Gilmour Academy Orientation / PD

6: Inheritance & Algorithms

We introduce our first complex algorithms and start to pull back the scaffolding a bit. Students will get their first real experience designing aggregate objects with encapsulated properties.

Learning Targets

  • I can secure my class variables with encapsulation.

  • I can create a constructor to initialize an object.

  • I can override a parent method.

  • I can build unit tests for all methods in an aggregate object.

Inheritance

Keep it DRY and OOP-y

DRY stands for Don't Repeat Yourself. So far, that's meant that if you need to repeat a bit of code, you make a loop instead of sloppily copying and pasting the same commands a few times. Now we're seeing that OOP, or Object-Oriented Programming, has was to build classes off of one another. That means we can avoid repeating (aka DRY code) by using inheritance.

Abstract Classes

Polymorphism Practice

Polymorphism is my favorite CS term because it sounds so much fancier than it really is (sort of a theme here). In class we'll build an abstract class called Shape with some encaspulated instance variables like length and width, abstract methods like .area(). Then we'll inherit from those methods when we create types of shapes like Circle, Rectangle, Triangle and so on. Let's look closer at why this is an example of inheritance and polymorphism.

Why this is smart, object-oriented programming

Creating a parent class called Shape that all your shapes will inherit means that the common properties and functions can live in one place. If new features or changes needed to happen across all the elements in your app, you would have a logical place to check.

But why do we declare abstract methods?

Let's take a look at a shortened Shape class:

class Shape {
    
    // a single example encaspulated instance variable
    private float length;
    
    // accessor method
    public float length(){
        // using this. is not required but helpful when writing
        return this.length; 
    
    // mutator method
    public void length(float length){
        // now using this. is required to tell the local and instance var apart
        this.length = length;

    // example abstract method. Each shape will need to make or "implement" 
    pubilc abstract float area();
        
}

All the encapsulation shown between lines 3 - 14 should be familiar by now. Seek additional practice including your teacher's office hours if not. The cool new thing here is line 17. Because the parent is declaring this method, all its children (i.e class Circle extends Shape { )will have to build their own area method. The Circle class will return 3.14 * (length * length); for its area() implementation.

So how exactly is this business "polymorphic?"

Because all of my app's shapes will have a common parent, I can create a collection of Shape like: ArrayList<Shape> myShapes = new ArrayList<>();

That allows me to to loop through the whole collection and report out their areas:

for (Shape x : myShapes) {
    System.out.println("This shape's area is: " + x.area() + " cm2");
}

Multiple Inheritance?

What if I wanted to build a huge space game with thousands of types of ships. I'd make rules for cargo ships and warships. There may be times when I want multiple inheritances to be applied. Well there are some limits here in Java. You can have multiple vertical levels, like Ship --> Warship --> Battleship but a single ship can't inherit from two places simultaneously. Instead, there are Interfaces.

Elevens

Please download the guide to this old project from CollegeBoard.

Elevens 1

Let's create a simple Card object and test its encapsulated properties. 

/**
 * This is a class that tests the Card class.
 */
public class CardTester {

	/**
	 * The main method in this class checks the Card operations for consistency.
	 *	@param args is not used.
	 */
	public static void main(String[] args) {
		/* *** TO BE IMPLEMENTED IN ACTIVITY 1 *** */
	}
}

Elevens 2

Now we'll build our aggregate object, the Deck. 

/**
 * This is a class that tests the Deck class.
 */
public class DeckTester {

	/**
	 * The main method in this class checks the Deck operations for consistency.
	 *	@param args is not used.
	 */
	public static void main(String[] args) {
		/* *** TO BE IMPLEMENTED IN ACTIVITY 2 *** */
	}
}

Elevens 3: Shuffling Algorithm

This algorithm uses a lot of structures we'll see when we have to start sorting items rather than shuffling. The problem with a perfect shuffle is that after 4 passes, all the cards are back in the original order.

/**
 * This class provides a convenient way to test shuffling methods.
 */
public class Shuffler {

	/**
	 * The number of consecutive shuffle steps to be performed in each call
	 * to each sorting procedure.
	 */
	private static final int SHUFFLE_COUNT = 1;

	/**
	 * Tests shuffling methods.
	 * @param args is not used.
	 */
	public static void main(String[] args) {
		System.out.println("Results of " + SHUFFLE_COUNT +
								 " consecutive perfect shuffles:");
		int[] values1 = {0, 1, 2, 3};
		for (int j = 1; j <= SHUFFLE_COUNT; j++) {
			perfectShuffle(values1);
			System.out.print("  " + j + ":");
			for (int k = 0; k < values1.length; k++) {
				System.out.print(" " + values1[k]);
			}
			System.out.println();
		}
		System.out.println();

		System.out.println("Results of " + SHUFFLE_COUNT +
								 " consecutive efficient selection shuffles:");
		int[] values2 = {0, 1, 2, 3};
		for (int j = 1; j <= SHUFFLE_COUNT; j++) {
			selectionShuffle(values2);
			System.out.print("  " + j + ":");
			for (int k = 0; k < values2.length; k++) {
				System.out.print(" " + values2[k]);
			}
			System.out.println();
		}
		System.out.println();
	}


