How to access 3D histogram data - OpenCV Java

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I am writing a java program to calculate a 3-Dimensional histogram of R,G,B channels. I am unable to find out how to access the resulting 3 dimensional histogram (the Mat object which represents the histogram). My code is like below -

        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);       
        Mat sourceMat = Highgui.imread(args[0]);        
        List<Mat> sourceImages = new Vector<Mat>();         
        sourceImages.add(sourceMat);        
        Mat histogram = new Mat();              
        MatOfInt histSizes = new MatOfInt(10,10,10);        
        MatOfFloat histRanges = new MatOfFloat(0,256,0,256,0,256);  
        MatOfInt channels = new MatOfInt(0,1,2);
        Imgproc.calcHist(sourceImages,channels,new Mat(), histogram, histSizes, histRanges);

the above calcHist method runs and returns the histogram (which is basically a Mat object) which has 3 dimensions and 1000 (10x10x10) total values.

The question is how to access this histogram? Rows and Colums return -1 (as expected for a 3D Mat). Can someone help?