Objects, Classes and Constructors

Question 1

How many classes in a file can be a public class?

Answer 1

In Java, only one class per file can be public.

If a class is declared public, the file name must match the class name, with the extension .java. For example, if you have a public class named MyClass, the file containing the class must be named MyClass.java.

Question 2

Constructors are a special kind of method. What are their three peculiarities?

Answer 2

A constructor must have the same name as the class itself.
Constructors do not have a return type, not even void.
Constructors are invoked using the “new” operator when an object is created. (Constructors play the role of initializing objects.)

Question 3

___ represents an entity in the real world that can be distinctly identified

Answer 3

A class

Question 4

__ is a construct that defines objects of the same type

Answer 4

A class

Question 5

An object is an instance of a __

Answer 5

class

Question 6

__ is invoked to create an object

Answer 6

A constructor

Question 7

Given the declaration Circle x = new Circle(), which of the following statement is most accurate
x contains an int value
x contains an object of the Circle type
x contains a reference to a Circle object
you can assign an int value to x

Answer 7

x contains a reference to a Circle object

Question 8

The default value for data field of a boolean type, numeric type, object type is ____, _____, ______ respectively

Answer 8

false, 0, null

Question 9

what is the syntax for creating an object (workerOne) from the class Workers using a default constructor

Answer 9

Workers workerOne = new Workers();

Question 10

what are the 3 types of constructors?

Answer 10

Default Constructor
No-Arg (no-argument) Constructor
Parameterized Constructor

Question 11

what is the syntax for creating an object (workerOne) from the class Workers using a parameterized constructor which needs name (“Bob”) and age (“35”) arguments

Answer 11

Workers workerOne = new Workers(“Bob”, 35);

Question 12

what does overloading mean?

Answer 12

That you can have more than one method (including constructors) with the same name as long as the method is unique.

Question 13

what are the criteria that can make a method(including constructors) unique?

Answer 13

Different number or order of arguments/parameters
AND / OR
Different parameter/arguments types

Question 14

What is dot notation used for?

Answer 14

to access the instance data and methods of an object once it is created. commonly:
Update a class member value.
Access a class member value.
Call a method.

Question 15

what are the members of a class?

Answer 15

variables (aka: attributes or fields),
methods,
constructors.

Question 16

what are the 4 types of access modifiers?

Answer 16

public
private
protected
default

Question 17

What are access modifiers used for?

Answer 17

to specify the visibility of a class, its data members, and methods to other parts of the code.

Question 18

What is an abstract data type(ADT)?

Answer 18

Another name for the data type you’ve created when you define a class.

Question 19

What type of naming convention is used for classes?

Answer 19

Classes are in pascal case which starts with a capital letter and then squeezes words together which all begin with upper case.

Question 20

What is a “use case”?

Answer 20

Use cases describe what a system is supposed to do.

Question 21

What is “composition” when referring to Java classes?

Answer 21

A relationship expressed as “has a” or “belongs to” is called composition. Virtually all Java programs use composition to combine classes together to accomplish their tasks.

Question 22

What is a Unified Modeling Language(UML) class diagram?

Answer 22

UML class diagrams are a visual way to show what attributes and methods are included in a class. You can translate a class diagram into Java code or you can use a tool to generate class diagrams from existing Java code.

Question 23

What is the syntax to check if two object (objectOne and objectTwo) are of the same class and output the Boolean result?

Answer 23

System.out.println(objectOne.getClass() == object.Two.getClass());

Question 24

What is the @Override annotation and why can it be useful?

Answer 24

The @Override annotation indicates that the child class method is over-writing its base class method. The @Override annotation can be useful for two reasons. It extracts a warning from the compiler if the annotated method doesn’t actually override anything. It can improve the readability of the source code.

Question 25

what’s one way to check if an object o is a member of the Person class and return false if it’s not

Answer 25

if (!(o instanceof Person)) return false;

Question 26

Explain the (Person) part of this line from a method that overrides the .equals() for the class Person: Person other = (Person) o;

Answer 26

The (Person) part of the line is called an explicit cast, which is a way of telling the compiler that we know o is actually a Person object (or a subclass of Person).

If o were not a Person object or a subclass of Person, then a ClassCastException would be thrown at runtime. However, we check this earlier in the code with the instanceof keyword, so we know that o is a Person object (or a subclass of Person) before we cast it to a Person object in this line.