With the java class attached, try:

t=javaObject('eu.simuline.octave.OctaveInterfaceTester')

Of course t.aStr returns null, i.e. [], because not initialized.
Initialize in the following way:

arr=javaArray('java.lang.String',3)
arr(1)='xx'
arr(2)='yy'
arr(3)='zz'
t.aStr=arr

and check:

t.aStr

which really yields

ans =
<Java object: java.lang.String[]>

So all ok until now.
Normally, it should be much easier to initialize t.aStr.
Try

t.aStr=['ab';'cd';'ef']

which should initialize with another String[], namely['ab';'cd';'ef'] according to
http://de.mathworks.com/help/matlab/matlab_external/passing-data-to-a-java-method.html

but trying

t.aStr

yields

ans = [](0x0)

representing the null-value.
This is easy to see:
ClassHelper.castArgument just returns null if it cannot cast.
That way, the field becomes initialized with null.

This works for all reference types because for these null is untyped.
So, the bug is hidden.
It would be better to throw a ClassCastException in ClassHelper.castArgument.