	/**
	 * Apply a "perfect shuffle" to the argument.
	 * The perfect shuffle algorithm splits the deck in half, then interleaves
	 * the cards in one half with the cards in the other.
	 * @param values is an array of integers simulating cards to be shuffled.
	 */
	public static void perfectShuffle(int[] values) {
		/* *** TO BE IMPLEMENTED IN ACTIVITY 3 *** */
	}

	/**
	 * Apply an "efficient selection shuffle" to the argument.
	 * The selection shuffle algorithm conceptually maintains two sequences
	 * of cards: the selected cards (initially empty) and the not-yet-selected
	 * cards (initially the entire deck). It repeatedly does the following until
	 * all cards have been selected: randomly remove a card from those not yet
	 * selected and add it to the selected cards.
	 * An efficient version of this algorithm makes use of arrays to avoid
	 * searching for an as-yet-unselected card.
	 * @param values is an array of integers simulating cards to be shuffled.
	 */
	public static void selectionShuffle(int[] values) {
		/* *** TO BE IMPLEMENTED IN ACTIVITY 3 *** */
	}
}

Elevens 4-6

In Activity 4, we'll add the shuffler method to the Deck class. It's the same process but we'll adapt using arrays to ArrayLists. We'll just skip activities 5 and 6 outright as we'll cover that material elsewhere. In case you're curious, the one cool thing we skip over for now is assert statements in Java:

Elevens 7

import java.util.List;
import java.util.ArrayList;

/**
 * This class represents a Board that can be used in a collection
 * of solitaire games similar to Elevens.  The variants differ in
 * card removal and the board size.
 */
public abstract class Board {

	/**
	 * The cards on this board.
	 */
	private Card[] cards;

	/**
	 * The deck of cards being used to play the current game.
	 */
	private Deck deck;

	/**
	 * Flag used to control debugging print statements.
	 */
	private static final boolean I_AM_DEBUGGING = false;

	/**
	 * Creates a new <code>Board</code> instance.
	 * @param size the number of cards in the board
	 * @param ranks the names of the card ranks needed to create the deck
	 * @param suits the names of the card suits needed to create the deck
	 * @param pointValues the integer values of the cards needed to create
	 *                    the deck
	 */
	public Board(int size, String[] ranks, String[] suits, int[] pointValues) {
		cards = new Card[size];
		deck = new Deck(ranks, suits, pointValues);
		if (I_AM_DEBUGGING) {
			System.out.println(deck);
			System.out.println("----------");
		}
		dealMyCards();
	}

	/**
	 * Start a new game by shuffling the deck and
	 * dealing some cards to this board.
	 */
	public void newGame() {
		deck.shuffle();
		dealMyCards();
	}

	/**
	 * Accesses the size of the board.
	 * Note that this is not the number of cards it contains,
	 * which will be smaller near the end of a winning game.
	 * @return the size of the board
	 */
	public int size() {
		return cards.length;
	}

	/**
	 * Determines if the board is empty (has no cards).
	 * @return true if this board is empty; false otherwise.
	 */
	public boolean isEmpty() {
		for (int k = 0; k < cards.length; k++) {
			if (cards[k] != null) {
				return false;
			}
		}
		return true;
	}

	/**
	 * Deal a card to the kth position in this board.
	 * If the deck is empty, the kth card is set to null.
	 * @param k the index of the card to be dealt.
	 */
	public void deal(int k) {
		cards[k] = deck.deal();
	}

	/**
	 * Accesses the deck's size.
	 * @return the number of undealt cards left in the deck.
	 */
	public int deckSize() {
		return deck.size();
	}

	/**
	 * Accesses a card on the board.
	 * @return the card at position k on the board.
	 * @param k is the board position of the card to return.
	 */
	public Card cardAt(int k) {
		return cards[k];
	}

	/**
	 * Replaces selected cards on the board by dealing new cards.
	 * @param selectedCards is a list of the indices of the
	 *        cards to be replaced.
	 */
	public void replaceSelectedCards(List<Integer> selectedCards) {
		for (Integer k : selectedCards) {
			deal(k.intValue());
		}
	}

	/**
	 * Gets the indexes of the actual (non-null) cards on the board.
	 *
	 * @return a List that contains the locations (indexes)
	 *         of the non-null entries on the board.
	 */
	public List<Integer> cardIndexes() {
		List<Integer> selected = new ArrayList<Integer>();
		for (int k = 0; k < cards.length; k++) {
			if (cards[k] != null) {
				selected.add(Integer.valueOf(k));
			}
		}
		return selected;
	}

	/**
	 * Generates and returns a string representation of this board.
	 * @return the string version of this board.
	 */
	public String toString() {
		String s = "";
		for (int k = 0; k < cards.length; k++) {
			s = s + k + ": " + cards[k] + "\n";
		}
		return s;
	}

