package com.nps.base.xue.DataStructure.stack.utils;

import java.util.Scanner;
import java.util.Stack;

/*
 * @author XueWeiWei
 * @date 2019/7/7 9:02
 */
public class PostfixEvaluator {
    private final static char ADD = '+';
    private final static char SUBTRACT = '-';
    private final static char MULTIPLY = '*';
    private final static char DIVIDE = '/';

    private Stack<Integer> stack;

    /**
     *  创建一个 evaluator
     */

    public PostfixEvaluator() {
        stack = new Stack<Integer>();
    }

    public int evaluate(String expr){
        int op1,op2, result=0;
        String token;
        Scanner parser = new Scanner(expr);

        while (parser.hasNext()){
            token = parser.next();

            if (isOperator(token)){
                op2 = (stack.pop()).intValue();
                op1 = (stack.pop()).intValue();
                result = evaluateSingleOperator(token.charAt(0),op1,op2);
                stack.push(new Integer(result));
            }else {
                stack.push(new Integer(Integer.parseInt(token)));
            }
        }
        return result;
    }

    private boolean isOperator(String token){
        return (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/"));
    }

    private int evaluateSingleOperator(char operator, int op1 , int op2){
        int result = 0;

        switch (operator){
            case ADD:
                result = op1 + op2;
                break;
            case SUBTRACT:
                result = op1- op2;
                break;
            case MULTIPLY:
                result = op1 * op2;
                break;
            case DIVIDE:
                result = op1 / op2;
                break;
        }

        return result;
    }
}

　　

package com.nps.base.xue.DataStructure.stack.test;

import com.nps.base.xue.DataStructure.stack.utils.PostfixEvaluator;

import java.util.Scanner;

/*
 * @author XueWeiWei
 * @date 2019/7/7 9:01
 */
public class PostfixTester {
    public static void main(String[] args) {
        String expression,again;
        int result;

        Scanner in = new Scanner(System.in);

        do {
            PostfixEvaluator evaluator = new PostfixEvaluator();
            System.out.println("e.g. 5 4 +  3 2 1 - + *");
            System.out.println();
            expression = in.nextLine();

            System.out.println("输入的表达式：" + expression);
            result = evaluator.evaluate(expression);
            System.out.println(">>>>>结果：  " + result);

            System.out.println("是否输入下一个表达式？[Y/N]");
            again = in.nextLine();

        }while (again.equalsIgnoreCase("y"));
    }
}