2 Calendar Update

Thusday's CDT section will now start at 5 to match the sceduled course time (banner)

• Eastern timezone sessions, and Tuesdays will continue at sceduled times

• Make sure to keep up with the calendar for links and timings

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 jeed playground on the 125 homepage for the interactive running

This

• always refers to the current instance that is executing the method
• in the example below this is used to distinguish name(the instance variable), and name(the method param)
• checkstyle is your friend

public class Dog {
  private String name;
  Dog(String name) {
    this.name = name;
  }
}

Static

• Attaching the static keyword to attach the method to the class, rather than the instance
• static class methods cannot access instance variables

public class Building {
  private String name;
  private int height;
  public Building(String name, int height) {
    this.name = name;
    this.height = height;
  }
  public static String getName() {
    return name;
  }
}
Building siebel = new Building("Siebel", 4);
// Will this work? why/why not?
System.out.println(siebel.getName());

Final

• keyword disallows a variable from being modified
• Perfect for constants

1) Inheritance

• Inheritance can be useful when we have objects that share functionality
• class SubClass extends BaseClass
• class Chuchu extends Pet

What do you think happens here?

public class Animal {
    public Animal(){
        System.out.println("New animal created!");
    }
}
public class Cat extends Animal {
    public Cat(){
        System.out.println("New cat created!");
    }
}
public class Example {
    public static void main(String[] args){
        Animal c1 = new Cat();
    }
}

What gets printed here?

class Base {
    final public void show() {
       System.out.println("Base::show() called");
    }
}
  
class Derived extends Base {
    public void show() {
       System.out.println("Derived::show() called");
    }
}
  
class Main {
    public static void main(String[] args) {
        Base b = new Derived();;
        b.show();
    }
}

Private/Protected

• Private variables in the parent class can't be accessed by child classes
• Child classes can acess protected variables in the parent class

Super

• the super keyword is used in the child constructor to call the parent constructor

• it has to be called at the start of the constructor.

public class Animal {
    protected String name;
    protected String sound;
    public Animal(String n, String s){
        name = n; 
        sound = s; 
    }
}

public class Cat extends Animal {
   public double cutenessLevel; 
   protected boolean hasClaws; 
}

Exercise #1 (10 minutes)

• using the sample code in the previous slide, create a constructor in the Cat class using super
• What other variables could fit in the class structure?

Exercise #2 (10 minutes)

• Create a class named Rectangle containing (doubles) length and width. Then create two methods to print the area and perimeter.

• Let class 'Square' inherit Rectangle with its constructor having a parameter for its side (suppose s), and call the constructor of its parent class as 'super(s,s)'.

class Rectangle{
 int length;
 int breadth;

 // don't forget Your constructor!
 public void findArea(){
  // code here
 }
 public void findPerimeter() {
  //code here
 }
}

class Square extends Rectangle{
  // code here
  // don't forget Your constructor!
}

Polymorphisim

• polymorphism is the provision of a single interface to entities of different types

• We can use instanceof to check if an object belongs to a class

"Is a" vs

• polymorphism is the provision of a single interface to entities of different types
  • every instance of any Java class "is a" Object
  • Dogs are (Pets are (Animals are Objects))

polymorphism example

• Say we have a class Animal with the function speak()
• Say we have class Cat that extends Animal, with a function meow()
• Which method(s) can be called on a Cat object? Which can be called on an Animal?

Upcasting vs Downcasting

• Upcasting means we are taking a child object and casting it up, as the parent object
  • ie) casting Object Cat as an Object Animal
• Downcasting means the opposite, taking a parent object, and casting it as a child
  • ie) Animal myAnimal = new Cat();
  • NOTE: if we aren't careful, we can ClassCastException
  • the compiler is your BFF

Exercise 3, ordering off the kids menu

Fix the following upcast:

public class Person{
    int age;
    Person(int setAge) {
        age = setAge;
    }
}
public class Child extends Person{
    int age;
    boolean kid;
    Child(int setAge, boolean setKid) {
        age = setAge;

    }
}

Spoilers :P

public class Animal {
  protected String name;
  protected String sound;
  public Animal(String n, String s){
    name = n; 
    sound = s; 
  }
}

public class Cat extends Animal {
 public double cutenessLevel; 
 protected boolean hasClaws; 
 public Cat(String n, String s, double c, boolean h) {
   super(n, s);
   cutenessLevel = c;
   hasClaws = h;
 } 
}

public class Rectangle {
 int l;
 int b;
 public Rectange(int setL, int setB) {
   l = setL;
   b = setB;
 }
 public void findArea() {
   return l * b;
 }
 public void findPerimeter(){
   return 2 * (l + b);
 }
}

public class Square extends Rectangle {
  public Square(int setSize) {
    super(setSize, setSize);
  }
}