# Interpreter Design Pattern — Java Example

**Pattern:** Behavioral → Interpreter
**Article:** https://ankurm.com/interpreter-design-pattern-java/

## What this example shows

A math expression evaluator built as an abstract syntax tree. `Expression` declares `interpret()` — every AST node implements it. `NumberExpression` is the only terminal: a leaf with no children that returns its value directly. `AddExpression`, `SubtractExpression`, and `MultiplyExpression` are non-terminals: each holds two child expressions and applies its operator after recursively interpreting them. `Main` is the client — it assembles three small trees by hand (`(5 + 3) * 2`, `10 - (4 + 2)`, `(3 * 4) + (10 - 6)`) and evaluates each by calling `interpret()` on the root.

## How to run

```bash
javac interpreter/*.java -d out/interpreter
java -cp out/interpreter interpreter.Main
```

Requires Java 25.

## Post Section ↔ File Mapping

| Post Section | File(s) |
|---|---|
| Implementation: Math Expression Evaluator — the AbstractExpression | `Expression.java` |
| Implementation: Math Expression Evaluator — the TerminalExpression | `NumberExpression.java` |
| Implementation: Math Expression Evaluator — AddExpression | `AddExpression.java` |
| Implementation: Math Expression Evaluator — SubtractExpression | `SubtractExpression.java` |
| Implementation: Math Expression Evaluator — MultiplyExpression | `MultiplyExpression.java` |
| Implementation: Math Expression Evaluator — wiring it together | `Main.java` |

Article: https://ankurm.com/interpreter-design-pattern-java/
All patterns: https://ankurm.com/design-patterns-java/