MVC "Pull" Pattern.

MVCPull.java

package mvc.java.pull;

import java.util.Observable;
import java.util.Observer;
import java.util.Random;

public class MVCPull {

    public static void main(String[] args) {
        // The Model doesn't know anything about the controller neither the view
        RandomNumberModel rnm = new RandomNumberModel();

        // The Controller knows about the Model but not the View
        RandomNumberController rnc = new RandomNumberController(rnm);

        // The View knows about the Controller and has a reference to the Model in the update() method.
        RandomNumberView rnv = new RandomNumberView(rnc);

        // The Observable (Model) registers the observer (View)
        rnm.addObserver(rnv);

        int i = 0;
        while (i < 5) {
            /*
            Imagine that this could be an ActionEvent
            triggered by some Button. i.e. "generateRandomNumberButton"
            */
            rnv.generateRandomNumber();
            i++;
        }

        /*
          Output

          1. 66
          2. 17
          3. 75
          4. 94
          5. 39

          *The output will vary because its randomness.
         */
    }
}

/**
 * In general, in the MVC pattern all the dependencies point to the Model.
 * The Model doesn't know anything about the controller neither the view.
 */
class RandomNumberModel extends Observable {
    private Random random;
    private int number;

    public RandomNumberModel() {
        this.random = new Random();
    }

    /**
     * Generates an integer random number between 0 and 100,
     * notifying its observers and passing its state.
     */
    public void generateRandomNumber() {
        this.number = this.random.nextInt(101);
        setChanged();
        notifyObservers(); // Pushes "this" by default.
    }

    public int getRandomNumber() {
        return number;
    }
}

/**
 * The Controller knows about the Model but not the View
 */
class RandomNumberController {
    private RandomNumberModel rnm;

    public RandomNumberController(RandomNumberModel rnm) {
        this.rnm = rnm;
    }

    /**
     * Encapsulates the call. ("Tell", don't "Ask" Principle)
     */
    public void generateRandomNumber() {
        this.rnm.generateRandomNumber();
    }
}

/**
 * Because the "Observer Pull Pattern" the View knows
 * about the Controller and the Model.
 */
class RandomNumberView implements Observer {
    private RandomNumberController rnc;
    private int position;
    private int randomNumber;

    public RandomNumberView(RandomNumberController rnc) {
        this.rnc = rnc;
        this.position = 1;
    }

    public void generateRandomNumber() {
        this.rnc.generateRandomNumber();
    }

    /**
     * @param o   MVCPullFromScratch.RandomNumberModel Object
     * @param arg Integer random number (The Model "pushes" its state)
     */
    public void update(Observable o, Object arg) {
        this.randomNumber = ((RandomNumberModel) o).getRandomNumber(); // <- o is casted
        this.updatePosition();
        this.printRandomNumber();
    }

    private void updatePosition() {
        this.position++;
    }

    /**
     * Prints the number to the standard output stream
     */
    private void printRandomNumber() {
        System.out.printf("%-1d. %s\n", this.position, this.randomNumber);
    }
}

Class Diagram - MVC "Pull" Pattern