public class MyCalculator { public static void main(String[] args) {  String obj = "6+(8/2)+6/3+1*8 + 30";  ArrayList<String> arrayList = strFormat(obj);  System.out.println(obj + "=" + calculator(arrayList)); } /**  * 采用后續表達式計算結果  * 1、當前字符串為數字時，直接入棧  * 2、當前字符串為計算符時，取出棧中的前兩個計算  * 3、將計算結果再放入到棧中，棧中最后剩余的一個元素就是要求的結果  */ private static int calculator(ArrayList<String> obj) {  ArrayList<String> result = transform(obj);  System.out.println(result);  Stack<Integer> stack = new Stack<>();  for (int i = 0; i < result.size(); i++) {   String symbol = result.get(i);   if (isDigital(symbol)) { //數字直接入棧    stack.push(Integer.parseInt(symbol));   } else { // 處理操作符    int num1, num2;    num1 = stack.pop(); //取出兩個數    num2 = stack.pop();    switch (symbol) {     case "+":      stack.push(num2 + num1);      break;     case "-":      stack.push(num2 - num1);      break;     case "*":      stack.push(num2 * num1);      break;     case "/":      stack.push(num2 / num1);      break;     default:      break;    }   }  }  return stack.pop(); } /**  * 中序遍歷改為后續遍歷  */ private static ArrayList<String> transform(ArrayList<String> arrayList) {  Stack<String> stack = new Stack<>();  ArrayList<String> result = new ArrayList<>();  for (int index = 0; index < arrayList.size(); index++) {   String symbol = arrayList.get(index);   if (isDigital(symbol)) { //如果是數字直接輸出    result.add(symbol);   } else if (symbol.equals(")")) {    String tmp;    while (!(tmp = stack.pop()).equals("(")) { // 匹配成功后停止     result.add(tmp);    }   } else {    if (stack.isEmpty()) {     stack.push(symbol);     continue;    }    String tmp = stack.peek();    while (outPriority(symbol) <= inPriority(tmp)) { //優先級小于棧內優先級，一直出棧     result.add(tmp);     stack.pop();     if (stack.isEmpty()) {      break;     }     tmp = stack.peek();    }    stack.push(symbol);   }  }  //將剩余的出棧  while (!stack.isEmpty()) {   result.add(stack.pop());  }  return result; } /**  * 首先對String 進行格式化 轉化成ArrayList  * @param src 3*5+8;  * @return ArrayList 3 * 5 + 8  */ private static ArrayList<String> strFormat(String src) {  if (src == null || src.equals("")) {   return null;  }  ArrayList<String> arrayList = new ArrayList<>();  StringBuilder comChar = new StringBuilder();  for (int i = 0; i <src.length(); i++) {   char ch = src.charAt(i);   if (ch == ' ') {    continue; //去除空格   }   if (!Character.isDigit(ch)) {    if (!comChar.toString().trim().equals("")) {     arrayList.add(comChar.toString().trim());     comChar.delete(0, comChar.length());    }    arrayList.add(ch + "");    continue;   }   comChar.append(ch);  }  if (!comChar.toString().trim().equals("")) {   arrayList.add(comChar.toString().trim());  }  return arrayList; } /**  * 判斷是否為數字  * @param symbol 782 或者 + - * /  * @return true or false  */ private static boolean isDigital(String symbol) {  return !symbol.equals("+") && !symbol.equals("-")    && !symbol.equals("*") && !symbol.equals("/")    && !symbol.equals("(") && !symbol.equals(")"); } private static int inPriority(String ch) {  switch (ch) {   case "+":   case "-":    return 2;   case "*":   case "/":    return 4;   case ")":    return 7;   case "(":    return 1;   default:    return 0;  } } private static int outPriority(String ch) {  switch (ch) {   case "+":   case "-":    return 3;   case "*":   case "/":    return 5;   case ")":    return 1;   case "(":    return 7;   default:    return 0;  } }}