	/**
	 * Determine whether or not the game has been won,
	 * i.e. neither the board nor the deck has any more cards.
	 * @return true when the current game has been won;
	 *         false otherwise.
	 */
	public boolean gameIsWon() {
		if (deck.isEmpty()) {
			for (Card c : cards) {
				if (c != null) {
					return false;
				}
			}
			return true;
		}
		return false;
	}

	/**
	 * Method to be completed by the concrete class that determines
	 * if the selected cards form a valid group for removal.
	 * @param selectedCards the list of the indices of the selected cards.
	 * @return true if the selected cards form a valid group for removal;
	 *         false otherwise.
	 */
	public abstract boolean isLegal(List<Integer> selectedCards);

	/**
	 * Method to be completed by the concrete class that determines
	 * if there are any legal plays left on the board.
	 * @return true if there is a legal play left on the board;
	 *         false otherwise.
	 */
	public abstract boolean anotherPlayIsPossible();

	/**
	 * Deal cards to this board to start the game.
	 */
	private void dealMyCards() {
		for (int k = 0; k < cards.length; k++) {
			cards[k] = deck.deal();
		}
	}
}

Elevens 9

/**
 * This is a class that plays the GUI version of the Elevens game.
 * See accompanying documents for a description of how Elevens is played.
 */
public class ElevensGUIRunner {

	/**
	 * Plays the GUI version of Elevens.
	 * @param args is not used.
	 */
	public static void main(String[] args) {
		Board board = new ElevensBoard();
		CardGameGUI gui = new CardGameGUI(board);
		gui.displayGame();
	}
}

You will need to add this folder to your codebase.

Concepts to Review

  • Loops

    • Searching / counting while traversing an array and an ArrayList

      • Traverse to find max and mins: public static int findLargest(int[] nums){

      • Interact with an ArrayList

        public static void makeLowerCase(ArrayList<String> commands){

      • Keep count

        public static int countNames(ArrayList<String> names, String target){

      • Shuffle methods:

        public static void selectShuffle(int[] nums){ public static void perfectShuffle(Card[] cards){

    • Nested loops

      • Find common elements between two collections:

        public static ArrayList<String> findCommonNames(String[] roster1, String[] roster2){

  • Classes

    • Encapsulation with accessor and mutator methods

    • Implementing an abstract class (with encapsulation)

      • OOP: When should you move properties and methods to an abstract layer?

        • Name three properties or methods that would be suitable for a Human class and three that would go on an abstract Mammal class.

      • How do you declare an abstract class and an abstract method?

        • Declare an abstract class called Athlete, give it private properties of name, number, position, and is_active.

        • Make a constructor.

        • Make accessor methods for each property.

      • How do you declare a class to inherit the properties from an abstract class?

        • Create classes called SoccerAthlete and BasketballAthlete that both inherit from Athlete.

        • Give each class a toString method that returns the String, "My name is {name} and I'm a {position} in soccer. I'm #{number}." Only if they're active. If they're inactive, the returning String should read, "I used to be play basketball as a {position}."

      • How can you take advantage of polymorphism?

        • Create a runner class with a main method that creates an array of Athletes. Loop through them and have each one introduce themselves.

Challenge

Submit a completed NumSet class:

import java.util.ArrayList;


public class NumSet {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        /*
        ROUND 1 tests
        */
        
        // Create a random int array of a given length, low and high end of range
        int[] randArray = randArray(15, 0, 100);
        
        // Create a random Integer ArrayList of given length, low and high range
        ArrayList<Integer> randArrL = randArrL(8, 5, 50);
        
        // How many similar elements are in a given array and ArrayList
        System.out.print("There are this many similar elements: ");
        System.out.println(compareNums(randArray, randArrL));
        
        // printPretty takes an int array and prints it out nicely
        printPretty(randArray);
        // printPretty takes an Integer ArrayList and prints it out nicely
        printPretty(randArrL);
        
        /*
        ROUND 2 tests
        */
        
        // shuffle randomizes an int array (then calls printPretty)
        shuffle(randArray);
        
        // shuffle randomizes an Integer ArrayList (then calls printPretty)
        shuffle(randArrL);
        
        // divide all numbers by two
        divByTwo(randArray);
        divByTwo(randArrL);
        
        //sumArray
        sumArray(randArray);
        sumArray(randArrL);
        
    }
    /*
    ROUND 1 code
    */
    
    // TODO: randArray
    
    
    // TODO: randArrL
    
    
    // TODO: compareNums
    
    
    // TODO: prettyPretty (overloaded)
    
    /*
    ROUND 2 code
    */
    
    // TODO: shuffle array
    
    
    // TODO: shuffle ArrayList
    
    
    // TODO: divByTwo (overloaded)
    
    
    // TODO: sumArray (overloaded)
}

Studying for the Test

Your Elevens test will be a semi-randomized selection of multiple choice questions all taken from the same sources used for our Do Nows. There will be one free response question similar to the homework. Both of these parts of the test reflect the two portions of the AP test. Therefore, the best way to study would be to review AP practice problems from our relevant topics:

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