행위패턴 : Interpreter 패턴

분류 : 행위패턴(Behavioral Patterns)

정의 :

언어에서 문장을 해석할때 사용되는 패턴

용도 :

• SQL 같은 데이터베이스 쿼리 언어에 상용
• 통신 프로토콜을 설명하는데 자주사용됨.

소스

import java.util.Map;

public interface Expression {
	public int interpret (Map variables);

}


import java.util.Map;

public 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);
    }
}


import java.util.Map;

public class Number implements Expression {

	private int number;
    public Number(int number)       { this.number = number; }
    public int interpret(Map variables)  { return number; }
    

}

import java.util.Map;

public 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);
    }

}


import java.util.Map;

public 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); 
    }
}

import java.util.Map;
import java.util.Stack;

public 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);
    }
}


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);
    }

}

참고자료

http://en.wikipedia.org/wiki/Interpreter_pattern