Bridge pattern - Example

If you have a question database, you may want to develop a program to display it based on the user selection.

The following is a simple example to show how to use a Bridge pattern to decouple the relationship among the objects.

// abstraction

interface Question { 

    public void nextQuestion();

    public void priorQuestion();

    public void newQuestion(String q);

    public void deleteQuestion(String q);

    public void displayQuestion();

    public void displayAllQuestions();

}

// implementation

class QuestionManager {

  protected Question questDB; //instantiate it later

  public String catalog;

  public QuestionManager(String catalog) {

      this.catalog = catalog;

  }

  public void next() {

      questDB.nextQuestion();

  }

  public void prior() {

      questDB.priorQuestion();

  }

  public void newOne(String quest) {

      questDB.newQuestion(quest);

  }

  public void delete(String quest) {

      questDB.deleteQuestion(quest);

  }

  public void display() {

      questDB.displayQuestion();

  }

  public void displayAll() {

      System.out.println("Question Catalog: " + catalog);

      questDB.displayAllQuestions();

  }

}

// further implementation

class QuestionFormat extends QuestionManager {

    public QuestionFormat(String catalog){

        super(catalog);

    }

    public void displayAll() {

        System.out.println("\n~~~~~~~~~~~~~~~~~~~~~~~~");

        super.displayAll();

        System.out.println("~~~~~~~~~~~~~~~~~~~~~~~~");

    }

}

// decoupled implementation

class JavaQuestions implements Question {

    private List<String> questions = new ArrayList<String>();

    private int current = 0;

    public JavaQuestions() {

        //load from a database and fill in the container

        questions.add("What is Java? ");

        questions.add("What is an interface? ");

        questions.add("What is cross-platform? ");

        questions.add("What is UFT-8? ");

        questions.add("What is abstract? ");

        questions.add("What is Thread? ");

        questions.add("What is multi-threading? ");

    }

    public void nextQuestion() {

        if( current <= questions.size() - 1 )

            current++;

    }

    public void priorQuestion() {

        if( current > 0 )

            current--;

    }

    public void newQuestion(String quest) {

        questions.add(quest);

    }

    public void deleteQuestion(String quest) {

        questions.remove(quest);

    }

    public void displayQuestion() {

        System.out.println( questions.get(current) );

    }

    public void displayAllQuestions() {

        for (String quest : questions) {

            System.out.println(quest);

        }

    }

}

class TestBridge {

    public static void main(String[] args) {

        QuestionFormat questions = 

                new QuestionFormat("Java Language");

        questions.questDB = new JavaQuestions();  

        // can be hooked up with other question class

        //questions.questDB = new CsharpQuestions();

        //questions.questDB = new CplusplusQuestions();

        questions.display();

        questions.next();    

        questions.newOne("What is object? ");

        questions.newOne("What is reference type?");

        questions.displayAll();

  }

}