CS 199 EMP

Even More Practice

2 meetings every week Tuesday and Thursday

  • Early CST - Early risers and Eastern timezones

  • Evening CST - Western timezones (and really late Eastern folks)

  • Calendar contains zoom links

  • Attend 10 ones for credit.

Format

  • 3 practice problems every session

  • focused on practical understanding of concepts covered recently in class

  • slides and material available after class

How to follow along

  • Small breakout rooms of working together

  • Work for 10-15 minutes together, feel free to use paper, whiteboards, online share tools.

  • Use the code playground on the 125 homepage for the interactive running

Polymorphism Casting

From Tuesday: Upcasting

public class Book { }
public class Fiction extends Book { }
public class Romance extends Fiction { }

  • Up casting is going up the inheritance chain.

  • Romance can be cast up to Fiction, Book, Object. Fiction can be cast up to Book and Object

  • Everything can basically be cast into an Object since everything inherits from that

From Teuesday: Default methods and equals use cases

  • In java all objects by default have toString(), hashCode() and equqls(Object o)

  • They're not often that good so we override them to work as we want using @Override

  • For all objects we use equals() and for all primitives/null we use ==

From Teuesday: Equals - algorithm

  1. Check if null

  2. Check if correct type using instanceof

  3. Downcast it to the right type and then check all instance variables using a mix of == and equals()

  4. Don't forget @Override

Copycat

  • Java has no built-in way to copy an object
  • Complier has no way of telling how you would want to copy Objects you defined!
    • Other languages, like C++ will create a (bad) default constructor for you in the absence of a real cc
  • To copy Objects, we need to build a copy constructor
  Cat(Cat c) {  
      height = c.height; 
      cuteness = c.cuteness; 
  }

Shallow VS Deep copying

public class GradeBook { 
	private int[] scores; 
	public GradeBook(int[] values) { 
		scores = values; 
	}
}
  • In a shallow copy we simply copy over all the refferences to the original
  • In other words, our 'copy' just reffers to the original object, nothing new is created
  • This means that when the original data changes, so does the object
  • A deep copy will require making a new object
public class GradeBook { 
  private int[] scores; 
  public GradeBook(int[] values) { 
    scores = values; 
  }
}

The above example actually yields the following, values and scores are both references to the same array:

Shallow Copy

Deep Copy

A true deep copy would yield the following:
Deep Copy

Exercise 1

Use the starter code from the GradeBook class example to fix the copy constructor to make a deep copy

Starter Code

public class GradeBook { 
  private int[] scores; 
  
  public GradeBook(int[] values) { 
    scores = values; 
  }
}

Dot Notation

public class Bird {
  public int wings = 0;
  public String color = "blue";
}

void color_bird(Bird canary) {
  canary.color = "yellow";
}

void redBull(Bird canary) {
  canary.wings += 2;
}

Bird bigBird = new Bird();
System.out.println(bigBird.wings);
System.out.println(bigBird.color);

color_bird(bigBird);
redBull(bigBird);

System.out.println(bigBird.wings);
System.out.println(bigBird.color);

Exercise 2 (practice with dot notation)

  • create a class Student with attributes grad_year and major.
  • create one function void upgrade() that will change an only an ECE major to a CS major
  • create anoter function void graduate() that will change their grad year to 2020.
  • No starter code for this one, start from scratch! ;)

Weekly recap

  • Remaining time is for you to ask questions about literally anything the class has covered so far!
  • Don't hesitate :)

Solutions

public class GradeBook { 
  private int[] scores; 
  
  public GradeBook(int[] values) { 
    scores = new int[values.length];
    for (int i = 0; i < values.length; i++) {
      scores[i] = values[i];
    }
  }
}

public class Student {
  private int gradYear;
  private String major;
  public class Student(int g, String m) {
    gradYear = g;
    major = m;
  }

  public void upgrade() {
    if (major.equals("ECE")) {
      major = "CS";
    }
  }

  public void graduate() {
    gradYear = 2020;
  }
}