Java计算器核心算法代码实现

private String getResult(String str) {
    //分割
    String[] Str = segment(str);
    //中缀转后缀
    String newStr = infToSuf(Str);
    //后缀计算
    String result = sufToRes(newStr);
    return sufToRes(result);
  }

private static String[] segment(String str) {
    String[] exp = new String[str.length()+1];
    //找最近的索引并截取字符串
    int l = str.length();
    for(int i = 0;i < l+1;i++) {
      int index;
      int[] ind = new int[6];
      ind[0] = str.indexOf('+');
      ind[1] = str.indexOf('-');
      ind[2] = str.indexOf('*');
      ind[3] = str.indexOf('/');
      ind[4] = str.indexOf('(');
      ind[5] = str.indexOf(')');
      if(ind[1] == 0) {
        Arrays.sort(ind);
        int t;
        for(t = 0;t <6;t++) {
          if(ind[t] >= 0)
            break;
        }
        int r = ind[t+1];
        exp[i] = str.substring(0,r);
        i++;
        exp[i] = str.substring(r,r+1);
        str = str.substring(r+1);
      }else if(((ind[1]-ind[4]) == 1) && (ind[4]==0)) {
        Arrays.sort(ind);
        int t ;
        for(t = 0;t <6;t++) {
          if(ind[t] >= 0)
            break;
        }
        int r = ind[t+1];
        exp[i] = str.substring(0,1);
        i++;
        exp[i] = str.substring(1,r+2);
        i++;
        exp[i] = str.substring(r+2,r+3);
        str = str.substring(r+3);
      }else {
        Arrays.sort(ind);
        int t;
        for(t = 0;t <6;t++) {
          if(ind[t] >= 0)
            break;
        }
        if(t==6)
          break;
        index = ind[t];
        if(index!=0) {
          exp[i] = str.substring(0,index);
          i++;
        }
        exp[i] = str.substring(index,index+1);
        str = str.substring(index+1);
      }
    }
    int j = 0;
    int k = 0;
    for(; exp[j]!=null ;j++){}
    if(!exp[j-1].equals(")")) {
      exp[j]=str;
      str = "";
      k = j;
    }else {
      k = j-1;
    }
    String[] expp = new String[k+1];
    for(int t = 0; t < k+1;t++) {
      expp[t] = exp[t];
    }
    return expp;
    //System.out.println("分割的字符串:");
  }

private static String infToSuf(String[] exp) {
    String newStrs = "";
    //初始化栈
    Stack<String> stack = new Stack<>();
     /*
           判断并放入后缀表达式中：
       for循环遍历整个str进行判断
           循环结束若栈不为空全部出栈
     */
    int l = exp.length;
    for(int i = 0; i < l; i++) {
      if ((stack.empty()) && (exp[i].equals("+") || exp[i].equals("-") || exp[i].equals("*") || exp[i].equals("/"))) {
        stack.push(exp[i]);
      } else if (exp[i].equals("(")) {
        stack.push(exp[i]);
      } else if (exp[i].equals("*") || exp[i].equals("/")) {
        while (stack.peek().equals("*") || stack.peek().equals("/")) {
          newStrs = newStrs.concat(stack.pop()+" ");
          if(stack.isEmpty()) {
            break;
          }
        }
        stack.push(exp[i]);
      } else if (exp[i].equals("+") || exp[i].equals("-")) {
        while (!(stack.isEmpty())&&((stack.peek()).equals("*") || (stack.peek()).equals("/") || (stack.peek()).equals("+") || (stack.peek()).equals("-"))) {
          newStrs = newStrs.concat(stack.pop()+" ");
          if(stack.isEmpty()) {
            break;
          }
        }
        stack.push(exp[i]);
      } else if (exp[i].equals(")")) {
        int t = stack.search("(");
        for (int k = 1; k < t; k++) {
          newStrs = newStrs.concat(stack.pop()+" ");
        }
        String tstr = stack.pop();
      } else {
        newStrs = newStrs.concat(exp[i]+ " ");
      }
    }
    while (!stack.empty()) {
      if (!stack.peek().equals("(") || !stack.peek().equals(")")) {
        newStrs = newStrs.concat(stack.pop()+" ");
      } else if (stack.peek().equals("(") || stack.peek().equals(")")) {
        String tstr = stack.pop();
      }
    }
//   System.out.println("后缀:"+newStrs);
    return newStrs;
  }

  private static String sufToRes(String sufStr) {
    String[] exp = sufStr.split(" ");
    Stack<String> stack = new Stack<>();
    String Res = "";
    for(int i = 0;i < exp.length; i++) {
      if(!exp[i].equals("+") && !exp[i].equals("-") && !exp[i].equals("*") && !exp[i].equals("/")){
        stack.push(exp[i]);
      }else if(exp[i].equals("+")) {
        BigDecimal b2 = new BigDecimal(stack.pop());
        BigDecimal b1 = new BigDecimal(stack.pop());
        BigDecimal b3 = b1.add(b2);
        stack.push(b3.toString());
      }else if(exp[i].equals("-")) {
        BigDecimal b2 = new BigDecimal(stack.pop());
        BigDecimal b1 = new BigDecimal(stack.pop());
        BigDecimal b3 = b1.subtract(b2);
        stack.push(b3.toString());
      }else if(exp[i].equals("*")) {
        BigDecimal b2 = new BigDecimal(stack.pop());
        BigDecimal b1 = new BigDecimal(stack.pop());
        BigDecimal b3 = new BigDecimal(0);
        if(b1.compareTo(BigDecimal.ZERO)== 0|| b2.compareTo(BigDecimal.ZERO) == 0) {
          b3 = BigDecimal.ZERO;
        }else {
          b3 = b1.multiply(b2);
        }
        stack.push(b3.toString());
      }else if(exp[i].equals("/")){
        BigDecimal b2 = new BigDecimal(stack.pop());
        BigDecimal b1 = new BigDecimal(stack.pop());
        BigDecimal b3 = new BigDecimal(0);
        double d1 = b1.doubleValue();
        double d2 = b2.doubleValue();
        if(d1%d2 == 0){
          b3 = (b1.divide(b2));
          stack.push(b3.toString());
        }else {
          b3 = b1.divide(b2,10, RoundingMode.HALF_UP);
          stack.push(b3.toString());
        }
      }
    }
    Res = stack.pop();
    boolean flag = false;
    for (int m = 0; m < Res.length() - 1;m++) {
      if(Res.charAt(m) == '.'){
        flag = true;
      }
    }
    if(flag) {
      for(int m = Res.length()-1;m >= 0;m--) {
        if(Res.charAt(m) == '0'){
        }else {
          Res = Res.substring(0,m+1);
          break;
        }
      }
      if(Res.charAt(Res.length()-1) == '.') {
        Res = Res.substring(0,Res.length()-1);
      }
    }
    return Res;
  }