Get android Ethernet MAC Address (not wifi interface)

Assuming your ethernet interface is eth0, try opening and reading the file /sys/class/net/eth0/address.

This is my solution based on the Joel F answer. Hope it helps someone!

/*
 * Load file content to String
 */
public static String loadFileAsString(String filePath) throws java.io.IOException{
    StringBuffer fileData = new StringBuffer(1000);
    BufferedReader reader = new BufferedReader(new FileReader(filePath));
    char[] buf = new char[1024];
    int numRead=0;
    while((numRead=reader.read(buf)) != -1){
        String readData = String.valueOf(buf, 0, numRead);
        fileData.append(readData);
    }
    reader.close();
    return fileData.toString();
}

/*
 * Get the STB MacAddress
 */
public String getMacAddress(){
    try {
        return loadFileAsString("/sys/class/net/eth0/address")
            .toUpperCase().substring(0, 17);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}

this way to use java fix it; maybe can help you

NetworkInterface netf = NetworkInterface.getByName("eth0");
byte[] array = netf.getHardwareAddress();
StringBuilder stringBuilder = new StringBuilder("");
String str = "";
for (int i = 0; i < array.length; i++) {
    int v = array[i] & 0xFF;
    String hv = Integer.toHexString(v).toUpperCase();
    if (hv.length() < 2) {
        stringBuilder.append(0);
    }
    stringBuilder.append(hv).append("-");                   
}
str = stringBuilder.substring(0, stringBuilder.length()- 1);

Check also /sys/class/efuse/mac at least on Amlogic platforms.