行为型（十） - 解释器模式

一、概念

解释器模式（Interpreter Pattern）：解释器模式为某个语言定义它的语法（或者叫文法）表示，并定义一个解释器用来处理这个语法。实际应用中较少用到的行为模式。

二、实现

举个常见的面试题，输入这样一个字符串“6 3 5 7 - + * ”，中间空格隔开。前面是运算的数，后面是运算符。先取前两个数6 3，再取一个运算符-，6-3=3，然后用结果继续和后面的数进行计算3+5=8，8*7=56。下面是实现代码：
1、表达式

public interface Expression {
    long interpreter();
}

public class NumberExpression implements Expression{
    private long number;

    public NumberExpression(long number) {
        this.number = number;
    }

    public NumberExpression(String number) {
        this.number = Long.parseLong(number);
    }

    @Override
    public long interpreter() {
        return number;
    }
}

public class AddExpression implements Expression {
    private Expression exp1;
    private Expression exp2;

    public AddExpression(Expression exp1, Expression exp2) {
        this.exp1 = exp1;
        this.exp2 = exp2;
    }

    @Override
    public long interpreter() {
        return exp1.interpreter() + exp2.interpreter();
    }
}

public class SubstractionExpression implements Expression {
    private Expression exp1;
    private Expression exp2;

    public SubstractionExpression(Expression exp1, Expression exp2) {
        this.exp1 = exp1;
        this.exp2 = exp2;
    }

    @Override
    public long interpreter() {
        return exp1.interpreter() - exp2.interpreter();
    }
}

public class MultiplicationExpression implements Expression {
    private Expression exp1;
    private Expression exp2;

    public MultiplicationExpression(Expression exp1, Expression exp2) {
        this.exp1 = exp1;
        this.exp2 = exp2;
    }

    @Override
    public long interpreter() {
        return exp1.interpreter() * exp2.interpreter();
    }
}

public class DivisionExpression implements Expression {
    private Expression exp1;
    private Expression exp2;

    public DivisionExpression(Expression exp1, Expression exp2) {
        this.exp1 = exp1;
        this.exp2 = exp2;
    }

    @Override
    public long interpreter() {
        return exp1.interpreter() / exp2.interpreter();
    }
}

2、ExpressionInterpreter运算实现类

public class ExpressionInterpreter {
    private Deque<Expression> numbers = new LinkedList<>();

    public long interpreter(String expression) {
        String[] elements = expression.split(" ");
        Expression result = null;
        int lastNumIndex = (elements.length + 1) / 2;
        for (int i = 0; i < lastNumIndex; i++) {
            numbers.addLast(new NumberExpression(elements[i]));
        }
        for (int j = lastNumIndex; j < elements.length; j++) {
            Expression num1 = numbers.pollFirst();
            Expression num2 = numbers.pollFirst();
            
            switch (elements[j]) {
                case "+":
                    result = new AddExpression(num1, num2);
                    break;
                case "-":
                    result = new SubstractionExpression(num1, num2);
                    break;
                case "*":
                    result = new MultiplicationExpression(num1, num2);
                    break;
                case "/":
                    result = new DivisionExpression(num1, num2);
                    break;
        }
        long temp = result.interpreter();
        numbers.addFirst(new NumberExpression(temp));       
    }

    System.out.println("result: " + result.interpreter());
    return result.interpreter();
}

3、测试类

public class Client {
    public static void main(String[] args) {
        ExpressionInterpreter interpreter = new ExpressionInterpreter();
        interpreter.interpreter("6 3 5 7 - + *");
    }
}

4、运行结果