Interpreter Design Pattern in Java

In this article, I am going to discuss the Interpreter Design Pattern in Java with Examples. Please read our previous article where we discussed the Command Design Pattern in Java with Examples. The Interpreter Design Pattern falls under the category of Behavioral Design Pattern. In this article, we will explore the fundamental principles and benefits of the Interpreter design pattern, showcasing its significance in various software development scenarios.

What is Interpreter Design Pattern?

In software development, interpreting and processing language-based expressions or grammar is a common requirement. The Interpreter design pattern provides a structured approach to solving this challenge by defining a language and its grammar using a set of classes and objects. The pattern allows for the interpretation and evaluation of expressions, enabling the creation of powerful domain-specific languages or query languages.

The pattern consists of four primary components: the abstract expression class, the concrete expression class, the context, and the client. The abstract expression class defines the common interface for interpreting expressions, while the concrete expression classes implement specific grammar rules. The context contains information and a state relevant to the interpretation process, and the client constructs the expression tree and triggers the interpretation.

The Interpreter Design Pattern Provides a way to evaluate language grammar or expression. This pattern is used in SQL Parsing, Symbol Processing Engines, etc.

Example to Understand Interpreter Design Pattern

Let us understand the Interpreter Design Pattern with an example. Please have a look at the following image. On the left-hand side, you can see the Context. The Context is nothing but the value that we want to interpret. Here, the context value is the current date. On the right-hand side, you can see the Date expression or you can say the grammar. We have different types of date expressions such as (MM-DD-YYYY, DD-MM-YYYY, YYYY-MM-DD, and DD-YYYY).

Suppose, you want the date in MM-DD-YYYY format then what you need to do is, you need to pass the Context value and the Date Expression you want (i.e. MM-DD-YYYY) to the interpreter. What the interpreter will do is, it will convert the context value into the date expression format you passed to it. So, basically, the interpreter contains the logic or grammar to convert the context object into a specific readable format.

Implementing Interpreter Design Pattern in Java

A real-world example where the Interpreter pattern can be applied is in a program that converts one expression into another. The Interpreter pattern can be used to implement the parsing and interpretation of the expression. It involves defining a grammar that represents the syntax and structure of the expression. The interpreter will then evaluate the expressions based on the grammar rules. By using the Interpreter pattern, the compiler or interpreter can effectively parse and interpret the expression. The UML Diagram of this example is given below using Interpreter Design Pattern.

Step 1: Create a new directory to store all the class files of this project.

Step 2: Open VS Code and create a new project, called interpreter.

Step 3: In the project, create a new file called Pattern.java. Add the following code to the file:

public interface Pattern
{
    public String conversion (String expression);
}

This is the interface from which other concrete classes will extend.

Step 4: In the project, create a new file called InfixToPostfixPattern.java. Add the following code to the file:

import java.util.Stack;

public class InfixToPostfixPattern implements Pattern
{
    @Override
    public String conversion (String expression)
    {
        Stack<Character> operators = new Stack<Character>();
        char symbol;  
        String postfix = "";

        for(int i = 0; i<expression.length(); i++)  
        {  
            symbol = expression.charAt(i);  
    
            if (Character.isLetter(symbol))  
                postfix = postfix + symbol;  
            else if (symbol=='(')  
                operators.push(symbol);  
            else if (symbol==')')  
            {  
                while (operators.peek() != '(')    
                    postfix = postfix + operators.pop();  
                
                operators.pop();
            }
            else    
            {  
                while (!operators.isEmpty() && !(operators.peek()=='(') && prec(symbol) <= prec(operators.peek()))  
                    postfix = postfix + operators.pop();  
                    operators.push(symbol);  
            }  
        }

        while (!operators.isEmpty())  
            postfix = postfix + operators.pop();  
        
        return postfix; 
    }

    public static int prec(char x)  
    {  
    if (x == '+' || x == '-')  
    return 1;  
    if (x == '*' || x == '/' || x == '%')  
    return 2;  
    return 0;  
    } 
}

We have added the following pieces of code:

Imported the java.util.Stack package.
Added a function to convert the infix expression and return the postfix expression.
Added a helper function for the conversion() function.

