设计模式（15）——解释器(Interpreter)

解释器模式是什么？

解释器模式简单来说就是用语法规则来解释一种语言。需要定义语法，以及一个解释器。

解释器的适用范围

解释器模式适用于有一个语言需要解释执行，而且可以将该语言中的句子表示为一个抽象语法树。在下列情况下，效果最好： 1. 文法简单。 2. 效率不是一个关键问题。

示例

加，减的文法解释示例：

import java.util.Map;

interface Expression {
    public int interpret(Map<String,Expression> variables);
}

class Number implements Expression {
    private int number;
    public Number(int number)       { this.number = number; }
    public int interpret(Map<String,Expression> 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<String,Expression> 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<String,Expression> 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<String,Expression> variables)  { 
        if(null==variables.get(name)) return 0; //Either return new Number(0).
        return variables.get(name).interpret(variables); 
    }
}

解析用：

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

class Evaluator implements Expression {
    private Expression syntaxTree;

    public Evaluator(String expression) {
        Stack<Expression> expressionStack = new Stack<Expression>();
        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<String,Expression> context) {
        return syntaxTree.interpret(context);
    }
}

测试：

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<String,Expression> variables = new HashMap<String,Expression>();
        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);
    }
}