Java实现计算器的代码

用Java 实现的计算器，原理看代码注释，具体内容如下

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));
 }


 /**
  * 采用后续表达式计算结果
  * 1、当前字符串为数字时，直接入栈
  * 2、当前字符串为计算符时，取出栈中的前两个计算
  * 3、将计算结果再放入到栈中，栈中最后剩余的一个元素就是要求的结果
  */
 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)) { //数字直接入栈
    stack.push(Integer.parseInt(symbol));
   } else { // 处理操作符
    int num1, num2;
    num1 = stack.pop(); //取出两个数
    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();
 }

 /**
  * 中序遍历改为后续遍历
  */
 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)) { //如果是数字直接输出
    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)) { //优先级小于栈内优先级，一直出栈
     result.add(tmp);
     stack.pop();
     if (stack.isEmpty()) {
      break;
     }
     tmp = stack.peek();
    }
    stack.push(symbol);
   }
  }
  //将剩余的出栈
  while (!stack.isEmpty()) {
   result.add(stack.pop());
  }
  return result;
 }

 /**
  * 首先对String 进行格式化 转化成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;
 }

 /**
  * 判断是否为数字
  * @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;
  }
 }
}

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