### Question:
ST_Array extends STBase implements Cloneable { public ST_Array mtxMul(ST_Array array) { if (this.array.size() != 6 || array.size() != 6) { throw new IllegalArgumentException("矩阵乘法数组规模必须 6元素(a b c d e f)"); } double[][] a = this.toMtx(); double[][] b = array.toMtx(); double[][] res = new double[3][3]; for (int k = 0; k < 3; k++) { for (int i = 0; i < 3; i++) { for (int j = 0; j < 3; j++) { res[i][j] += a[i][k] * b[k][j]; } } } return new ST_Array( fmt(res[0][0]), fmt(res[0][1]), fmt(res[1][0]), fmt(res[1][1]), fmt(res[2][0]), fmt(res[2][1]) ); } ST_Array(Serializable... arr); static ST_Array unitCTM(); ST_Array mtxMul(ST_Array array); static ST_Array getInstance(String arrStr); ST_Array add(String item); List<String> getArray(); Double[] toDouble(); Integer[] toInt(); ST_Array setArray(List<String> array); int size(); double[][] toMtx(); void printMtx(); @Override ST_Array clone(); @Override String toString();  }### Answer:
@Test void mtxMul() { ST_Array a = new ST_Array(0, 1, -1, 0, 0, 0); ST_Array b = new ST_Array(2, 0, 0, 1, 0, 0); ST_Array c = new ST_Array(1, 0, 0, 1, 0, 3); ST_Array d = a.mtxMul(b).mtxMul(c); d.printMtx(); }