方法中缺少返回语句错误 [英] Missing return statement error in a method

问题描述

我正在尝试编写一个返回我的计算机 MAC 地址字符串的静态方法(函数本身在这里找到:http://www.mkyong.com/java/how-to-get-mac-address-in-java/).我遇到了静态函数的 return 方面的问题.我得到的错误是 缺少返回语句.我该如何补救?

I am trying to write a static method that returns a string of my computer's MAC address (the function itself was found here: http://www.mkyong.com/java/how-to-get-mac-address-in-java/). I am having issues with the return aspect of the static function. The error I get is the missing return statement. How do I remedy this?

static String returnMacAddress(){
        InetAddress ip;
        try{
            ip = InetAddress.getLocalHost();

            NetworkInterface network = NetworkInterface.getByInetAddress(ip);
            byte[] mac = network.getHardwareAddress();

            System.out.print("Current MAC address: ");

            StringBuilder stringBuilder = new StringBuilder();
            for(int i = 0; i < mac.length; i++){
                stringBuilder.append(String.format("%02X%s", mac[i], (i < mac.length - 1) ? "-" : ""));
            }
            return stringBuilder.toString();
        }catch(UnknownHostException e){
            e.printStackTrace();
        } catch(SocketException e){
            e.printStackTrace();
        }
    }

推荐答案

所有分支都必须返回一些东西，只需在末尾添加一个return null;:

All branches must return something, just add a return null; at the end:

static String returnMacAddress(){             // 1.
    InetAddress ip;
    try{                                      // 2.
        ip = InetAddress.getLocalHost();      // 3. (until return stmt)

        NetworkInterface network = NetworkInterface.getByInetAddress(ip);
        byte[] mac = network.getHardwareAddress();

        System.out.print("Current MAC address: ");

        StringBuilder stringBuilder = new StringBuilder();
        for(int i = 0; i < mac.length; i++){
            stringBuilder.append(String.format("%02X%s", mac[i], (i < mac.length - 1) ? "-" : ""));
        }
        return stringBuilder.toString();       // 4.
    }catch(UnknownHostException e){            // 5.
        e.printStackTrace();                   // 6.
    } catch(SocketException e){
        e.printStackTrace();
    }
    return null;                               // 7.
}

这是语法正确的 - 但你必须考虑这意味着什么语义，如果这是所需的操作:

This is syntactically correct - but you have to think about what this means semantically, and if that is the desired operation:

• 异常 - 您真的只想在 System.err 上打印它们吗?
  • 如果您只需要有效的地址，您是否要打印它们?

  编辑在这种情况下，控制如何流动 - 正如 OP 询问最后的 return null 是否会在成功执行中否定先前的值:

  EDIT How the control flows in this case - as OP asked if the return null at the end would negate the previous value, in a successful execution:

  • enter 方法 - 新堆栈帧(1. 在代码中)
    • 进入 try 块(2. 在代码中)
      • 在 try 中处理指令(3. 在代码中)
      • return语句:停止执行block，将值返回到上一个栈帧(代码中的4.)
      • (现在不是这样，但是如果有一个 finally 块，现在就会执行，甚至会覆盖返回的值...)
      • enter method - new stack frame (1. in code)
        • enter try block (2. in code)
          • process instructions in try (3. in code)
          • return statement: stop execution of block, the value is returned to the previous stack frame (4. in code)
          • (not a case now, but if there was a finally block, that would be executed now, and that could even overwrite the returned value...)

          在不成功的情况下，(例如UnknownHostException):

          In unsuccessful case, (UnknownHostException for example):

          • enter 方法 - 新堆栈帧(1. 在代码中)
            • 进入 try 块(2. 在代码中)
              • 在 try 中处理指令(3. 在代码中)
              • 抛出异常
              • 进程捕获块(日志异常，6. in code)
              • (现在不是这样，但是如果有一个 finally 块，现在就会执行，甚至会覆盖返回的值...)
              • process catch block (log exception, 6. in code)
              • (not a case now, but if there was a finally block, that would be executed now, and that could even overwrite the returned value...)

              如您所见，在成功的情况下，return null; 语句，即使它在真正的返回"之后，也不会影响返回值.每当达到返回时，当前块的执行就会停止.(如果实际上下文中有一个，则相应的 finally 块将获得控制权).

              As you see, in successful case, the return null; statement, even though it is after the "real return", doesn't influence the returned value. Whenever a return is reached, the eecution of the current block is stopped. (and if there is one in the actual context, the appropriate finally block will get the control).

              finally 块很棘手:仔细阅读它，这将是有用的知识.

              The finally block is tricky though: read up on that, it will be useful knowledge.

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