这是一段C#代码，帮我换成java代码

将这段C#代码转换为Java代码需要考虑几个关键点，包括数据类型的转换、异常处理和方法调用的差异。以下是转换后的Java代码：

import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;

public class CardReader {

    public String readCard() {
        int comHandle = 0;
        int ret = 0;
        byte macAddr = 0;
        char[] recordInfo = new char[1024];

        try {
            comHandle = K720CommOpenWithBaud("com2", 9600);
            if (comHandle < 1) {
                throw new Exception("打开端口com2失败");
            }

            for (int i = 0; i < 16; i++) {
                char[] recordInfo1 = new char[1000];
                int nRet = K720AutoTestMac(comHandle, (byte) i, recordInfo1);
                if (nRet == 0) {
                    macAddr = (byte) i;
                    break;
                }
            }

            ret = K720SendCmd(comHandle, macAddr, "FC8", 3, recordInfo); // K720_SendCmd(ComHandle, MacAddr, "FC8", 3)
            if (ret != 0) {
                throw new Exception("查询发卡器状态失败");
            }

            byte[] stateInfo = new byte[3];
            ret = K720Query(comHandle, macAddr, stateInfo, recordInfo);
            if (ret != 0) {
                throw new Exception("查询传感器状态失败");
            }

            byte[] statesInfo = new byte[50];
            ret = K720SensorQuery(comHandle, macAddr, statesInfo, recordInfo);
            if (ret != 0) {
                return "";
            }

            if (statesInfo[3] == 0x30 || statesInfo[3] == 0x34 || statesInfo[3] == 0x38 || statesInfo[3] == 0x31 || statesInfo[3] == 0x39) {
                return "";
            }

            byte[] test = new byte[128];
            ret = K720SendCmd(comHandle, macAddr, "FC7", 3, recordInfo);
            if (ret != 0) {
                throw new Exception("发卡到读卡位置失败");
            }

            Thread.sleep(300);
            ret = K720S70DetectCard(comHandle, macAddr, recordInfo);

            String strPassword = "013014633987";
            byte[] password = new byte[6];
            stringToArray(strPassword, password);

            ret = K720S70LoadSecKey(comHandle, macAddr, (byte) 0x01, (byte) 0x30, password, recordInfo);

            List<Byte> cardNumber = new ArrayList<>();
            byte[] bData = new byte[16];
            ret = K720S70ReadBlock(comHandle, macAddr, (byte) 1, (byte) 2, bData, recordInfo);
            for (byte b : bData) {
                cardNumber.add(b);
            }

            if (ret != 0) {
                throw new Exception("获取卡号失败");
            }

            ret = K720SendCmd(comHandle, macAddr, "FC0", 3, recordInfo);
            Thread.sleep(300);

            if (ret != 0) {
                throw new Exception("发卡失败");
            }

            StringBuilder result = new StringBuilder();
            for (byte b : bData) {
                result.append(String.format("%02x", b));
            }

            return result.toString();

        } catch (Exception ex) {
            ret = K720SendCmd(comHandle, macAddr, "FC0", 3, recordInfo);
            Thread.sleep(300);
            Util.recordException(this.getClass().toString() + "readCard", ex.toString());
            return "";
        } finally {
            K720CommClose(comHandle);
        }
    }

    private void stringToArray(String str, byte[] array) {
        for (int i = 0; i < str.length() && i < array.length; i++) {
            array[i] = (byte) str.charAt(i);
        }
    }

    // 假设这些方法已经实现
    private int K720CommOpenWithBaud(String port, int baudRate) {
        // 实现细节
        return 0;
    }

    private int K720AutoTestMac(int handle, byte mac, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720SendCmd(int handle, byte mac, String cmd, int len, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720Query(int handle, byte mac, byte[] stateInfo, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720SensorQuery(int handle, byte mac, byte[] statesInfo, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720S70DetectCard(int handle, byte mac, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720S70LoadSecKey(int handle, byte mac, byte secKey, byte block, byte[] password, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720S70ReadBlock(int handle, byte mac, byte sector, byte block, byte[] data, char[] info) {
        // 实现细节
        return 0;
    }

    private int K720CommClose(int handle) {
        // 实现细节
        return 0;
    }

    private static class Util {
        public static void recordException(String method, String exception) {
            // 实现细节
        }
    }
}

### 主要变化点：
1. **数据类型**：C#中的`char`在Java中仍然是`char`，但数组处理方式有所不同。
2. **字符串处理**：使用`StringBuilder`来构建最终的字符串结果。
3. **异常处理**：Java中的异常处理与C#类似，但语法略有不同。
4. **线程睡眠**：使用`Thread.sleep`替代`Thread.Sleep`。
5. **字节数组转换**：添加了`stringToArray`方法来将字符串转换为字节数组。

请根据实际需求进一步完善和测试这些方法的实现。