[JDK17]逆波兰计算器的简单实现(含一般表达式转逆波兰表达式)

将中缀表达式转成后缀表达式的工具类:

package Stack.PoLandCaculation;import java.util.ArrayList;
import java.util.Collections;
import java.util.Stack;/*** 将中缀表达式转成后缀表达式的工具类*/
public class PoLandTransform {/*** 一般表达式转后缀表达式* @param commonExpression 一般表达式* @return 返回后缀表达式字符串数组,数组每一个元素都是一个数或者运算符*/public static ArrayList<String> toPostfixExpression(String commonExpression){ArrayList<String> list = PoLandTransform.toStringArray(commonExpression);Stack<String> s1 = new Stack<>();Stack<String> s2 = new Stack<>();for (int i = 0; i < list.size(); i++) {//当遍历到数字时，直接压入s2栈String str = list.get(i);if (str.matches("\\d+(\\.\\d+)?")) {s2.push(str);} else {//遍历到运算符//s1为空 或者 遍历到'(' 或者 s1栈顶是'('，运算符直接压入s1栈if (s1.isEmpty() || str.equals("(") || s1.peek().equals("(")) {s1.push(str);} else if (str.equals(")")) {//当遍历到')'时//弹出s1中的运算符并压入到s2，直到s1栈顶是'('。最后弹出s1栈顶的'('while (!s1.peek().equals("(")) s2.push(s1.pop());s1.pop();} else {//当遍历到加减乘除这四个运算符//先计算当前运算符与s1栈顶运算符的优先级int priority = PoLandTransform.priority(str, s1.peek());if (priority == 1) {//当前运算符比栈顶运算符优先级高//直接压入s1栈s1.push(str);} else {//比s1栈顶优先级低//弹出一个s1元素并压入s2，然后当前运算符再次与弹出一次后的s1栈顶元素对比s2.push(s1.pop());//回滚索引i--;}}}}//s1栈剩余的运算符依次弹入s2栈中while (!s1.isEmpty()) s2.push(s1.pop());//s2栈依次弹出，添加到新的list中ArrayList<String> newList = new ArrayList<>();while (!s2.isEmpty()) newList.add(s2.pop());//将list转置后返回Collections.reverse(newList);return newList;}/*** 识别算术表达式，并返回处理好的算式列表* @param expression 算术表达式* @return 返回处理好的表达式列表*/private static ArrayList<String> toStringArray(String expression){//表达式匹配到非法字符（匹配到加减乘除和数字以及小数点以外的字符）if (expression.replaceAll(" ","").matches("[^\\d\\+\\-\\*\\/\\.]")){throw new RuntimeException("表达式存在非法字符！");}ArrayList<String> list = new ArrayList<>();//将表达式转char数组方便后续处理char[] exp = expression.replaceAll(" ","").toCharArray();//定义numStr方便后续拼接多位数数字String numStr = "";for (int i = 0; i < exp.length; i++) {if (exp[i] >= '0' && exp[i] <= '9' || exp[i] == '.'){//遍历到数字或者小数点，都拼接到numStrnumStr += exp[i];if (i == exp.length - 1){//遍历到最后一个数字，记得把最后一个数字也添加到列表listlist.add(numStr);}}else if(exp[i] == '+' || exp[i] == '-' || exp[i] == '*' || exp[i] == '/' || exp[i] == '(' || exp[i] == ')'){//遍历到运算符，先把拼接好的numStr添加进list，然后重置numStrif (!numStr.isEmpty()) list.add(numStr);numStr = "";//把运算符也添加进listlist.add(""+exp[i]);}}return list;}/*** 计算运算符优先级* @param cur 当前运算符* @param stackEle 栈顶运算符* @return 返回0代表当前运算符优先级低或者相等，返回1代表当前运算符优先级高*/private static int priority(String cur,String stackEle){if (stackEle.equals("*") || stackEle.equals("/")){return (cur.equals("+") || cur.equals("-")) ? 0 : 1;}return (cur.equals("+") || cur.equals("-")) ? 0 : 1;}
}

逆波兰计算器：

package Stack.PoLandCaculation;import java.util.ArrayList;
import java.util.Stack;/*** 逆波兰表达式计算器*/
public class PoLandCaculation {/*** 将一般算式转换成逆波兰表达式后返回算式计算结果* @param expression 一般算式* @return 返回计算结果*/public static Double poLandCaculate(String expression){//获取后缀表达式ArrayList<String> postfixList = PoLandTransform.toPostfixExpression(expression);Stack<Double> stack =new Stack<>();postfixList.forEach(str ->{//遍历到数字直接入栈if (str.matches("\\d+(\\.\\d+)?")){stack.push(Double.parseDouble(str));}else {//遍历到运算符//栈顶弹出两个元素，与当前运算符计算后重新入栈try {Double result = PoLandCaculation.calculate(stack.pop(), stack.pop(), str);stack.push(result);}catch (ArithmeticException e){throw new ArithmeticException("出现算术异常！");}catch (RuntimeException e){throw new RuntimeException(e.getMessage());}}});//栈中最后剩下的元素就是计算结果，返回结果return stack.pop();}/*** 根据运算符进行计算并返回结果* @param num1* @param num2* @param operator* @return* @throws RuntimeException*/private static Double calculate(Double num1,Double num2,String operator) throws RuntimeException{return switch (operator){case "+"-> num2+num1;case "-"-> num2-num1;case "*"-> num2*num1;case "/"-> num2/num1;default -> throw new RuntimeException("未知异常");};}
}

测试代码：

package Stack.PoLandCaculation;import java.util.Scanner;public class PoLandCaculationTest {public static void main(String[] args) {Scanner scan = new Scanner(System.in);while (true) {System.out.println("--------------------------------------------------------------------------------------");System.out.println("[输入e退出程序]");System.out.println("请输入计算表达式(支持小数运算、多位数运算、小括号运算)：");System.out.println("--------------------------------------------------------------------------------------");String expression = scan.next();if (expression.equalsIgnoreCase("e")) return;Long startTime = System.currentTimeMillis();Double result = null;try {result = PoLandCaculation.poLandCaculate(expression);} catch (ArithmeticException e) {System.out.println(e.getMessage());}catch (Exception e){e.printStackTrace();}Long endTime = System.currentTimeMillis();System.out.println("计算结果为：["+result+"]");System.out.println("本次计算用时："+ (endTime-startTime) +" ms");}}
}

测试图：