Question

I’m using MATLAB to access a postgresql database. I’m running into problems trying to access a column of type boolean[]:

x;#% x is a <1x1 org.postgresql.jdbc4.Jdbc4Array>

When accessing real[] values, I can take the following approach:

double(x.getArray());

Unfortunately, with a boolean[] this leads to the following error message:

Undefined function 'toDouble' for input arguments of type 'logical'.

So I figured converting to logical first may work:

logical(x.getArray());

Except this doesn’t work either.

Error using logical
Conversion to logical from java.lang.Boolean[][] is not possible.

The problem may arise because java.lang.Boolean doesn’t derive from java.lang.Number, however the MATLAB docs on conversion of java return types make it seem like this shouldn’t be a problem.

Am I doing something wrong here? How do I get from the fetch result to a logical array usable in MATLAB? If all else fails, I can rebuild the tables with arrays of numeric types instead of boolean[], but it seems like this should be possible without going that far.

Answer 1

I suspect that this is due to the difference in Java between Boolean (an object) and boolean, a primitive. Matlab conversions are really set up to handle primitives, not necessarily their associated object wrappers.

I think that this conversion requires a loop, as follows:

%Setup test
b = java.lang.Boolean.FALSE;
array = java.lang.reflect.Array.newInstance(b.getClass(), [5 6]);
for ix1=1:length(array); 
    for ix2 = 1:(length(array(1))); 
        array(ix1, ix2)=b; 
    end; 
end;
%Now `array` is an initialize 2D Java array of type Boolean.

%To convert to a Matlab logical array
matArray = false(length(array), length(array(1)));  %Initialize
for ix1=1:size(matArray, 1); 
    for ix2 = 1:size(matArray, 2); 
        matArray(ix1, ix2)=(array(ix1, ix2).booleanValue());
        %The  `booleanValue()` method converts from `Boolean` class  to `boolean` primitive
    end; 
end;