concern for any implementation of this pattern. Introducing your own grammar requires extensive error checking, which will be time consuming for the programmer to implement, and needs careful design in order to run efficiently at runtime. Also, as the grammar becomes more complicated, the maintainence effort is increased. 

Next Up

As we've mentioned the Composite pattern in this article when dealing with Abstract Syntax Trees, I'll cover Composite in the next article.

Enjoy the Whole "Design Patterns Uncovered" Series:

Creational Patterns

• Learn The Abstract Factory Pattern
• Learn The Builder Pattern
• Learn The Factory Method Pattern
• Learn The Prototype Pattern
• Learn The Singleton Pattern

Structural Patterns

• Learn The Adapter Pattern
• Learn The Bridge Pattern
• Learn The Composite Pattern
• Learn The Decorator Pattern
• Learn The Facade Pattern
• Learn The Flyweight Pattern
• Learn The Proxy Pattern

Behavioral Patterns

• Learn The Chain of Responsibility Pattern
• Learn The Command Pattern
• Learn The Interpreter Pattern
• Learn The Iterator Pattern
• Learn The Mediator Pattern
• Learn The Memento Pattern
• Learn The Observer Pattern
• Learn The State Pattern
• Learn The Strategy Pattern
• Learn The Template Method Pattern
• Learn The Visitor Pattern