byte[] mac = NetworkInterface.getNetworkInterfaces().nextElement().getHardwareAddress();

String macAddress = "";
for (int i = 0; i < mac.length; i++) {
    macAddress += String.format("%02X%s", mac[i], (i < mac.length - 1) ? ":" : "");
}

WifiManager manager = (WifiManager) getSystemService(Context.WIFI_SERVICE);
WifiInfo info = manager.getConnectionInfo();
String macAddress = info.getMacAddress();