AP Computer Science A:: Writing Classes

What are the two main components typically found inside a well-designed Java class?

- **A)** Only constructors, with no other methods allowed
- **B)** Only a single \(main\) method
- **C)** A class can contain only one instance variable and no methods
- **D)** INSTANCE VARIABLES (data/state the object holds) and METHODS (behaviors the object can perform)

What is ENCAPSULATION, as it applies to writing Java classes?

- **A)** Making every instance variable and method \(public\) so anything can access them directly
- **B)** A technique used only for primitive data types, never for objects
- **C)** A synonym for inheritance
- **D)** Bundling an object's data (instance variables) together with the methods that operate on it, while restricting DIRECT outside access to that data (typically by making instance variables \(private\))

Why are instance variables conventionally declared \(private\) in a well-designed class?

- **A)** To prevent outside code from directly accessing or modifying them, forcing all interaction to go through the class's own methods, which can validate or control how the data changes
- **B)** \(private\) instance variables cannot be given values at all
- **C)** Java requires all instance variables to be \(private\) or the code will not compile
- **D)** Declaring variables \(private\) makes the program run faster

What is the purpose of an ACCESSOR method (also called a 'getter'), such as \(public\ int\ getAge()\ \{\ return\ age;\ \}\)?

- **A)** Accessor methods must always return \(void\)
- **B)** To allow outside code to READ the value of a \(private\) instance variable, without allowing direct modification of it
- **C)** To delete an instance variable
- **D)** To allow outside code to directly CHANGE a \(private\) instance variable's value

What is the purpose of a MUTATOR method (also called a 'setter'), such as \(public\ void\ setAge(int\ newAge)\ \{\ age\ =\ newAge;\ \}\)?

- **A)** To permanently prevent a variable from ever being changed
- **B)** To allow outside code to READ a variable's value without changing it
- **C)** To allow outside code to CHANGE the value of a \(private\) instance variable in a controlled way, often including validation logic
- **D)** Mutator methods must always return the new value as an \(int\)

Why might a MUTATOR method be considered better design than making an instance variable directly \(public\), even though both ultimately allow the value to be changed?

- **A)** A mutator method can include VALIDATION logic (e.g., rejecting a negative age) before actually changing the variable, whereas a \(public\) variable can be set to ANY value with no restriction or checking whatsoever
- **B)** Mutator methods and public variables provide identical levels of control with no meaningful difference
- **C)** Mutator methods can never actually change a variable's value
- **D)** Public variables always include automatic validation, while mutator methods never do

What is the purpose of a CONSTRUCTOR in a class definition?

- **A)** A synonym for an accessor method
- **B)** A special method (same name as the class, no return type) that runs automatically when \(new\) creates an object, used to initialize that object's instance variables to appropriate starting values
- **C)** A method used to permanently destroy an object
- **D)** A method that must be called manually every time an object's data needs to change

Can a single Java class have MULTIPLE constructors?

- **A)** Yes, through CONSTRUCTOR OVERLOADING -- multiple constructors with the SAME name (the class name) but DIFFERENT parameter lists, allowing objects to be created in different ways
- **B)** No, Java allows exactly one constructor per class, no exceptions
- **C)** Yes, but only if all constructors have identical parameter lists
- **D)** A class can have multiple constructors only if none of them accept any parameters