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In my opinion the most universal way to store the keypoints is to first convert them to a data-interchange format like JSON.

After you are able to do that conversion you have a lot of flexibility to store it. JSON is easily converted to a String and/or sent through a network connection.

With OpenCV C++ you are able to store data as YAML, but that is not available for Android yet like it was pointed by Andrey Pavlenko. JSON here has the same purpose as YAML.

To parse JSON in Java you can use this easy to use library Google GSON.

And here is my first attempt to do exactly that (I did a simple test and it worked, let me know if there are problems):

 public static String keypointsToJson(MatOfKeyPoint mat){
    if(mat!=null && !mat.empty()){          
        Gson gson = new Gson();

        JsonArray jsonArr = new JsonArray();            

        KeyPoint[] array = mat.toArray();
        for(int i=0; i<array.length; i++){
            KeyPoint kp = array[i];

            JsonObject obj = new JsonObject();

            obj.addProperty("class_id", kp.class_id); 
            obj.addProperty("x",        kp.pt.x);
            obj.addProperty("y",        kp.pt.y);
            obj.addProperty("size",     kp.size);
            obj.addProperty("angle",    kp.angle);                          
            obj.addProperty("octave",   kp.octave);
            obj.addProperty("response", kp.response);

            jsonArr.add(obj);               
        }

        String json = gson.toJson(jsonArr);         

        return json;
    }
    return "{}";
}

public static MatOfKeyPoint keypointsFromJson(String json){
    MatOfKeyPoint result = new MatOfKeyPoint();

    JsonParser parser = new JsonParser();
    JsonArray jsonArr = parser.parse(json).getAsJsonArray();        

    int size = jsonArr.size();

    KeyPoint[] kpArray = new KeyPoint[size];

    for(int i=0; i<size; i++){
        KeyPoint kp = new KeyPoint(); 

        JsonObject obj = (JsonObject) jsonArr.get(i);

        Point point = new Point( 
                obj.get("x").getAsDouble(), 
                obj.get("y").getAsDouble() 
        );          

        kp.pt       = point;
        kp.class_id = obj.get("class_id").getAsInt();
        kp.size     =     obj.get("size").getAsFloat();
        kp.angle    =    obj.get("angle").getAsFloat();
        kp.octave   =   obj.get("octave").getAsInt();
        kp.response = obj.get("response").getAsFloat();

        kpArray[i] = kp;
    }

    result.fromArray(kpArray);

    return result;
}

In my opinion the most universal way to store the keypoints is to first convert them to a data-interchange format like JSON.

After you are able to do that conversion you have a lot of flexibility to store it. JSON is easily converted to a String and/or sent through a network connection.

With OpenCV C++ you are able to store data as YAML, but that is not available for Android yet like it was pointed by Andrey Pavlenko. JSON here has the same purpose as YAML.

To parse JSON in Java you can use this easy to use library Google GSON.

And here is my first attempt to do exactly that (I did a simple test and it worked, let me know if there are problems):

 public static String keypointsToJson(MatOfKeyPoint mat){
    if(mat!=null && !mat.empty()){          matToJson(Mat mat){        
    JsonObject obj = new JsonObject();

    if(mat.isContinuous()){
        int cols = mat.cols();
        int rows = mat.rows();
        int elemSize = (int) mat.elemSize();    

        byte[] data = new byte[cols * rows * elemSize];

        mat.get(0, 0, data);

        obj.addProperty("rows", mat.rows()); 
        obj.addProperty("cols", mat.cols()); 
        obj.addProperty("type", mat.type());

        // We cannot set binary data to a json object, so:
        // Encoding data byte array to Base64.
        String dataString = new String(Base64.encode(data, Base64.DEFAULT));

        obj.addProperty("data", dataString);            

        Gson gson = new Gson();

        JsonArray jsonArr = new JsonArray();            

        KeyPoint[] array = mat.toArray();
        for(int i=0; i<array.length; i++){
            KeyPoint kp = array[i];