Step 5: In the project, create a new file called InfixToPrefixPattern.java. Add the following code to the file:

import java.util.Stack;

public class InfixToPrefixPattern implements Pattern
{
    static boolean isOperator(char c)
    {
        return (!(c >= 'a' && c <= 'z') &&
                !(c >= '0' && c <= '9') &&
                !(c >= 'A' && c <= 'Z'));
    }
    
    static int getPriority(char C)
    {
        if (C == '-' || C == '+')
            return 1;
        else if (C == '*' || C == '/')
            return 2;
        else if (C == '^')
            return 3;
        return 0;
    }
    
    @Override
    public String conversion(String expression)
    {
        Stack<Character> operators = new Stack<Character>();
    
        Stack<String> operands = new Stack<String>();
    
        for (int i = 0; i < expression.length(); i++)
        {
            if (expression.charAt(i) == '(')
                operators.push(expression.charAt(i));

            else if (expression.charAt(i) == ')')
            {
                while (!operators.empty() && operators.peek() != '(')
                {
                    String op1 = operands.peek();
                    operands.pop();
    
                    String op2 = operands.peek();
                    operands.pop();

                    char op = operators.peek();
                    operators.pop();

                    operands.push(op + op2 + op1);
                }
    
                operators.pop();
            }
    
            else if (!isOperator(expression.charAt(i)))
                operands.push(expression.charAt(i) + "");
    
            else
            {
                while (!operators.empty() &&
                getPriority(expression.charAt(i)) <=
                getPriority(operators.peek()))
                {
    
                    String op1 = operands.peek();
                    operands.pop();
    
                    String op2 = operands.peek();
                    operands.pop();
    
                    char op = operators.peek();
                    operators.pop();

                    operands.push(op + op2 + op1);
                }
    
                operators.push(expression.charAt(i));
            }
        }
    
        while (!operators.empty())
        {
            String op1 = operands.peek();
            operands.pop();
    
            String op2 = operands.peek();
            operands.pop();
    
            char op = operators.peek();
            operators.pop();
    
            operands.push(op + op2 + op1);
        }
    
        return operands.peek();
    }    
}

We have added the following pieces of code:

Imported the java.util.Stack package.
Added a function to convert the infix expression and return the prefix expression.
Added a helper function for the conversion() function.

Step 7: In the project, create a new file called InterpreterPatternDemo.java. This class will contain the main() function. Add the following code to InterpreterPatternDemo.java:

public class InterpreterPatternDemo
{
    public static void main(String[] args)
    {
        String infix = "a+b+c*d-e";
        System.out.println("Infix: " + infix);
        System.out.println("Postfix: " + new InfixToPostfixPattern().conversion(infix));
        System.out.println("Prefix: " + new InfixToPrefixPattern().conversion(infix));
    }   
}

The main function defines an infix expression. It then converts and prints them to infix and postfix respectfully.

Step 8: Compile and execute the application. Ensure compilation is successful. Verify that the program works as expected.

Congratulations! You now know how to implement interpreter patterns!

UML Diagram of Interpreter Design Pattern:

Now, let us see the Interpreter Design Pattern UML Diagram Components with our Example so that you can easily understand the UML Diagram.

The classes can be described as follows:

Object: This class contains the definitions for the functions. These functions will later be implemented in the concrete classes.
ConcreteObject: This class implements the functions defined in aforementioned interface.
DriverClass: This class contains the main() function and is responsible for handling the simulation of the program.

