Interpreter Pattern - Example

Example

The following Reverse Polish notation example illustrates the interpreter pattern. The grammar
expression ::= plus | minus | variable | number
plus ::= expression expression '+'
minus ::= expression expression '-'
variable ::= 'a' | 'b' | 'c' | ... | 'z'
digit = '0' | '1' | ... '9'
number ::= digit | digit number

 defines a language which contains reverse Polish expressions like:
a b +
a b c + -
a b + c a - -

 Following the interpreter pattern there is a class for each grammar rule.

import java.util.Map; interface Expression { public int interpret(Map variables); } class Number implements Expression { private int number; public Number(int number) { this.number = number; } public int interpret(Map variables) { return number; } } class Plus implements Expression { Expression leftOperand; Expression rightOperand; public Plus(Expression left, Expression right) { leftOperand = left; rightOperand = right; } public int interpret(Map variables) { return leftOperand.interpret(variables) + rightOperand.interpret(variables); } } class Minus implements Expression { Expression leftOperand; Expression rightOperand; public Minus(Expression left, Expression right) { leftOperand = left; rightOperand = right; } public int interpret(Map variables) { return leftOperand.interpret(variables) - rightOperand.interpret(variables); } } class Variable implements Expression { private String name; public Variable(String name) { this.name = name; } public int interpret(Map variables) { if(null==variables.get(name)) return 0; //Either return new Number(0). return variables.get(name).interpret(variables); } }

While the interpreter pattern does not address parsing a parser is provided for completeness.

import java.util.Map; import java.util.Stack; class Evaluator implements Expression { private Expression syntaxTree; public Evaluator(String expression) { Stack expressionStack = new Stack; for (String token : expression.split(" ")) { if (token.equals("+")) { Expression subExpression = new Plus(expressionStack.pop, expressionStack.pop); expressionStack.push( subExpression ); } else if (token.equals("-")) { // it's necessary remove first the right operand from the stack Expression right = expressionStack.pop; // ..and after the left one Expression left = expressionStack.pop; Expression subExpression = new Minus(left, right); expressionStack.push( subExpression ); } else expressionStack.push( new Variable(token) ); } syntaxTree = expressionStack.pop; } public int interpret(Map context) { return syntaxTree.interpret(context); } }

Finally evaluating the expression "w x z - +" with w = 5, x = 10, and z = 42.

import java.util.Map; import java.util.HashMap; public class InterpreterExample { public static void main(String args) { String expression = "w x z - +"; Evaluator sentence = new Evaluator(expression); Map variables = new HashMap; variables.put("w", new Number(5)); variables.put("x", new Number(10)); variables.put("z", new Number(42)); int result = sentence.interpret(variables); System.out.println(result); } }


Read more about this topic:  Interpreter Pattern

Famous quotes containing the word example:

    “Our intellect is not the most subtle, the most powerful, the most appropriate, instrument for revealing the truth. It is life that, little by little, example by example, permits us to see that what is most important to our heart, or to our mind, is learned not by reasoning but through other agencies. Then it is that the intellect, observing their superiority, abdicates its control to them upon reasoned grounds and agrees to become their collaborator and lackey.”
    —Marcel Proust (1871–1922)