Questions and Answers
What is the result of the following code: Android a = new Android(x); a.setServoCount(y); System.out.println(a.getServoCount());
The value of y
Which of the following can be used in the printInfo method in the Artwork class to produce the intended output?
What happens when executing the code: EBike eB = new EBike(4);?
Which of the following can be used to compare if hotChocolate and coffee have the same temperature?
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What is the output of the code segment: Hero j = new SuperHero('JavaHero', 50); j.powerUp(10); System.out.println(j.showPower());
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Under which condition will the statement myPet.speak(); compile and run without error?
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Which of the following best explains why the code segment involving Book and AudioBook does not compile?
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Is it true that the statement b1.equals(b2) checks if b1 and b2 have the same lengths and widths?
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Which of the following can be used to correctly initialize a variable of type B?
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What will be printed by the code segment: Dog fido = new UnderDog();? What is the result of the call fido.act()?
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What error occurs in the code segment involving the Road and Highway classes?
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What happens when the message method is removed from class A in the code segment?
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What is true about the setPrice method in the context of Vehicle and Car classes?
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Which of the following best describes the effect of executing the statement Bird b = new Hawk(5, 8);
?
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What, if anything, is printed when the code segment ClassA obj = new ClassB(); obj.showValue();
is executed?
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Which of the following statements will NOT compile?
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What code segment can replace super()
so that the Square class constructor initializes the Rectangle class instance variables height and width to x?
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The following code segment appears in a method in a class other than Book or TextBook. What best describes the effect of executing the code segment Book b = new TextBook("Psychology");
?
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Which of the following should be used to replace so that all instance variables are initialized with parameters in the Boot class?
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Under which of the following conditions will the statement someApple.printColor();
print "Red"?
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Which of the following best explains why the code segment does not produce the intended output?
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Which of the following best explains why the code segment does not compile: Game g1 = new BoardGame("checkers"); BoardGame g2 = new Game("chess");
?
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Which of the following replacements will cause a compile-time error?
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Which of the following can be used so that the TextBook constructor compiles without error?
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Which of the following must be true so that the code segment will compile without error: Coffee myCup = new Coffee(); myCup.setSize("large");
?
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Which of the following proposed constructors for the NamedPoint class would be legal?
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Which of the following best explains why the code segment does not compile: Bike b = new EBike(250); System.out.println(b.getNumOfWatts());
?
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Which of the following explanations best describes why the code segment does not compile: FamilyMembership m1 = new Membership("123");
?
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What is printed as a result of executing the following code segment?
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Which of the following declarations will compile without error?
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What is printed as a result of executing the statement Derived d1 = new Derived();
?
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Which code segment can be used to ensure the value 20 will be printed?
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Study Notes
Class Definitions and Inheritance
-
Bird Class: Contains a private variable
beakStrength
. The constructor initializes this variable with the provided input. -
Hawk Class: Inherits from
Bird
and initializes bothbeakStrength
(through the superclass constructor) and its own variabletalonStrength
. - Binding a
Bird
type variable to aHawk
object instantiates aHawk
and initializes both strength attributes via their constructors.
Method Overriding and Polymorphism
-
ClassA and ClassB:
ClassB
overridesgetValue
fromClassA
. AClassA
reference can point to aClassB
object, executingshowValue
prints "B" due to method overriding. - In Thing1 and Thing2, method calls illustrate how superclass methods can also be called from subclass overrides, affecting output.
Access Control and Constructor Conflicts
- Constructors that don't invoke the superclass constructors properly will lead to compilation errors. For instance,
Sub s3 = new Sub(5);
will not compile because it doesn't match any constructor defined. - Properly calling superclass constructors is essential for initializing inherited instance variables.
Object Types and ArrayLists
- Java's type system requires that the declared variable type matches the object it references. For example, a
Game
object cannot be referenced as aBoardGame
type.
Compilation Errors
- Using a method defined in one class on an instance of an unrelated class will lead to a compile-time error (e.g.,
two.methodB()
). - Field access restrictions due to visibility can also result in compilation errors.
Constructors in Subclasses
- When creating subclass constructors, ensure superclass constructors are called correctly to initialize inherited fields. For example,
super(the_author, the_title);
is required when initializingTextbook
.
Printing Output and Method Execution
- Methods from subclasses can override those in the superclass, affecting what gets printed during execution.