Advantages of Interpreter Design Pattern in Java

The advantages of using the Interpreter Design Pattern in Java are as follows:

Formalizing Language and Grammar: The Interpreter pattern allows for the formal definition of languages and grammar in object-oriented terms. By defining a set of classes that represent the language elements and grammar rules, the pattern provides a clear structure and syntax for expressing and processing complex expressions. This formalization simplifies the interpretation process and promotes code readability and maintainability.
Custom Domain-Specific Languages: The Interpreter pattern facilitates the creation of custom domain-specific languages (DSLs). By defining the grammar and semantics of a DSL using the pattern, developers can design languages tailored to specific problem domains. DSLs enable non-technical users or domain experts to express complex concepts or operations in a natural and intuitive manner, enhancing productivity and collaboration.
Extensibility and Easy Modification: The Interpreter pattern promotes extensibility and easy modification of the language and grammar. New expressions or language constructs can be added by introducing new concrete expression classes. This flexibility allows for the evolution of languages to accommodate changing requirements, without the need to modify existing code or disrupt the interpretation process.
Separation of Concerns: The pattern promotes the separation of concerns by dividing the interpretation logic from the client code. The client constructs the expression tree and triggers the interpretation, while the interpretation itself is delegated to the interpreter classes. This separation enhances code modularity, maintainability, and testability by isolating the interpretation logic from other parts of the system.
Reusability of Grammar Rules: The Interpreter pattern encourages the reuse of grammar rules across different expressions. As the grammar rules are encapsulated in separate concrete expression classes, they can be shared and reused in various contexts. This reusability reduces duplication of code, enhances code consistency, and simplifies the maintenance of grammar rules.

Disadvantages of Interpreter Design Pattern in Java

The disadvantages of using the Interpreter Design Pattern in Java are as follows:

Complexity and Learning Curve: Implementing the Interpreter pattern can introduce complexity and a steeper learning curve. Defining language and grammar requires a thorough understanding of the problem domain and the ability to decompose it into appropriate expression classes. Additionally, maintaining and extending the set of expression classes as the language evolves can become challenging, especially in scenarios with complex or rapidly changing grammar.
Performance Overhead: The Interpreter pattern may introduce performance overhead, particularly in situations where expressions are complex or the interpretation process involves extensive computations. Each expression typically needs to be traversed and evaluated recursively, which can impact the overall execution time. Careful consideration should be given to performance optimization techniques, such as caching or optimizing the evaluation process, to mitigate this potential drawback.
Limited Flexibility: The Interpreter pattern may lack flexibility in certain scenarios, especially when dealing with dynamic languages or expressions that require runtime modifications. Since the interpretation process is tightly coupled with the defined expression classes, making changes to the language or grammar at runtime can be challenging. Adding new grammar rules or modifying existing ones may require altering the class hierarchy and potentially affecting other parts of the system.
Maintenance Effort: As the complexity of the grammar and the number of expressions classes increase, the maintenance effort for the Interpreter pattern can become significant. Adding or modifying expressions may involve updating multiple classes and ensuring consistency across the entire system. This maintenance overhead can impact code readability, increase the chances of introducing bugs, and require additional testing efforts.
Potential Scalability Issues: The Interpreter pattern might face scalability challenges when dealing with large expressions or high volumes of interpreted requests. The recursive nature of expression evaluation can lead to stack overflow errors or excessive memory usage, particularly in deeply nested or complex expressions. Scaling the interpretation process to handle increased workloads or large-scale systems may require careful optimization and consideration of alternative approaches.
Lack of Standardization: The Interpreter pattern lacks a standardized implementation approach, which can lead to inconsistencies across different implementations. The design and organization of expression classes, the interpretation process, and the integration with the rest of the system can vary significantly based on the specific implementation choices. This lack of standardization can make it harder for developers to understand and work with different Interpreter pattern implementations.

In the next article, I am going to discuss Iterator Design Pattern in Java with Examples. Here, in this article, I try to explain Interpreter Design Pattern in Java with Examples. I hope you understood the need for and use of the Interpreter Design Pattern in Java.