How to iterate over non zero values in a sparse ma

问题:

The code example on the COLT website shows:

DoubleFactory2D factory;
factory = DoubleFactory2D.sparse;

DoubleMatrix2D matrix = factory.make(3,4);
    for (int row = matrix.rows(); --row >= 0;) {
     for (int column = matrix.columns(); --column >= 0;) {
       value = matrix.getQuick(row, column);
     }
    }

but this does not take advantage of the sparsity of the matrix. I'd like to iterate over non zero values only.