- Compiling methods referenced on variables depends on the actual object being instantiated, not just the declared type.
Object Instantiation and Implicit Calls
- Creating an instance of a subclass automatically calls the superclass's no-argument constructor unless specified otherwise, allowing for inherited field initialization without explicit calls.
Advanced Object Interaction
- Multiple variables and their interactions can lead to various expected outputs when printed. Each method's presence and visibility play a crucial role in what gets executed.
Method Interaction and Visibility
- When invoking methods, the context (whether accessing via a superclass or subclass) affects the outcome, especially when dealing with method overriding.
Key Output Expectations
- Statement outputs can be predicted based on which methods are overridden, and variables which hold values help understand behavior across method calls in inheritance hierarchies.### Class Initialization and Constructor Behavior
-
Instance Variable Initialization: In an EBike constructor, the instance variable
numBatteries
is initialized with the parameterbatteries
. If not explicitly referenced, it may lead to uninitialized variables. -
Superclass Constructor: Failure to call the
super
constructor in subclasses like EBike can result in uninitialized instance variables from the superclass, leading to compile-time errors. - Constructor Requirements: Superclasses must have constructors defined, a missing constructor in the superclass can cause execution failures.
Object Equality and Method Overriding
-
Method Overriding Using
equals
: Theequals
method in the Beverage class compares temperature values, returning true if they match. UsinghotChocolate.equals(coffee)
will setsame
to true if both beverages have the same temperature. -
Behavior of
showPower
Method: When invokingj.powerUp(10)
on a SuperHero type object, the overridden method doubles the power increment, printing the result as 70.
Polymorphism and Dynamic Binding
-
Dynamic Method Invocation: The
speak
method in the Pet class displays behavior determined at runtime. For instance, whenmyPet.speak()
is called, it can run without errors for Dog and Cat types due to polymorphism. -
Correct Method Calls: For methods like
doNothing
, valid calls depend on the class type. Example: Usinge2.doNothing(e2, e2);
is a proper instantiation for an object of type Example2.
Inheritance and Method Invocation
- Super Calls in Constructor: In class B, a constructor must explicitly call the superclass constructor if it's designed to initialize superclass variables.
- Ambiguous Calls: Compiler errors may arise due to ambiguous method calls, especially when multiple classes share method names with similar signatures.
Object Behavior and Access Modifiers
-
Private Variables Access: Private instance variables in classes cannot be accessed directly outside the class; methods like
getLength()
must be utilized. - Runtime vs Compile-time Errors: Understanding when an error is compile-time (due to method definitions and signatures) versus runtime (due to logic flaws) is crucial in debugging.
Object Instantiation and Type Compatibility
-
Instantiation Mismatches: Attempting to instantiate subclasses without proper references leads to compilation errors. For example,
Highway r3 = new Road("Sullivan Street");
will fail as Highway cannot hold a Road reference directly. -
Overridden Methods in Subclasses: The behavior of methods can change based on references; for instance, a
Ticket
reference to aDiscountTicket
will invoke the overriddengetPrice
method.
Overriding and Method Behavior
-
Super Keyword Usage:
super.getPrice()
calls the parent class method, allowing the subclass to modify its return value while still accessing the original logic.
Summary of Expected Outputs
- Output Results: Correct understanding of printed outputs is key. For example, a DiscountTicket will print "Price is 5.0" due to method overriding that alters how the price is calculated.
Important Definitions
- Method Overloading and Overriding: Distinction between methods with the same name but different signatures (overloading) versus the same name in different contexts/classes (overriding).
-
Object Equality and Hashing: Custom implementations of
equals
andhashCode
are crucial for collections and are often overridden to ensure proper functioning.
Error Investigation
- Referencing Non-Existing Methods: Always check if the methods you intend to use on an object are defined within its class or inherited from its ancestors; undefined methods result in compile-time errors.
Note on Code Compilation
- Understanding when a code segment will compile without error is essential for efficient object-oriented programming; different types of references can result in valid or invalid operations based on class definitions.
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Description
Test your knowledge with these multiple-choice questions from AP Computer Science A Chapter 9. Explore object-oriented programming concepts, including class definitions, inheritance, and method overriding. Prepare effectively for your exam and reinforce your understanding of essential programming principles.