@Phil – as you discovered, this conversion is automatically done by Matlab. I am not familiar with a way to override it. The best choice is to store the scalar along with some dummy value, and only use the first item in the array when you fetch it back from the hashmap.

>> map.put('key2', int32([5,0]));
>> data = map.get('key2')
data =
  2×1 int32 column vector
   5
   0
>> data(1)
ans =
  int32
   5

See the following simple example:

% initialize hashmap
map = java.util.LinkedHashMap();

% add a k/v pair with integer vector value
map.put('key1', int32([5 6 7]));

% add a k/v pair with integer scalar value
map.put('key2', int32(5));

% get the value for key1 - it's an int32, as one would expect
class(map.get('key1'))

% get the value for key2 - it's a double - Matlab automatically converted this
% is there any way to get this value's type before it is converted?
% conversion, so that I cam 
class(map.get('key2'))

Cheers,
John

@Christopher – primitive Java arrays (e.g. double[]) are automatically converted into a column vector in Matlab.

When you store your data in a Java object using automated Matlab-Java type-conversions, Java strips away any such meta-info. Matlab has no way of knowing whether the array should be a row or column array, because this meta-info is not stored by Java, so the decision to create a column Matlab array is just as valid as a decision to create a row array.

If you wish to store the Matlab data including the meta-info, then you need to serialize your Matlab data before storing it, and deserialize it back when you read it: http://undocumentedmatlab.com/blog/serializing-deserializing-matlab-data

>> dummyQueue = java.util.LinkedList
dummyQueue =
[]

>> vecIn = [1,2,3]
vecIn =
     1     2     3

>> dummyQueue.add(vecIn)
ans =
     1

>> vecOut = dummyQueue.remove()
vecOut =
     1
     2
     3

@Sepehr – while Hashtable does indeed accept arrays as keys, the automatic conversion between Java and Matlab data types causes hash.get not to find the key. Instead, use a simpler data type such as a number or string.

You also need to remember that all numeric data in Java is signed, unlike Matlab which uses uint8 for its serialization. Therefore, you need to first convert into a signed int16 before storing in the Java array, and then to convert back to uint8 when loading back into Matlab from the hash:

>> data = int16(getByteStreamFromArray([2,3]));
>> hash.put('mykey', data);
...
>> data2 = getArrayFromByteStream(uint8(hash.get('mykey')))
data2 =
     2     3

>> hash = java.util.Hashtable;
>> key = getByteStreamFromArray([1,2]);
>> value = getByteStreamFromArray([2,3]);
>> hash.put(key,value)
ans =
     []

>> hash.containsKey(key)
ans =
     0

Also, when I use strings as keys and byte arrays as values, the retrieved values are scrambled! e.g.

>> hash = java.util.Hashtable;
>> hash.put('mykey',getByteStreamFromArray([2,3]));
>> getArrayFromByteStream(hash.get('mykey'))
Error using getArrayFromByteStream
The input must be a uint8 byte stream.

Any suggestions?

To store a Matlab class object in a Java collection, you would need to first serialize the object into a byte array that can then be stored in the Java collection object (and deserialize it when it is extracted).

Comparators, AFAIK, can only be specified in Java code. I don’t think you can specify a Matlab function to act as a comparator function that Java understands.

@Rajpal – either use the Matlab Builder for Java toolbox ($$$) or use one of the free alternatives such as JMI, JNI, JNA, COM etc. The toolbox costs money but saves you a lot of programming and QA time, so it could well be worth the investment.

You can find more details in my Matlab-Java programming book (Chapter 9).