在spring boot中用post请求写出四则混合运算代码，写出三种这样的代码

在Spring Boot中，我们通常使用`@RestController`和`@PostMapping`来处理HTTP POST请求，并可以利用`@RequestBody`来获取客户端发送的数据。以下是三个示例，展示如何实现四则混合运算的POST请求处理：

### 示例1:

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class MathCalculatorController {

    @PostMapping("/calculate")
    public String calculate(@RequestBody String formula) {
        String[] parts = formula.split(" ");
        double leftNum = 0, rightNum = 0;
        String operator;

        try {
            leftNum = Double.parseDouble(parts[0]);
            operator = parts[1];
            rightNum = Double.parseDouble(parts[2]);

            switch (operator) {
                case "+":
                    return Double.toString(leftNum + rightNum);
                case "-":
                    return Double.toString(leftNum - rightNum);
                case "*":
                    return Double.toString(leftNum * rightNum);
                case "/":
                    if (rightNum != 0) {
                        return Double.toString(leftNum / rightNum);
                    } else {
                        return "Error: Division by zero";
                    }
                default:
                    return "Error: Invalid operator";
            }
        } catch (NumberFormatException e) {
            return "Error: Invalid number format";
        }
    }
}

### 示例2:

@PostMapping("/calculate-with-validation")
public ResponseEntity<String> validateAndCalculate(@Valid @RequestBody CalculateRequest request) {
    double result = request.calculate();
    return ResponseEntity.ok().body(Double.toString(result));
}

class CalculateRequest {
    private double num1;
    private String operator;
    private double num2;

    // getters and setters

    public double calculate() {
        switch (operator) {
            // ...同样处理四则运算
        }
    }
}

### 示例3:

@RestController
public class AdvancedMathController {

    @PostMapping("/complex-calculation")
    public Result complexCalculation(@RequestBody CalculationData data) {
        // 数据验证
        if (!data.isValid()) {
            return new Result(false, "Invalid input");
        }

        // 计算逻辑
        double result = calculate(data.getExpression());
        return new Result(true, result);
    }

    private double calculate(String expression) {
        // 使用正则表达式解析并计算
    }

    static class CalculationData {
        private String expression;
        // getters and setters
        // 提供验证逻辑
    }
}

在这几个例子中，用户可以通过POST请求发送一个包含数字和运算符的字符串到指定的URL，服务器会解析这个字符串并执行相应的运算。每个示例都有其特定的结构和复杂度，比如第二个示例增加了输入验证和对象封装，第三个示例则展示了更复杂的表达式和数据结构处理。