AP CSA 3.7 Exercise 2: Class Variables and Methods Applied Practice

Unit 3: Class Creation · Lesson 3.7 · Exercise 2

Class Variables and Methods: Applied Practice

Six scenarios on static: what belongs to the class rather than to any object, and what static code is forbidden from touching.

Back to the Lesson 3.7 pageAll Unit 3 lessons

How this one is different

The exercise-1 editor asks you to use static members. These six ask which things are shared, since a static field changed by one object is changed for every object.

Applied Practice

6 questions · scenario driven · every answer is recorded for your teacher

Question 1 of 6Analyze
A class has a static int counter incremented in its constructor. Four objects are created.
What does counter hold?
Correct. Correct, and counting instances is the textbook use of a static field.
Incorrect. Four increments occurred.
Incorrect. The behavior is fully determined.
Incorrect. Per-object copies are what instance variables give.
Question 2 of 6Apply
A static method tries to read an instance variable of its own class.
What happens?
Incorrect. There is no field for it to read.
Incorrect. The compiler rejects it before it can run.
Incorrect. Java tracks no such "most recent" object.
Correct. Correct. A static method can access instance data only through an object it is given.
Question 3 of 6Analyze
An instance method calls a static method of the same class.
Is that allowed?
Incorrect. Any instance method may do it.
Incorrect. The restriction runs one way only.
Correct. Correct: instance to static is fine; static to instance needs an object.
Incorrect. That would make it a different kind of method.
Question 4 of 6Apply
A value is the same for every object of a class and never changes, such as a maximum size.
How should it be declared?
Incorrect. That stores an identical copy in every object for no reason.
Correct. Correct. This is exactly the case static final exists for.
Incorrect. A local variable disappears when the constructor ends.
Incorrect. Public mutable state on every object is the worst of the options.
Question 5 of 6Evaluate
A static field is used to hold data specific to one particular object.
What goes wrong?
Correct. Correct, and the bug is silent: the first object simply starts reporting the second one's data.
Incorrect. It is legal, merely wrong.
Incorrect. static has nothing to do with mutability.
Incorrect. True only by accident, and it fails the moment a second object appears.
Question 6 of 6Apply
main is declared static.
Why must it be?
Incorrect. The return type is unrelated to staticness.
Incorrect. The parameter type is unrelated too.
Incorrect. Speed is not the reason.
Correct. Correct, and it explains why calling an instance method directly from main fails.

Where to go next

Try the Lesson 3.7 coding exercise Pick another Unit 3 lesson

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