设计模式（15）——解释器(Interpreter)

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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);    }}

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