Lesson 8: Mutator Methods

We now know how to access the values stored in private instance variables, but what if we needed to change them after we have already created the object? Let's revisit the Painter class and the methods it used to modify the values stored in its instance variables.

Discuss: Click through the animated slide to display the prompts. Use the Retrieve-Pair-Share strategy to discuss the prompts.

• What did it mean for a method to return void?
• What methods in the Painter class were void methods?
• Why did these methods return void?

Discussion Goal: Students recall that void methods do not return a value and identify the methods in the Painter class that were void methods, such as move() and paint(). Students note that these methods were meant to perform an action and not provide any information.

We learned about accessor methods in the previous lesson that we could use to get the value stored in an instance variable. We can also use mutator methods to modify a value stored in an instance variable.

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 code in this program?
• What do you wonder about the code in this program?

Discussion Goal: Students notice that the methods change the values stored in the instance variables. Students may wonder how these methods are able to change the values since instance variables are private.

Display: Show the video – Mutator Methods.

Mutator methods are void methods, which means they do not return a value and therefore cannot be called as part of an expression. As void methods, they can perform an action, like changing the state of an object.

Do This: Click through the animated slide to demonstrate writing and calling a mutator method.

When designing a class, programmers make decisions about what data to make accessible and modifiable from an external class. Data can be either accessible or modifiable, or it can be both or neither.

Discuss: Click through the animated slide to display the prompts. Use the Hold That Thought strategy to discuss the prompts.

• What is different about the way mutator methods work in comparison to accessor methods?
• Should all instance variables have mutator methods? When would we not want an instance variable to be modified?
• Are there values that should be considered invalid for certain instance variables?

Discussion Goal: Students identify similarities between mutator methods and accessor methods, such as both working with the instance variables of a class. Students also identify differences, such as mutator methods taking a parameter to set a new value to the instance variable and does not return a value while accessor methods do not have a parameter and return the current value stored in the instance variable.

We can use an if statement to check for invalid values. For example, we may want to make sure that a user doesn't enter a negative value for a price or a quantity.

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

• Where have you used operators like less than, greater than, or equal?

Discussion Goal: Students recall using operators in math or previous computer science courses.

Group: Place students in pairs.

Do This: Direct students to Level 2 on Code Studio to investigate the program with a partner. Students make the changes to the program as prompted.

Do This: Click through the animated slide to introduce relational operators.

Do This: Click through the animated slide to define Boolean expression.

Do This: Click through the animated slide to define sentinel value.

Do This: Direct students to Level 3 on Code Studio to complete Levels 3 through 5. Students write mutator methods in the Dessert class on Level 3, then continue to Level 4 to use the Dessert class mutator methods. On Level 5, students complete a choice level to use the Scanner class and a sentinel value to repeatedly take input from a user.

Students use the Dessert classes they have developed in previous lessons to write accessor methods for their instance variables. 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 class to the Backpack.

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

We saw some new types of errors today! Let's discuss these errors we encountered and update our Debugging Wall with the strategies we used to debug them.

Discuss: Click through the animated slide to display the prompts. Use the Hold That Thought strategy to discuss the prompts.

• What errors did you encounter while writing your code?
• How did you find and fix the error?

Discussion Goal: Students share errors that they encountered while writing their code and strategies they used to debug the errors.

Ask students to consider both syntax and logic errors they might have encountered, such as not specifying a parameter for a mutator method or setting an invalid value for an instance variable.

Do This: Have students choose a strategy as a class and add it to the Debugging Wall.

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 2.4
• Topic Questions 3.1
• Topic Questions 5.5

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