软件体系结构的第二次实验（解释器风格与管道过滤器风格）

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class calc {

    private static String[] SIGNS = {"+", "-", "*", "/"};
    private static Character[] STANDARD_CHAT = {‘+‘, ‘-‘, ‘*‘, ‘/‘, ‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘, ‘ ‘};
    private static List<Character> STANDARD_LIST = new ArrayList<Character>(Arrays.asList(STANDARD_CHAT));

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        String expr = scanner.nextLine();
        //包含quit退出运算
        while (!expr.contains("quit")) {
            //词法+语法分析
            if (analyzer(expr)) {
                System.out.println(calculator(expr));
            } else {
                System.out.println("表达式输入错误");
            }
            expr = scanner.nextLine();
        }

    }

    private static Double calculator(String expr) {

        //去首尾空格
        expr = expr.trim();
        for (String sign : SIGNS) {
            Integer index;
            if ("-".equals(sign)) {
                index = expr.lastIndexOf(sign);
            } else {
                index = expr.indexOf(sign);
            }
            //不包含当前运算符
            if (index == -1) {
                continue;
            }

            String firstHalf = expr.substring(0, index);
            String secondHalf = expr.substring(index + 1, expr.length());
            switch (sign) {
                case "+":
                    return calculator(firstHalf) + calculator(secondHalf);
                case "-":
                    return calculator(firstHalf) - calculator(secondHalf);
                case "*":
                    return calculator(firstHalf) * calculator(secondHalf);
                case "/":
                    return calculator(firstHalf) / calculator(secondHalf);
            }

        }

        return Double.valueOf(expr);
    }

    private static boolean analyzer(String expr) {
        //词法分析，输入的表达式是否包含非法字符
        for (int i = 0; i < expr.length(); i++) {
            if (!STANDARD_LIST.contains(expr.charAt(i))) {
                return false;
            }
        }
        //语法分析，是否包含运算符
        boolean haveSign = false;
        for (String sing:SIGNS) {
            if (expr.contains(sing)){
                haveSign = true;
            }
        }
        return haveSign;
    }
}