code: public void convertToDense() {
          switch ( mat.getType() ) {
              case DSCC: {
                  DMatrix m = new DMatrixRMaj(mat.getNumRows(), mat.getNumCols());
                  ConvertDMatrixStruct.convert((DMatrix) mat, m);
                  setMatrix(m);
              } break;
              case FSCC: {
                  FMatrix m = new FMatrixRMaj(mat.getNumRows(), mat.getNumCols());
                  ConvertFMatrixStruct.convert((FMatrix) mat, m);
                  setMatrix(m);
              } break;
              case DDRM:
              case FDRM:
              case ZDRM:
              case CDRM:
                  break;
              default:
                  throw new RuntimeException("Not a sparse matrix!");
          }
      }
docs: Switches from a sparse to dense matrix