Object-Oriented Programming Basics

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Questions and Answers

What are classes in object-oriented programming primarily used for?

Housing a set of methods to perform tasks. (correct)

Executing procedures in a linear fashion.

Storing primitive data types only.

Hiding data from users.

Which of the following best describes an object in software development?

A fixed data structure that cannot change.

A collection of primitive types without any methods.

A standalone program unrelated to classes.

A reusable software component with attributes and behaviors. (correct)

What are instance variables in a class?

Attributes that hold the state of an object. (correct)

Static variables that cannot be modified.

Methods that return values to the calling environment.

Unique objects created from a class.

What is one advantage of using object-oriented programming over structured programming?

It promotes modular design and easier modification. Signup and view all the answers

What is the significance of constructors in a class?

They initialize objects of the class. Signup and view all the answers

What is the purpose of 'set' and 'get' methods in a class?

To control access to and manipulate private instance variables. Signup and view all the answers

What does it mean for a class to have overloaded constructors?

It can have multiple constructors with different parameter lists. Signup and view all the answers

Which of the following does NOT accurately describe reference types in Java?

They contain only primitive values. Signup and view all the answers

What is a key role of a method within a class?

It performs tasks while hiding the implementation from users. Signup and view all the answers

What must occur before a program can execute the tasks defined by a class's methods?

An object of the class must be instantiated. Signup and view all the answers

What does the term 'instance' refer to in object-oriented programming?

A specific representation of a class. Signup and view all the answers

Why is reusability of classes important in programming?

It saves time and leverages tested components. Signup and view all the answers

Which of the following accurately describes attributes in an object?

Attributes are unique properties held by each object. Signup and view all the answers

What does an account object primarily represent within a program?

The balance of money in the account Signup and view all the answers

Which statement is true regarding instance variables?

They define unique attributes for each instance of a class. Signup and view all the answers

In the context of classes and objects, which of the following illustrates instantiation?

Account myAccount = new Account(); Signup and view all the answers

What is the valid range for the 'hour' variable in the Time1 class?

0 - 23 Signup and view all the answers

What is the primary purpose of the Unified Modeling Language (UML)?

To model object-oriented systems visually Signup and view all the answers

What will happen if the setTime method is provided with a minute value of 65?

The method will throw an IllegalArgumentException. Signup and view all the answers

What does the toUniversalString method return?

A String formatted as HH:MM:SS Signup and view all the answers

Which of the following is NOT a primitive type in Java?

String Signup and view all the answers

Why are most instance variables in the Time1 class declared as private?

To restrict access to the class itself. Signup and view all the answers

What does a reference type variable store in memory?

The address of an object Signup and view all the answers

Why can primitive-type variables not invoke methods?

They are not linked to any objects Signup and view all the answers

What is the output format of the toString method in the Time1 class?

H:MM:SS AM/PM Signup and view all the answers

What is the result of declaring instance variables as private?

Data hiding or information hiding Signup and view all the answers

What exception is thrown if the setTime method receives an invalid hour?

IllegalArgumentException Signup and view all the answers

Which of the following statements about access modifiers is correct?

Public methods can access private variables. Signup and view all the answers

Which part of an object do methods often require to perform their tasks?

Arguments passed during method calls Signup and view all the answers

What defines the separation between primitive types and reference types in Java?

Primitive types hold single values while reference types refer to objects Signup and view all the answers

What is the purpose of the 'this' keyword in the setTime method?

To differentiate between class and local variables. Signup and view all the answers

Which statement accurately describes an object in object-oriented programming?

It encapsulates both data and methods that operate on that data Signup and view all the answers

What keyword is used to create a new object in a class instance creation expression?

new Signup and view all the answers

What does a constructor do when an object is created?

It initializes the object’s instance variables. Signup and view all the answers

Which of the following correctly describes a driver class?

A class that creates an object of another class and calls its methods. Signup and view all the answers

How do you call a method of an object in Java?

object.method(); Signup and view all the answers

What does the command 'javac *.java' do?

Compiles all Java files in the current directory. Signup and view all the answers

What is the primary purpose of UML class diagrams?

To summarize a class’s attributes and operations graphically. Signup and view all the answers

What does the term 'instantiating an object' refer to?

Creating a new instance of a class. Signup and view all the answers

Which of the following is true about empty parameter list methods?

They do not require additional information to perform their task. Signup and view all the answers

What is contained in the top compartment of a UML class diagram?

The class's name Signup and view all the answers

Which sign is used to denote public operations in a UML class diagram?

<ul> <li></li> </ul> Signup and view all the answers

How are private attributes represented in UML class diagrams?

With a minus sign (-) Signup and view all the answers

What is indicated by a colon in the context of UML class diagrams?

The return type or parameter type Signup and view all the answers

Which of the following statements is true regarding import declarations in Java?

