@Override
    public StructType schema() {
        return df.schema();
    }
@Test
    public void testSchema() {
        System.out.println("schema");
        System.out.println(df.schema());
        System.out.println(df.structure());
        System.out.println(df);
        smile.data.type.StructType schema = DataTypes.struct(
                new StructField("age", DataTypes.IntegerType),
                new StructField("birthday", DataTypes.DateType),
                new StructField("gender", DataTypes.ByteType, new NominalScale("Male", "Female")),
                new StructField("name", DataTypes.StringType),
                new StructField("salary", DataTypes.object(Double.class))
        );
        assertEquals(schema, df.schema());
    }