Lesson 9: Printing Objects

We have learned a lot of new terms so far! Let's review some of these terms through a game of CS Bingo.

Distribute: Give each student a unique CS Bingo Card.

Do This: Review the instructions for playing CS Bingo.

Do This: Play the music clip to cue the CS Bingo activity. Choose a random definition and read it aloud to the class. Give students a moment to check their bingo card for the correct term, then state the correct term. Repeat until a student yells "Java."

To allow for multiple students to "win" the game, continue pulling definitions until a couple more students win as well.

When we want to get information about the state of an object, we have to call each accessor method. If there are multiple instance variables, this can result in multiple lines of code just to get information about an object. What if there was an easier way?

Do This: Review the lesson objectives.

Do This: Direct students to Level 1 on Code Studio to predict the program's outcome, then run the program to compare their predictions to the actual outcome.

Discuss: Click through the animated slide to display the prompts.

• What do you notice about the output of this program?
• What do you wonder about the output of this program?

Discussion Goal: Students notice that printing an object displays random letters and numbers to the console instead of the values stored in its instance variables. Students may wonder what the letters and numbers mean and why the values are not printed.

Display: Show the video – The toString() Method.

Do This: Click through the animated slide to explain why printing an object that doesn't have a toString() method in its class does not print the values stored in its instance variables.

Do This: Click through the animated slide to review the Object class and introduce its toString() method.

Discuss: Use the Hold That Thought strategy to discuss the prompt.

• If we could write our own version of the toString() method in our Dessert class, what information would we want it to include?

Discussion Goal: Students suggest including the instance variables in the toString() method.

Do This: Click through the animated slide to define overriding and demonstrate overriding the toString() method and printing an object.

Notice that this code segment did not explicitly call the object's toString() method. The object's toString() method is automatically called when we pass an object reference to a print statement or when we concatenate an object reference with a String.

Discuss: Is this information useful? Why or why not?

Discussion Goal: Students note that the output contains the information about the object, but it is not in a user-friendly format.

Have students consider how the output might look if there were more instance variables. Ask students if this format makes it easy to quickly identify the current state of an object and what would make it easier to read.

Do This: Explain that the super keyword can be used to call the superclass method.

Do This: Introduce escape sequences, and have students determine the output for each example:

• Printing a quotation mark ( \" )
• Printing a new line ( \n )
• Printing a backslash ( \\ )

Do This: Direct students to Level 2 on Code Studio to complete Levels 2, 3, and 4. Students write a toString() method in the Dessert class then continue to Level 3 to refactor the code. On Level 4, students write toString() methods in the Dessert subclasses.

Students use the Dessert classes they have developed in previous lessons to write toString() methods. These classes are revisited throughout the curriculum. Students expand on this problem and these classes to add new behaviors and create arrays of Dessert objects in future units.

This is a good time to commit our code and save our new version of our Dessert classes to the Backpack.

Do This: Play the music clip to cue committing their code and saving the new version of the Dessert classes to the Backpack.

You have learned to override a method from the Object class. We can also override methods in other classes.

Discuss: Use the Hold That Thought strategy to discuss the prompt.

• Are there other methods you might want to override? For instance, a method in the Painter class?
• What would you have the new version of the method do?

Discussion Goal: Students suggest methods in the Painter class they would want to override, such as move() to check if a Painter object can move first before moving forward or paint() to check if a Painter object has paint first before painting.

Do This: Review the concepts covered in this lesson.

Display: Key Vocabulary

Check For Understanding Question(s) and solutions can be found in each lesson on Code Studio. These questions can be used for an exit ticket.

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 2.6
• Topic Questions 9.7

Note: Some Learning Objectives and Essential Knowledge statements in the suggested Topic Questions are covered in later units.