gistfile1.java

/**
 * Interface for printers.
 * <p>
 * Printers are capable of printing a message, but the abstraction does not
 * require a printer to print to any particular destination.
 */
public interface Printer {

    /**
     * Prints the message.
     * <p>
     * Implementers are generally expected to call
     * {@link Message#accept(MessageVisitor)} and then feed each visited
     * message part to some destination of their choosing.
     *
     * @param message the message to print.
     */
    void print(Message message);
}

/**
 * Implementation of a printer which writes to the standard output stream.
 */
public class SystemOutPrinter implements Printer {

    @Override
    public void print(Message message) {
        message.accept(new MessageVisitorAdapter() {
            @Override
            public void visitMessageBody(String messageBody) {
                System.out.println(messageBody);
            }
        });
    }
}

/**
 * Interface holding the configuration for the application.
 */
public interface ApplicationConfig {

    /**
     * Gets the message to display.
     *
     * @return the message to display.
     */
    String getMessage();
}

/**
 * Default implementation of the application configuration interface, simply
 * feed the configuration into the constructor.
 */
public class DefaultApplicationConfig implements ApplicationConfig {

    /**
     * The message to display.
     */
    private final String message;

    /**
     * Constructs the application configuration object.
     *
     * @param message the message to display.
     */
    public DefaultApplicationConfig(String message) {
        this.message = message;
    }

    @Override
    public String getMessage() {
        return message;
    }
}

/**
 * Interface for message objects.
 */
public interface Message {

    /**
     * Accepts a visitor to each part of the message object.  Can be used to
     * get at the parts of the message to perform various optimisations on
     * them.
     *
     * @param visitor the visitor.
     */
    void accept(MessageVisitor visitor);
}

/**
 * Interface to be implemented when you wish to visit a message.
 *
 * @see Message#visit(MessageVisitor)
 */
public interface MessageVisitor {

    /**
     * Called when visiting the message body.
     *
     * @param messageBody the message body.
     */
    void visitMessageBody(String messageBody);
}

/**
 * Do-nothing implementation of {@link MessageVisitor} for forwards
 * compatibility.
 */
public abstract class MessageVisitorAdapter {

    @Override
    public void visitMessageBody(String messageBody) {
        // Do nothing.
    }
}

/**
 * Default implementation of a message.
 * <p>
 * Stores the message as an ordinary Java field.
 */
public class DefaultMessage implements Message {

    /**
     * The message body.
     */
    private final String messageBody;

    /**
     * Constructs the message.
     *
     * @param messageBody the message body.
     */
    public DefaultMessage(ApplicationConfig config) {
        messageBody = config.getMessage();
    }

    @Override
    public void accept(MessageVisitor visitor) {
        visitor.visitMessageBody(messageBody);
    }
}

/**
 * Interface to run the application itself.
 */
public interface Application {

    /**
     * Runs the application.
     */
    void run();
}

/**
 * Default implementation of the application itself.
 */
public class DefaultApplication implements Application {

    /**
     * The printer to use for displaying the message.
     */
    private final Printer printer;

    /**
     * The message to display.
     */
    private final Message message;

    /**
     * Constructs the application.
     *
     * @param printer the printer to use for displaying the message.
     * @param message the message to display.
     */
    public DefaultApplication(Printer printer, Message message) {
        this.printer = printer;
        this.message = message;
    }

    @Override
    public void run() {
        printer.print(message);
    }
}

/**
 * Main class.
 */
public class Main {

    /**
     * Main entry point.
     *
     * @param args command-line arguments (ignored.)
     */
    public static void main(String[] args) {
        MutablePicoContainer container = new DefaultPicoContainer();
        container.addComponent(Printer.class, SystemOutPrinter.class);
        container.addComponent(Message.class, DefaultMessage.class);
        container.addComponent(Application.class, DefaultApplication.class);
        container.addComponent(ApplicationConfig.class, new DefaultApplicationConfig("Hello, World!"));
        container.start();

        Application app = container.getComponent(Application.class);
        app.run();

        container.stop();
    }
}