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Relations Between Tables

As previously mentioned, the DVD might have a relation to a Director table, holding information about the director of the movie on the DVD. In this situation, you'd need two DAOs—one for the DVD and one for the Director—and a DTO that combines the data from these two tables. It then becomes the the transaction handler's job to get the information from the DAOs and return a DTO to the clients.

A problem arises when some clients want only the key of the director and others want more information about the director. You may end up with a single DTO containing all the director information (and possibly more) or a lot of nearly identical DTOs. Here's one approach to solve this problem: first define a DVD DTO that holds only the key for the director. Then, make an extension of the DVD DTO that holds the full director information. The following example shows how it works. First, the Director DTO:

package dk.hansen.dto;

import java.util.Date;

public class Director_DTO {

  private Key_DTO key;

  private String name;

  ... (other attributes, constructors, setters and getters)
}

Now, it's possible to use this class in a new DVD DTO:

package dk.hansen.dto;

public class DVD_DTO extends DVDKeyValue_DTO {

  protected Director_DTO director = new Director_DTO();

  public Key_DTO getDirectorKey() {
    return director.getKey();
  }

  public void setDirectorKey(Key_DTO directorKey) {
    director.setKey(directorKey);
  }

  public static void main(String[] args) {
    // For testing only...
    Director_DTO dir = new Director_DTO();
    dir.setKey(new Key_DTO("13"));
    dir.setName("Steven Spielberg");
 
    DVD_DTO dvd = new DVD_DTO();
    dvd.setKey(new Key_DTO("12"));
    dvd.setTitle("Jurassic Park");
    dvd.setDirectorKey(dir.getKey());
    
    System.out.println(dvd.getDirectorKey().getId());
  }
}

Note that the director member variable is actually defined as a complete Director DTO object, but you can only get to its key, since its getter—and setter—methods only reference the key. To get a DVD DTO that contains a complete Director object you extend the class as follows:

package dk.hansen.dto;

public class DVDWithDirector_DTO extends DVD_DTO {

  public void setDirector(Director_DTO director) {
    this.director = director;
  }

  public Director_DTO getDirector() {
    return director;
  }

  public static void main(String[] args) {
    Director_DTO dir = new Director_DTO();
    dir.setKey(new Key_DTO("13"));
    dir.setName("Steven Spielberg");

    DVDWithDirector_DTO dvd = new DVDWithDirector_DTO();
    dvd.setKey(new Key_DTO("123"));
    dvd.setTitle("Some Movie");
    dvd.setDirector(dir);

    System.out.println(dvd.getDirectorKey().getId());
    System.out.println(dvd.getDirector().getKey().getId());
    System.out.println(dvd.getDirector().getName());
  }
}

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