quizPrep.md

Week 7:

pog: PolymOrphismisGreat
Disclaimer: This document is meant to supplement other studying materials, not replace them

Concepts

• Everything covered last week has a high likelihood of being on an exam
• References and Polymorphism

You can use a code playground on the CS 125 website to follow along and test the code! https://cs125.cs.illinois.edu/

Conceptual Check

What is the difference between reference equality and object equality. How are they called?
When a function takes in an object what is it really taking in?

void one(FillerObject o) {
  o.addOne();
}


void one(int i) {
  i++;
}

What do arrays of objects store? Do you understand the behavioral implications when compared to numbers? See the example below

class Basic() {
  int i = 0;
  void incrementI() {
    i++;
  }
}

Basic[] example = new Basic[4];
//At this point we have created an array of what? Have we created ANY person objects.
  
for(int i = 0; i < example.length; i++) {
  example[i] = new Basic();
}
//How many objects are in example?
//what if i did:
Basic holder = new Basic();
for(int i = 0; i < example.length; i++) {
  example[i] = holder;
}

Do you understand the idea of making shallow and deep copies of the array shown above? Quiz Prep: Design a class that uses both static methods and variables

Practice Problems

1. Here is a book class:
   

public class Book {
  //make sure these are private on the exam 
  public String author;
  public String title;
  public int pageNumbers;
  //constructor is obviously public
  public Book(String setAuthor, String setTitle, int setPageNumbers) {
    author = setAuthor;
    title = setTitle;
    pageNumbers = setPageNumbers;
  }
}

Create a class that extends the book class in any way you see fit. Have a bookshelf that stores an array of Book objects. Downcast the book object to unlock the unique properties of the class you created.

public class Book {
  //make sure these are private on the exam 
  public String author;
  public String title;
  public int pageNumbers;
  //constructor is obviously public
  public Book(String setAuthor, String setTitle, int setPageNumbers) {
    author = setAuthor;
    title = setTitle;
    pageNumbers = setPageNumbers;
  }
  
  public static Book copy(Book o) {
    Book toReturn = new Book(o.title, o.author, o.pageNumbers);
    return toReturn;
  }
}

public class Fiction extends Book {
  public int fictionLevel;
  Fiction(String setAuthor, String setTitle, int setPageNumbers, int setFictionLevel) {
    super(setAuthor, setTitle, setPageNumbers);
    fictionLevel = setFictionLevel;
  }
}

2. Define a copy constructor for the Book object above. Your copy constructor should take in one book object. Create an array of only book objects and use it to create a new array with deep copies.

public class Book {
  //make sure these are private on the exam 
  public String author;
  public String title;
  public int pageNumbers;
  //constructor is obviously public
  public Book(String setAuthor, String setTitle, int setPageNumbers) {
    author = setAuthor;
    title = setTitle;
    pageNumbers = setPageNumbers;
  }
  
  public static Book copy(Book o) {
    Book toReturn = new Book(o.title, o.author, o.pageNumbers);
    return toReturn;
  }
}

public class Fiction extends Book {
  public int fictionLevel;
  Fiction(String setAuthor, String setTitle, int setPageNumbers, int setFictionLevel) {
    super(setAuthor, setTitle, setPageNumbers);
    fictionLevel = setFictionLevel;
  }
}

3. Create a Pet class with a method ``getType`` that returns the String "Pet". The pet class also has a method ``noise`` that returns a string, in this case ``null``. Create a class ``Dog`` that extends ``Pet``. ``Dog`` overrides ``getType`` and returns the String ``Dog``. Override `noise` to return ``Woof``.

Then try out:

Pet j = new Dog();
j.getType();
j.noise();

public class Pet {

  public String noise() {
    return null;
  }
  public String getType() {
    return "Pet";
  }
  
  
}

public class Dog extends Pet {

  public String getType() {
    return "Dog";
  }
  
  public String bark() {
    return "Bark";
  }
}

FeedBack: https://forms.gle/yJLaYFaBtthPf3AP6