Interpreter pattern is one of the behavioral design patterns and is used to define a grammatical representation for a language and provides an interpreter to deal with this grammar. The best example of this pattern is java compiler that interprets the java source code into byte code that is understandable by JVM. Google Translator is also an example of interpreter pattern where the input can be in any language and we can get the output interpreted in another language.

To implement interpreter pattern, we need to create Interpreter context engine that will do the interpretation work and then we need to create different Expression implementations that will consume the functionalities provided by the interpreter context. Finally we need to create the client that will take the input from user and decide which Expression to use and then generate output for the user.

When to use the Interpreter Design Pattern

Use the Interpreter pattern when there is a language to interpret, and you can represent statements in the language as abstract syntax trees. The Interpreter pattern works best when

  • The grammar is simple. For complex grammars, the class hierarchy for the grammar becomes large and unmanageable. Tools such as parser generators are a better alternative in such cases. They can interpret expressions without building abstract syntax trees, which can save space and possibly time.
  • Efficiency is not a critical concern. The most efficient interpreters are usually not implemented by interpreting parse trees directly but by first translating them into another form. For example, regular expressions are often transformed into state machines. But even then, the translator can be implemented by the Interpreter pattern, so the pattern is still applicable.

Interpreter Design Pattern in JDK:

  • java.util.Pattern
  • java.text.Normalizer
  • java.text.Format

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