Interpreter Design Pattern in Java: Complete Guide

In this article, we will do an in-depth exploration of Interpreter Design Pattern in Java. So, let’s get started.

Table of Contents

What is Interpreter Design Pattern in Java ?

Interpreter Design Pattern in Java is used for implementing a specialized language.It defines a grammatical representation for a language and provides an interpreter to deal with this grammar.

Example: Interpreter Design Pattern in Java

In the below example, we will use Interpreter Design Pattern for evaluating Reverse Polish Notation.

Reverse Polish Notation (Postfix)

Infix notation where the operator comes in between the operands. Example 1 + 2.
In Polish notation (Prefix notation), the operator comes before the operand. Example + 1 2
In reverse polish notation (postfix), the operator comes after the operand. Example 1 2 +

Class Diagram: Interpreter Design Pattern in Java

Java Code

// Expression Interface

package com.design.interpreter;

public interface Expression {
    public int interpret();

}

// NumberExpression Class

package com.design.interpreter;

public class NumberExpression implements Expression {

    private int number;

    public NumberExpression(int i) {
        number = i;
    }

    @Override
    public String toString() {
        return number + "";
    }

    public NumberExpression(String s) {
        number = Integer.parseInt(s);
    }

    @Override
    public int interpret() {
        return number;
    }

}

// PlusExpression Class

package com.design.interpreter;

public class PlusExpression implements Expression {

    private Expression leftExpression;
    private Expression rightExpresion;

    public PlusExpression(Expression leftExpression, Expression rightExpresion) {
        this.leftExpression = leftExpression;
        this.rightExpresion = rightExpresion;
    }

    @Override
    public int interpret() {
        return leftExpression.interpret() + rightExpresion.interpret();
    }

    @Override
    public String toString() {
        return "+";
    }

}

// MinusExpression Class

package com.design.interpreter;

public class MinusExpression implements Expression {

    private Expression leftExpression;
    private Expression rightExpresion;

    public MinusExpression(Expression leftExpression, Expression rightExpresion) {
        this.leftExpression = leftExpression;
        this.rightExpresion = rightExpresion;
    }

    @Override
    public int interpret() {
        return leftExpression.interpret() - rightExpresion.interpret();
    }

    @Override
    public String toString() {
        return "-";
    }

}

// MultiplyExpression Class

package com.design.interpreter;

public class MultiplyExpression implements Expression {

    private Expression leftExpression;
    private Expression rightExpresion;

    public MultiplyExpression(Expression leftExpression, Expression rightExpresion) {
        this.leftExpression = leftExpression;
        this.rightExpresion = rightExpresion;
    }

    @Override
    public int interpret() {
        return leftExpression.interpret() * rightExpresion.interpret();
    }

    @Override
    public String toString() {
        return "*";
    }

}

// InterpreterDemo Class

package com.design.interpreter;

import java.util.Stack;

public class InterpreterDemo {
    public static void main(String args[]) {
        String tokenString = "8 7 5 - 1 + *";
        Stack stack = new Stack < > ();

        String[] tokenList = tokenString.split(" ");
        for (String s: tokenList) {
            if (isOperator(s)) {
                Expression rightExpression = stack.pop();
                Expression leftExpression = stack.pop();
                Expression operator = getOperatorInstance(s, leftExpression,
                    rightExpression);
                int result = operator.interpret();
                System.out.println(leftExpression.toString() + operator + rightExpression + "=" + result);
                stack.push(new NumberExpression(result));
            } else {
                Expression i = new NumberExpression(s);
                stack.push(i);
            }
        }
        System.out.println("Result: " + stack.pop().interpret());
    }

    public static boolean isOperator(String s) {
        if (s.equals("+") || s.equals("-") || s.equals("*"))
            return true;
        else
            return false;
    }

    public static Expression getOperatorInstance(String s, Expression left,
        Expression right) {
        switch (s) {
            case "+":
                return new PlusExpression(left, right);
            case "-":
                return new MinusExpression(left, right);
            case "*":
                return new MultiplyExpression(left, right);
        }
        return null;
    }
}

// Output
7 - 5 = 2
2 + 1 = 3
8 * 3 = 24
Result: 24

Use-Cases: Interpreter Design Pattern in Java

Language Processing: Interpreting and executing domain-specific languages or custom query languages.
Regular Expressions: Implementing pattern matching and manipulation of strings using regular expressions.
Mathematical Expressions: Evaluating mathematical expressions and formulas dynamically.
Configurations: Interpreting configuration files or scripts to initialize system settings.
Query Languages: Processing queries in databases, search engines, or data manipulation languages.

Pros: Interpreter Design Pattern in Java

Extensibility: Adding new expressions or language features is easier without altering the existing code.
Modularity: Expressions are encapsulated into separate classes, promoting modularity and easy maintenance.
Customization: Clients can customize the behavior of the interpreter by creating new expressions.
Complexity Handling: Breaks down complex parsing and evaluation tasks into manageable components.
Domain-Specific Languages: Enables the creation of domain-specific languages tailored to specific problem domains.

Cons: Interpreter Design Pattern in Java

Performance Overhead: Interpreter pattern can introduce performance overhead due to object creation and traversal.
Complex Grammar: Managing complex grammars can lead to an increased number of expression classes.
Memory Usage: Can result in increased memory usage due to the creation of multiple expression objects.

Best Practices: Interpreter Design Pattern in Java

Define Grammar: Clearly define the grammar of the language to be interpreted.
Expression Hierarchy: Design a hierarchy of expression classes representing different elements of the language.
Composite Pattern: Implement the Composite pattern for expressions to handle complex grammars.
Context: Create a context class to hold the state and variables needed for interpretation.
Parser: Implement a parser to parse the input and create the expression tree.
Separation of Concerns: Keep the interpreter focused on interpreting and avoid mixing other responsibilities.
Error Handling: Implement robust error handling mechanisms for parsing and interpretation failures.
Testing: Thoroughly test different expressions and their combinations.
Reuse Existing Expressions: Utilize existing expression classes to create more complex expressions.
Documentation: Document the grammar, expression hierarchy, and usage of the interpreter.

Conclusion: Interpreter Design Pattern in Java

This article delved into the intricacies of the Interpreter Design Pattern in Java, which serves as a robust solution for implementing specialized languages and grammatical representations. By enabling the separation of algorithms from the objects they operate on, the pattern facilitates the addition of new operations without modifying existing object structures. Through a comprehensive example of evaluating Reverse Polish Notation, the article illustrated the practical application of the pattern in language processing scenarios.

The provided example demonstrated the power of the Interpreter Design Pattern in handling complex grammars and mathematical expressions. Additionally, the article highlighted the benefits, such as extensibility, modularity, and customization, that the pattern offers when applied to language processing tasks. However, it also addressed the potential trade-offs, including performance overhead and increased memory usage.

The best practices outlined in the article underscored the significance of clearly defining the grammar, designing a hierarchy of expression classes, and implementing the Composite pattern for handling complex grammars effectively. By following these guidelines and emphasizing separation of concerns, error handling, and thorough testing, developers can harness the full potential of the Interpreter Design Pattern to build flexible and maintainable language processing solutions in Java.

Must Read Design Pattern Articles:

Recommended Books:

“Design Patterns: Elements of Reusable Object-Oriented Software” by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (also known as the Gang of Four book)
“Head First Design Patterns” by Eric Freeman, Elisabeth Robson, Bert Bates, and Kathy Sierra