The Interpreter Design Pattern is a behavioral design pattern that provides a way to define and interpret languages or grammars. It involves creating an interpreter that can evaluate and execute expressions written in a specific language.

The primary goal of the Interpreter Design Pattern is to enable the processing of structured expressions according to a defined set of rules or grammar. It allows you to build an interpreter that can parse and interpret sentences or expressions, providing a way to perform actions or computations based on the input.

When an expression is evaluated using the Interpreter Design Pattern, it is recursively broken down into smaller sub-expressions until the terminal expressions are reached. The interpretation or execution of each expression is carried out based on the defined rules or semantics of the language.

Real World Analogy

Imagine you have a group of people speaking different languages, and you need a way to communicate between them. Each person represents an expression in their own language, and you want to interpret and understand their messages without having to learn all the languages yourself.

In this scenario, the interpreter acts as a language translator. It takes the expressions (messages) in one language, interprets their meaning, and translates them into the desired language for the other person to understand. The interpreter breaks down the expressions into smaller components, applies the language rules and semantics, and produces an understandable interpretation in the target language.

   Person A (Language A)        Interpreter          Person B (Language B)
+-----------------------+   +----------------+   +-----------------------+
|      Expression       |   |   Interpreter  |   |      Expression        |
|   (Language A)        |   |                |   |   (Language B)         |
|_______________________|   |________________|   |_______________________|
|                       |   |                |   |                       |
|    Hello, how are you?|   |   Interpreter  |   |   ¡Hola, ¿cómo estás? |
|                       |   |                |   |                       |
|_______________________|   |________________|   |_______________________|

Implementation

Components in Interpreter Design pattern

When an expression is interpreted, it is typically parsed and broken down into a hierarchical structure represented by the AST. The interpreter recursively traverses the AST, interpreting each expression until the desired result is obtained.

The pattern typically involves the following components:

1. Context: This holds the information or state that is shared among the expressions being interpreted. It provides the necessary data for the interpreter to evaluate the expressions.
2. Abstract Expression: This is an abstract class or interface that defines the common operations for all the expressions. It usually includes an interpret() method that needs to be implemented by the concrete expressions.
3. Terminal Expression: Terminal expressions represent the smallest units of the language or grammar. They implement the interpret() method to perform specific operations or actions. Terminal expressions do not have any sub-expressions.
4. Non-terminal Expression: Non-terminal expressions are composed of one or more terminal or other non-terminal expressions. They also implement the interpret() method, but in addition, they combine the interpretations of their child expressions to perform more complex operations.
5. Client: The client is responsible for constructing the abstract syntax tree (AST) by assembling the expressions in the desired order based on the grammar. The client initiates the interpretation process by calling the interpret() method on the root expression.

Example

Let’s consider a real-world example where the Interpreter Design Pattern can be applied in Java: a simple language parser for evaluating arithmetic expressions.

// Context class to hold the global information
class Context {
    // Additional context data if needed
}

// Abstract Expression class
abstract class Expression {
    public abstract int interpret(Context context);
}

// Terminal Expression for representing a number
class NumberExpression extends Expression {
    private int number;

    public NumberExpression(int number) {
        this.number = number;
    }

    public int interpret(Context context) {
        return number;
    }
}

// Non-terminal Expression for addition operation
class AddExpression extends Expression {
    private Expression leftOperand;
    private Expression rightOperand;

    public AddExpression(Expression leftOperand, Expression rightOperand) {
        this.leftOperand = leftOperand;
        this.rightOperand = rightOperand;
    }

    public int interpret(Context context) {
        return leftOperand.interpret(context) + rightOperand.interpret(context);
    }
}

// Client code that uses the Interpreter pattern
public class InterpreterExample {
    public static void main(String[] args) {
        // Create the expression: (5 + 3) + 2
        Context context = new Context();
        Expression expression = new AddExpression(
            new AddExpression(
                new NumberExpression(5),
                new NumberExpression(3)
            ),
            new NumberExpression(2)
        );

        // Interpret the expression
        int result = expression.interpret(context);
        System.out.println("Result: " + result);
    }
}

In this example, we have a simple language for arithmetic expressions that can handle addition. The Interpreter pattern is used to parse and evaluate the expressions. The Expression hierarchy includes NumberExpression for representing numbers and AddExpression for addition operation. The interpret() method is implemented for each expression type to evaluate the result.

The Context class holds any necessary global information or state that may be shared among the expressions during interpretation.

In the main() method, we create an expression (5 + 3) + 2 using the interpreter pattern. The expression is interpreted by calling the interpret() method on the root expression, and the result is printed to the console.

 ___________________                 _______________________
|       Context     |               |      Expression       |
|___________________|               |_______________________|
|                   |               |                       |
|___________________|               |_______________________|
|                   |               | + interpret(context)  |
|___________________|               |_______________________|
         ^                                     ^
         |                                     |
         |                                     |
         |                                     |
 ___________________            ____________________________
| NumberExpression  |          |      AddExpression         |
|___________________|          |____________________________|
|                   |          |                            |
|___________________|          |____________________________|
| - number          |          | - leftOperand : Expression |
|___________________|          | - rightOperand : Expression|
| + interpret(context)|        | + interpret(context)       |
|___________________|          |____________________________|

Applicability

The Interpreter Design Pattern has several real-world applications in various domains. Here are a few examples of how the pattern is used:

1. Database Query Languages: Interpreter pattern is widely used in database systems to interpret and execute query languages like SQL. The interpreter parses the SQL queries, evaluates the expressions, and performs the necessary database operations based on the interpreted results.
2. Regular Expressions: Regular expression engines often use the Interpreter pattern to interpret and match patterns against input strings. The interpreter breaks down the regular expression into smaller components and interprets them to determine if the pattern matches the given string.
3. Mathematical and Symbolic Computations: Symbolic mathematics systems and computer algebra systems often employ the Interpreter pattern to interpret and evaluate mathematical expressions. The interpreter processes the expressions by evaluating variables, performing arithmetic operations, and simplifying equations.
4. Configurations and Rules: Interpreter pattern can be used to interpret configuration files or rule-based systems. For example, in a workflow engine, the interpreter can parse and interpret configuration files containing rules and conditions to determine the next steps or actions in the workflow.
5. Template Engines: Template engines, such as those used in web development, utilize the Interpreter pattern to interpret template expressions and generate dynamic content. The interpreter processes the template expressions and replaces them with corresponding values or logic during the rendering process.
6. Domain-Specific Languages (DSLs): Interpreter pattern is commonly used to implement DSLs for specific domains. For instance, a financial trading system might have its own DSL to describe trading strategies. The interpreter would evaluate the expressions in the DSL to execute the desired trading actions.
7. Natural Language Processing: In natural language processing tasks, such as chatbots or virtual assistants, the Interpreter pattern can be used to interpret and process user commands or queries. The interpreter analyzes the input sentences, extracts meaning, and performs actions based on the interpreted results.

These are just a few examples of real-world applications where the Interpreter pattern is utilized. It demonstrates the versatility and usefulness of the pattern in various domains that involve processing, interpreting, and executing structured expressions or languages.

JVM Usages

• java.util.Pattern
• java.text.Normalizer
• All subclasses of java.text.Format
• javax.el.ELResolver

Pros & Cons

Parser vs Interpreter Design Pattern

https://sourcemaking.com/design_patterns/interpreter