Question 1

[True/False] A subclass is a subset of a superclass.

Accepted Answer

False. A subclass is an extension of a superclass and normally contains more detail information than its superclass.

Question 2

What keyword do you use to define a subclass?

Accepted Answer

The “extends” keyword is used to define a subclass that extends a superclass.

Question 3

What is single inheritance? What is multiple inheritance? Does Java support multiple inheritance?

Accepted Answer

Single inheritance allows a subclass to extend only one superclass. Multiple inheritance allows a subclass to extend multiple classes. Java does not allow multiple inheritance.

Question 4

How does a subclass invoke its superclass’s constructor?

Accepted Answer

A subclass can explicitly invoke a superclass’s constructor using the “super” keyword.

Question 5

[True/False] When invoking a constructor from a subclass, its superclass’s no-arg constructor is always invoked.

Accepted Answer

False. If a subclass’s constructor explicitly invokes a superclass’s constructor, the superclass’s no-arg constructor is not invoked.

Question 6

[True/False] You can override a private method defined in a superclass.

Accepted Answer

False. You can only override accessible instance methods.

Question 7

[True/False] You can override a static method defined in a superclass.

Accepted Answer

False. You can only override accessible instance methods.

Question 8

How do you explicitly invoke a superclass’s constructor from a subclass?

Accepted Answer

Use “super()” or “super(<args>)”. This statement must be the first in the constructor in the subclass.

Question 9

How do you invoke an overridden superclass method from a subclass?

Accepted Answer

Use super.method() or super.method(args).

Question 10

Explain the difference between method overloading and method overriding.

Accepted Answer

Method overloading defines methods of the same name in a class. Method overriding modifies the methods that are defined in the superclasses.

Question 11

If a method in a subclass has the same signature as a method in its superclass with the same return type, is the method overridden or overloaded?

Accepted Answer

It is a method overridden.

Question 12

If a method in a subclass has the same signature as a method in its superclass with a different return type, will this be a problem?

Accepted Answer

It will be a problem if the return type is not a compatible return type.

Question 13

If a method in a subclass has the same name as a method in its superclass with different parameter types, is the method overridden or overloaded?

Accepted Answer

It is method overloading.

Question 14

What is the benefit of using the @Override annotation?

Accepted Answer

It forces the compiler to check the signature of the overridden method to ensure that the method is defined correctly.

Question 15

What are the three (3) pillars of object-oriented programming (OOP)? What is polymorphism?

Accepted Answer

Encapsulation, Inheritance, and Polymorphism. In simple terms, polymorphism means that a variable of a super-type can refer to a sub-type object.

Question 16

What is dynamic binding?

Accepted Answer

A method may be implemented in several classes along the inheritance chain. The JVM decides which method is invoked at runtime. This is known as dynamic binding.

Question 17

Describe method matching.

Accepted Answer

The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method, IAT parameter type, # of parameters, and order of parameters at compile time.

Question 18

Describe method binding.

Accepted Answer

A method may be implemented in several subclasses. The JVM dynamically binds the implementation of the method at runtime, decided by the actual class of the object referenced by the variable.

Question 19

Can you assign new int[50], new Integer[50], new String[50], or new Object[50], into a variable of Object[] type?

Accepted Answer

new int[50] CANNOT be assigned to into a variable of Object[] type, but new Integer[50], new String[50], or new Object[50] are FINE.

Question 20

[True/False] You can always successfully cast an instance of a subclass to a superclass.

Accepted Answer

True. For example, casting apple to fruit is fine.

Question 21

[True/False] You can always successfully cast an instance of a superclass to a subclass.

Accepted Answer

False. For example, casting fruit to apple is not always successful unless a fruit is an apple.

Question 22

11.13.1. Write statements that create a MyStack and add number 11 to the stack.

Accepted Answer

MyStack stack = new MyStack(11);
stack.push(11); // 11 is auto boxed into new Integer(11)

Question 23

11.14.1. What modifier should you use on a class so that a class in the same package can access it, but a class (including a subclass) in a different package cannot access it?

Accepted Answer

default visibility modifier

Question 24

11.14.2. What modifier should you use so that a class in a different package cannot access the class, but its subclasses in any package can access it?

Accepted Answer

protected

Question 25

11.15.1. How do you prevent a class from being extended? How do you prevent a method from being overridden?

Accepted Answer

Use the “final” keyword.

Question 26

Object-oriented programming allows you to derive new classes from existing classes. This is called ________.

Accepted Answer

Inheritance

Question 27

[True/False] A subclass is usually extended to contain more functions and more detailed information than its superclass.

Accepted Answer

True.

Question 28

[True/False] “class A extends B” means A is a subclass of B.

Accepted Answer

True

Question 29

What modifier should you use on the members of a class so that they are not accessible to another class in a different package, but are accessible to any subclasses in any package?

Accepted Answer

protected

Question 30

The visibility of these modifiers increases in this order:

Accepted Answer

private; none (if no modifier is used); protected; and public

Question 31

A class design requires that a particular member variable must be accessible by any subclasses of this class, but otherwise not by classes which are not members of the same package. What should be done to achieve this?

Accepted Answer

The variable should be marked protected.

Question 32

[True/False] A method with no visibility modifier can be accessed by a class in a different package.

Accepted Answer

False.

Question 33

[True/False] “final class A { }” cannot be extended.

Accepted Answer

True.

Question 34

You can create an ArrayList using _________.

Accepted Answer

new ArrayList <> ()

Question 35

Invoking __________ removes all elements in an ArrayList x.

Accepted Answer

x.clear()

Question 36

Suppose ArrayList x contains two strings [Beijing, Singapore]. Define a method that will cause the list to become [Beijing, Chicago, Singapore].

Accepted Answer

x.add(1, “Chicago”)

Question 37

Suppose ArrayList x contains two strings [Beijing, Singapore]. Define a method that will cause the list to become [Beijing].

Accepted Answer

x.remove(“Singapore”) OR x.remove(1)

Question 38

Suppose ArrayList x contains two strings [Beijing, Singapore]. Define a method that will cause runtime errors.

Accepted Answer

x.set(2, “New York”); x.get(2); x.remove(2) - there is no element at index 2.

Question 39

Invoking ____________ returns the first element in an ArrayList x.

Accepted Answer

x.get(0)

Question 40

Invoking ____________ returns the number of the elements in an ArrayList x.

Accepted Answer

x.size()

Question 41

Object-Oriented programming allows you to derive new classes from existing classes. This is called _____.

Accepted Answer

inheritance

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Java (Chapter 11)

Inheritance