Classes compiled in the same directory are in the same package by default. Signup and view all the answers

What must be done to access classes outside of the default package?

Use explicit import declarations. Signup and view all the answers

What is the correct method of modeling a parameter in UML?

List the parameter name followed by its data type. Signup and view all the answers

Which statement is true about local variables in Java?

They require explicit initialization. Signup and view all the answers

Study Notes

Java Classes and Objects

Java uses classes to organize program units.
Classes contain methods that perform tasks.
Methods hold program statements that carry out tasks.
Methods hide underlying statements from the user.
An example of a class is Account.
Examples of methods include withdraw() and deposit().

Object Instantiation

Objects (instances) of a class are needed for tasks.
Before using an object, it must be built from its class's design.
An object is an instance of its class.
An example of instantiating an Account object: Account account1 = new Account();

Reusability

Classes can be reused to build multiple objects, similar to using design drawings to build multiple cars.
Reusing existing classes saves time and effort in program development.
Reusing classes contributes to more reliable and effective systems through thorough testing, debugging, and tuning prior developments.

Attributes and Instance Variables

A car has characteristics (attributes) like color, door count, fuel level, speed, and odometer readings.
Attributes are part of an object's design.
Each object has its own attributes.
An object's attributes are its instance variables.
An Account object has a balance attribute.
Each account knows only its own balance, not the balances of other accounts.

Encapsulation

Classes (and their objects) encapsulate attributes and methods, enclosing them within the class.
Objects can communicate; implementation details are hidden within each object.
Information hiding is vital for good software design.

Inheritance

Creating a new class from an existing class is called inheritance.
The new class (subclass) inherits characteristics from the existing class (superclass).
The new class can then add or modify characteristics of its own.
An example is a "convertible" being a type of "automobile."

Interfaces

Interfaces are collections of related methods, similar to functions.
Programmers can use tools and objects with similar functionality using interfaces.
An object's implementation details are hidden.
A class can implement multiple interfaces.
An example is a car implementing different interfaces for various functions.

Object-Oriented Analysis and Design (OOAD)

Developers determine the requirements (what the program must do) using a structured analysis process.
The design defines how the program will meet the requirements.
The design must be reviewed by experts and specialists before coding.
This entire process is called object-oriented analysis and design (OOAD).
The language Java is object-oriented.

Unified Modeling Language (UML)

UML is a graphical language for system modeling in an object-oriented way.
It's used before the programming language is chosen.

Primitive Types vs. Reference Types

Java's types are divided into primitive and reference types.
Primitive types (e.g., int, double) directly hold values.
Reference types (classes, String) hold memory addresses pointing to objects.
A reference type variable refers to an object in memory.
Variables of different types are used to invoke different methods.

Instance Variables, Methods, and Access Modifiers

Each class in Java becomes a new data type, used to declare variables and create objects.
Instance variables store the object's characteristics.
Declaring instance variables as private (e.g. as in private String name) hides implementation details of the object.
Access modifiers like public make methods accessible outside the class.
Private variables are only accessible within the same class.
Public methods are visible and accessible from outside the class.

Class Declarations

Every class begins with the keyword class.
Class names start with a capital letter.
Method and variable names start with a lowercase letter.
File names match class names and end with .java.

Instance Variables (Continued)

Objects have attributes; these are instance variables.
Instance variables are variables within a class declared outside the class's methods.
Each object has its own copy of instance variables.

Constructors

Class constructors prepare and initialize objects.
The compiler creates default constructors if none is defined.
Constructors are invoked using the new keyword.
They initialize the object's instance variables when the object is created.

Default and No-Argument Constructors

If classes contain constructors, the default constructor is not created implicitly.
If default initialization is needed, a no-argument constructor must be declared.
No-argument constructors are invoked without parameter lists.
Class Time1 represents time in a 24-hour format
Time2 can also represent time with constructors for handling different parameters/values

Overloaded Constructors

Multiple versions of a constructor can be offered within a class, handling different arguments/values.
The compiler distinguishes constructors by their parameters/signature.

Package Access

If no access modifier is used with a method or variable in a class, it's considered package-access.
Package-access members are directly accessible from other classes in the same package.

Compiling and Executing Multiple Classes

javac can compile multiple classes simultaneously using a list and or the wildcard *.
If all classes belong to the same project, the wildcard *.java can be used.

Account UML Class Diagram

UML diagrams are used to show the attributes and operations of a class.
These diagrams help describe the class's structure in a visual manner that's language-independent.
UML class diagrams use three components:
- The top compartment specifies the class name.
- The middle compartment lists attributes (variables).
- The bottom compartment describes operations (methods).

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Description

Test your understanding of the fundamental concepts of object-oriented programming, including classes, objects, instance variables, and methods. This quiz covers essential topics such as constructors, reusability, and the significance of attributes in programming.