Lesson 6: PainterPlus
45 minutes
Overview
Why would I use inheritance?
Students consider designing specialized types of classes and use inheritance to create the PainterPlus class. Students learn to create a new class and write a constructor that calls the superclass constructor using the super keyword. Students realize that while the PainterPlus class can perform the same behaviors as the Painter class, they can add new behaviors to expand its capabilities.
Standards
MOD-1 - Some objects or concepts are so frequently represented that programmers can draw upon existing code that has already been tested, enabling them to write solutions more quickly and with a greater degree of confidence
MOD-1.C - Identify, using its signature, the correct constructor being called.
- MOD-1.C.1 - A signature consists of the constructor name and the parameter list.
MOD-2 - Programmers use code to represent a physical object or nonphysical concept, real or imagined, by defining a class based on the attirbutes and/or behaviors of the object or concept
MOD-2.A - Designate access and visibility constraints to classes, data, constructors, and methods.
- MOD-2.A.5 - Constructors are designated public.
MOD-3 - When mulitple classes contain common attributes and behaviors, programmers create a new class containing the shared attributes and behaviors forming a hierachy. Modifications made at the highest level of the hierarchy apply to the subclasses.
MOD-3.B - Create an inheritance relationship from a subclass to the superclass.
- MOD-3.B.1 - A class hierarchy can be developed by putting common attributes and behaviors of related classes into a single class called a superclass.
- MOD-3.B.13 - A subclass will inherit all public methods from the superclass; these methods remain public in the subclass.
- MOD-3.B.14 - The keyword super can be used to call a superclass’s constructors and methods.
- MOD-3.B.2 - Classes that extend a superclass, called subclasses, can draw upon the existing attributes and behaviors of the superclass without repeating these in the code.
- MOD-3.B.3 - Extending a subclass from a superclass creates an “is-a” relationship from the subclass to the superclass.
- MOD-3.B.4 - The keyword extends is used to establish an inheritance relationship between a subclass and a superclass. A class can extend only one superclass.
- MOD-3.B.5 - Constructors are not inherited
- MOD-3.B.6 - The superclass constructor can be called from the first line of a subclass constructor by using the keyword super and passing appropriate parameters.
- MOD-3.B.7 - The actual parameters passed in the call to the superclass constructor provide values that the constructor can use to initialize the object’s instance variables.
MOD-3.C - Define reference variables of a superclass to be assigned to an object of a subclass in the same hierarchy.
- MOD-3.C.1 - When a class S “is-a” class T, T is referred to as a superclass, and S is referred to as a subclass.
Agenda
Objectives
Students will be able to:
- Explain the purpose and functionality of inheritance
- Write a constructor in a subclass that calls the constructor in the superclass
- Write a subclass that extends a superclass
Preparation
- Check the Teacher's Lounge for verified teachers on the CSA Forum to find additional strategies or resources shared by fellow teachers
Links
Heads Up! Please make a copy of any documents you plan to share with students.
For the students
- Inheritance - Video
- U1L6 Extra Practice - Handout
Vocabulary
superkeyword - a keyword used to refer to the superclass- constructor signature - the first line of the constructor which includes the
publickeyword, the constructor name, and the values to specify when an object is created - inheritance - an object-oriented programming principle where a subclass inherits the attributes and behaviors of a superclass
- subclass - a class that extends a superclass and inherits its attributes and behaviors
- superclass - a class that can be extended to create subclasses
Teaching Guide
Warm Up (10 minutes)
Managing Social Media Accounts
Remarks
We have learned a lot about creating classes and objects so far. Software engineers sometimes need to create specific types of objects that share common attributes and behaviors and have additional features. Let's take a look at an example scenario.
Discuss: Click through the animated slide to display the problem and prompts. Use the Retrieve-Pair-Share strategy to discuss the prompts.
- You are designing a program to keep track of the different social media services that you use. You create a class called
SocialMediato represent all service media accounts with the attributesusernameandfollowersas well as a method topostMessage. - Does this
SocialMediaclass represent all the types of social media accounts you use? - What other types of social media services are there? What are the similarities and differences between these types of social media services?
Discussion Goal: Students identify different types of social media services, such as services that let them post images and messages and services that let them post videos. Students suggest similarities and differences between the types of club members, such as character limits of some services or only posting videos and not messages.
Activity (30 minutes)
Creating a New Class (15 minutes)
Remarks
In this scenario, we might want to create types of social media services based on their specific capabilities. For example, we might want to create a VideoSocialMedia class with additional behaviors for posting and sharing videos. However, a video social media service might still share the same attributes and behaviors as other social media services. To solve this problem, we can use a concept called inheritance.
Do This: Review the lesson objectives.
Display: Show the video – Inheritance.
Remarks
We're going to use inheritance to create a new type of Painter called PainterPlus. The PainterPlus class will extend the Painter class and have its own unique behaviors.
Do This: Click through the animated slide to demonstrate that the PainterPlus class will inherit the attributes and behaviors from the Painter class.
Do This: Direct students to create a UML diagram for the PainterPlus class.
Remind students of the components of a UML diagram (attributes and behaviors). Emphasize that the PainterPlus class will share the same attributes and behaviors as the Painter class, so these do not need to be added to PainterPlus. Encourage students to brainstorm behaviors they would like to add to PainterPlus on their UML diagrams.
Remarks
You might be wondering why we don't just add what we want to the Painter class instead of extending it to create a subclass. In this case, since the Painter class is part of The Neighborhood package, we can't modify it. While the Painter class lets us navigate and paint The Neighborhood, it has limitations. Extending the Painter class to create PainterPlus lets us use the behaviors we already have and add new ones.
Creating PainterPlus Objects (20 minutes)
Do This: Click through the animated slide to show how to create a new class in Java Lab.
Remarks
While a subclass inherits the attributes and behaviors of the superclass, it does not inherit the constructors in a superclass. We will need to write a constructor in the PainterPlus class so we can create PainterPlus objects.
Do This: Click through the animated slide to review the constructor for the Painter class and how it is called to create a Painter object.
Remarks
Let's make our PainterPlus class!
Do This: Direct students to Level 1 on Code Studio to complete Levels 1, 2, and 3. Students complete a Check for Understanding on Level 1, then continue to Level 2 to write the PainterPlus class by creating a new file and writing the class header and constructor. Students create a PainterPlus object on Level 3.
Wrap Up (5 minutes)
PainterPlus Behaviors
Remarks
Right now, our PainterPlus objects can only do the same things a Painter object can do. In the next lesson, we will add a new method to the PainterPlus class so that our PainterPlus objects can do things that a Painter object can't do.
Discuss: What are some things you want a PainterPlus object to do that a Painter object can't do?
Discussion Goal: Students share ideas of methods they could add to the PainterPlus class, such as a method to turn right, navigate faster, or take all of the paint from a paint bucket.
Do This: Review the concepts covered in this lesson.
Display: Key Vocabulary
Assessment: Check for Understanding
Check For Understanding Question(s) and solutions can be found in each lesson on Code Studio. You can use these questions as an exit ticket.
AP Classroom Topic Questions
To assign questions from the AP Classroom Question Bank that align with this lesson, create a custom quiz in AP Classroom by searching the Question Bank for the Essential Knowledge statements listed at the top of this lesson plan. You can find instructions and video demonstrations to do this on AP Central.
The following Topic Questions in AP Classroom can be assigned as a formative assessment for this lesson:
- Topic Questions 9.1
- Topic Questions 9.4
Note: Some Learning Objectives and Essential Knowledge statements in the suggested Topic Questions are covered in later units.
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