            JsonObject obj = new JsonObject();

            obj.addProperty("class_id", kp.class_id); 
            obj.addProperty("x",        kp.pt.x);
            obj.addProperty("y",        kp.pt.y);
            obj.addProperty("size",     kp.size);
            obj.addProperty("angle",    kp.angle);                          
            obj.addProperty("octave",   kp.octave);
            obj.addProperty("response", kp.response);

            jsonArr.add(obj);               
        }

        String json = gson.toJson(jsonArr);         
gson.toJson(obj);

        return json;
    } else {
        Log.e(TAG, "Mat not continuous.");
    }
    return "{}";
}

public static MatOfKeyPoint keypointsFromJson(String Mat matFromJson(String json){
    MatOfKeyPoint result = new MatOfKeyPoint();

    JsonParser parser = new JsonParser();
    JsonArray jsonArr = parser.parse(json).getAsJsonArray();        
JsonObject JsonObject = parser.parse(json).getAsJsonObject();

    int size = jsonArr.size();

    KeyPoint[] kpArray rows = JsonObject.get("rows").getAsInt();
    int cols = JsonObject.get("cols").getAsInt();
    int type = JsonObject.get("type").getAsInt();

    String dataString = JsonObject.get("data").getAsString();       
    byte[] data = Base64.decode(dataString.getBytes(), Base64.DEFAULT); 

    Mat mat = new KeyPoint[size];

    for(int i=0; i<size; i++){
        KeyPoint kp = new KeyPoint(); 

        JsonObject obj = (JsonObject) jsonArr.get(i);

        Point point = new Point( 
                obj.get("x").getAsDouble(), 
                obj.get("y").getAsDouble() 
        );          

        kp.pt       = point;
        kp.class_id = obj.get("class_id").getAsInt();
        kp.size     =     obj.get("size").getAsFloat();
        kp.angle    =    obj.get("angle").getAsFloat();
        kp.octave   =   obj.get("octave").getAsInt();
        kp.response = obj.get("response").getAsFloat();

        kpArray[i] = kp;
    }

    result.fromArray(kpArray);
Mat(rows, cols, type);
    mat.put(0, 0, data);

    return result;
mat;
}

In my opinion the most universal way to store the keypoints a Mat is to first convert them it to a data-interchange format like JSON. JSON.

After you are able to do that conversion you have a lot of flexibility to store it. JSON is easily converted to a String and/or sent through a network connection.

With OpenCV C++ you are able to store data as YAML, but that is not available for Android yet like it was pointed by Andrey Pavlenko. JSON here has the same purpose as YAML.

To parse JSON in Java you can use this easy to use library Google GSON.

And here is my first attempt to do exactly that (I did a simple test and it worked, let me know if there are problems):

public static String matToJson(Mat mat){        
    JsonObject obj = new JsonObject();

    if(mat.isContinuous()){
        int cols = mat.cols();
        int rows = mat.rows();
        int elemSize = (int) mat.elemSize();    

        byte[] data = new byte[cols * rows * elemSize];

        mat.get(0, 0, data);

        obj.addProperty("rows", mat.rows()); 
        obj.addProperty("cols", mat.cols()); 
        obj.addProperty("type", mat.type());

        // We cannot set binary data to a json object, so:
        // Encoding data byte array to Base64.
        String dataString = new String(Base64.encode(data, Base64.DEFAULT));

        obj.addProperty("data", dataString);            

        Gson gson = new Gson();
        String json = gson.toJson(obj);

        return json;
    } else {
        Log.e(TAG, "Mat not continuous.");
    }
    return "{}";
}

public static Mat matFromJson(String json){
    JsonParser parser = new JsonParser();
    JsonObject JsonObject = parser.parse(json).getAsJsonObject();

    int rows = JsonObject.get("rows").getAsInt();
    int cols = JsonObject.get("cols").getAsInt();
    int type = JsonObject.get("type").getAsInt();

    String dataString = JsonObject.get("data").getAsString();       
    byte[] data = Base64.decode(dataString.getBytes(), Base64.DEFAULT); 

    Mat mat = new Mat(rows, cols, type);
    mat.put(0, 0, data);

    return mat;
}