Prompt:
What does this code do?

```java
public class ReadBigTableTable {

     private static final Logger LOG = LoggerFactory.getLogger(ReadBigTableTable.class);

     public interface BigTableReadOptions extends DataflowPipelineOptions {
         @Description("Bigtable instance name")
         @Default.String("quickstart-instance")
         String getBigTableInstance();

         void setBigTableInstance(String value);

         @Description("Bigtable table name")
         @Default.String("my-table")
         String getTableName();

         void setTableName(String value);

         @Nullable
         @Description("Bigtable Project ID")
         String getBigTableProject();

         void setBigTableProject(String value);
     }

     public static void main(String[] args) {

        BigTableReadOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().as(BigTableReadOptions.class);

         String project = (options.getBigTableProject() == null) ? options.getProject() : options.getBigTableProject();


         Pipeline p = Pipeline.create(options);

         p
                 .apply(BigtableIO.read()
                         .withInstanceId(options.getBigTableInstance())
                         .withProjectId(project)
                         .withTableId(options.getTableName())
                 )
                 .apply("Process Row", ParDo.of(new DoFn<Row, String>() {
                             @ProcessElement
                             public void processElement(ProcessContext c) {
                                 Row row = c.element();
                                 DateTimeFormatter format = DateTimeFormat.forPattern("yyyy/MM/dd HH:mm:ss");

                                 StringBuilder rowString = new StringBuilder("BigQuery Row: " + row.getKey().toStringUtf8());
                                 for (Family family : row.getFamiliesList()) {
                                     StringBuilder familyString = new StringBuilder("\t Family: " + family.getName());
                                     for (Column column : family.getColumnsList()) {
                                         StringBuilder columnString = new StringBuilder("\t\t Column: " + column.getQualifier().toStringUtf8());

                                         for (Cell cell : column.getCellsList()) {
                                             String timestamp = Instant.ofEpochMilli(cell.getTimestampMicros() / 1000).toString(format);
                                             String cellString = "\t\t\t Cell: " + cell.getValue().toStringUtf8()
                                                     + " at " + timestamp;

                                             columnString.append("\n").append(cellString);
                                         }
                                         familyString.append("\n").append(columnString);
                                     }
                                     rowString.append("\n").append(familyString);
                                 }

                                 LOG.info(rowString.toString());
                                 c.output(rowString.toString());
                             }
                         })
                 );

         p.run();
     }
 }
```

Response:
This code snippet uses the Apache Beam BigtableIO connector to read data from a Bigtable table.

The `BigTableReadOptions` interface defines the options that can be configured when running the pipeline. In this case, it specifies the Bigtable instance name, table name, and project ID. The `--bigTableInstance`, `--tableName`, and `--bigTableProject` command-line arguments set these values when running the pipeline. The `@Description` annotation provides a description of each option, while the `@Default.String` annotation sets their default values.

```java
public interface BigTableReadOptions extends DataflowPipelineOptions {
         @Description("Bigtable instance name")
         @Default.String("quickstart-instance")
         String getBigTableInstance();

         void setBigTableInstance(String value);

         @Description("Bigtable table name")
         @Default.String("my-table")
         String getTableName();

         void setTableName(String value);

         @Nullable
         @Description("Bigtable Project ID")
         String getBigTableProject();

         void setBigTableProject(String value);
     }
```

In the `main` method, the `PipelineOptionsFactory` class creates a `BigTableReadOptions` object from the command-line arguments. The `Pipeline.create` method then creates a new pipeline with the specified options. The `project` variable is set to the Bigtable project ID, or the default project ID if the `--bigTableProject` option is not provided.

```java
        BigTableReadOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().as(BigTableReadOptions.class);

         String project = (options.getBigTableProject() == null) ? options.getProject() : options.getBigTableProject();
        Pipeline p = Pipeline.create(options);
```

The pipeline reads data from the specified Bigtable table using the `BigtableIO.read()` method, which takes the instance ID, project ID, and table ID as parameters.

```java
 p
                 .apply(BigtableIO.read()
                         .withInstanceId(options.getBigTableInstance())
                         .withProjectId(project)
                         .withTableId(options.getTableName())
                 )
```

Next, the pipeline applies a `ParDo` transform to process each `Row` read from the Bigtable table. The `processElement` method of the `DoFn` class is used to process each row. The code iterates over the families, columns, and cells of the row to construct a string representation of the row data. The row key, family name, column qualifier, cell value, and timestamp are included in the output string. The code logs the string using the `LOG.info()` logging method and outputs it to the pipeline using the `c.output()` method.

```java
                 .apply("Process Row", ParDo.of(new DoFn<Row, String>() {
                             @ProcessElement
                             public void processElement(ProcessContext c) {
                                 Row row = c.element();
                                 DateTimeFormatter format = DateTimeFormat.forPattern("yyyy/MM/dd HH:mm:ss");

                                 StringBuilder rowString = new StringBuilder("BigQuery Row: " + row.getKey().toStringUtf8());
                                 for (Family family : row.getFamiliesList()) {
                                     StringBuilder familyString = new StringBuilder("\t Family: " + family.getName());
                                     for (Column column : family.getColumnsList()) {
                                         StringBuilder columnString = new StringBuilder("\t\t Column: " + column.getQualifier().toStringUtf8());

                                         for (Cell cell : column.getCellsList()) {
                                             String timestamp = Instant.ofEpochMilli(cell.getTimestampMicros() / 1000).toString(format);
                                             String cellString = "\t\t\t Cell: " + cell.getValue().toStringUtf8()
                                                     + " at " + timestamp;

                                             columnString.append("\n").append(cellString);
                                         }
                                         familyString.append("\n").append(columnString);
                                     }
                                     rowString.append("\n").append(familyString);
                                 }

                                 LOG.info(rowString.toString());
                                 c.output(rowString.toString());
                             }
                         })
                 );
```

Finally, the `run` method executes the pipeline.

```java
         p.run();
```
