Object-Oriented Programming Concepts
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Questions and Answers

What is the purpose of the TestCircle1 class?

  • To store the radius of a circle.
  • To implement the Circle class.
  • To provide utility functions for circles.
  • To serve as the main entry point for testing. (correct)

What is the default value of the age data field in a Student class?

  • undefined
  • false
  • 0 (correct)
  • null

If a data field of reference type in Java does not reference any object, what value will it hold?

  • undefined
  • 0
  • null (correct)
  • false

What must be true about the file name if both the TestCircle1 and Circle classes are in the same file?

<p>The file name must be TestCircle1.java. (A)</p> Signup and view all the answers

What happens to local variables in methods regarding default values in Java?

<p>They must be explicitly initialized before use. (C)</p> Signup and view all the answers

What radius was assigned to the object myCircle?

<p>5.0 (B)</p> Signup and view all the answers

Which method can be used to compute the area of the Circle1 objects?

<p>getArea() (A)</p> Signup and view all the answers

What is the radius assigned to the object yourCircle created with Circle1() constructor?

<p>1.0 (D)</p> Signup and view all the answers

What does the keyword 'this' reference when used in a method?

<p>The instance of the class (A)</p> Signup and view all the answers

If a class instance variable is hidden by a parameter name, what is necessary to access it within a method?

<p>Use 'this' to access the hidden variable (A)</p> Signup and view all the answers

How are the data of myCircle and yourCircle characterized?

<p>They have different data but share the same methods. (B)</p> Signup and view all the answers

Which statement about the hidden static variable is correct?

<p>It can be accessed using ClassName.StaticVariable. (C)</p> Signup and view all the answers

What does the Circle1(5.0) constructor specifically create?

<p>A Circle1 object with a radius of 5.0 (B)</p> Signup and view all the answers

What method in the Date class can be used to return the current date?

<p>getDate() (A)</p> Signup and view all the answers

Which statement is true about the no-arg constructor in the Date class?

<p>It creates an instance for the current date and time. (C)</p> Signup and view all the answers

What does the toString method in the Date class return?

<p>The date and time as a string. (B)</p> Signup and view all the answers

Which of the following statements is correct regarding the constructor requirements for Java?

<p>this(arg-list) must appear first in the constructor. (D)</p> Signup and view all the answers

What will happen if the this(arg-list) statement is not placed correctly in a constructor?

<p>The constructor will not behave as expected. (D)</p> Signup and view all the answers

Which of the following methods is NOT found in the Date class?

<p>getCurrentDate() (A)</p> Signup and view all the answers

Which aspect of the Date class does the no-arg constructor NOT affect?

<p>The ability to manipulate date formats. (C)</p> Signup and view all the answers

In Java, which statement is true regarding constructors?

<p>Constructors can call another constructor using this(). (C)</p> Signup and view all the answers

What is the purpose of the Circle1 constructor with no parameters?

<p>It sets the radius to a default value of 1. (B)</p> Signup and view all the answers

What does the getArea method return?

<p>The area of the circle. (A)</p> Signup and view all the answers

In the main method, what is the significance of creating the myCircle object with a radius of 5.0?

<p>It shows how to create an object with a customized radius. (C)</p> Signup and view all the answers

Which line in the code demonstrates modifying the radius of a circle object?

<p>yourCircle.radius = 100; (C)</p> Signup and view all the answers

What type of method is Circle() in the context of Circle1?

<p>It is a method, not a constructor. (C)</p> Signup and view all the answers

What does the output statement System.out.println("The area of the circle of radius " + myCircle.radius + " is " + myCircle.getArea()); do?

<p>Dislays the radius and calculated area of the <code>myCircle</code>. (A)</p> Signup and view all the answers

What happens if no argument is provided when creating a Circle1 object?

<p>A default radius of 1 is assigned. (C)</p> Signup and view all the answers

Which keyword is used to define a constructor in Java?

<p>public (D)</p> Signup and view all the answers

What is the output of yourCircle.getArea() after yourCircle.radius is modified to 100?

<p>$31415.92$ (C)</p> Signup and view all the answers

What Java concept is demonstrated by having multiple constructors in the Circle1 class?

<p>Overloading (D)</p> Signup and view all the answers

Why is it important to use the Math.PI constant in calculating the area of a circle?

<p>It provides a more accurate value for pi. (C)</p> Signup and view all the answers

How is the radius of the myCircle object used after its creation?

<p>It is accessed to print and calculate the area. (A)</p> Signup and view all the answers

What will happen if System.out.println("The area of the circle of radius " + myCircle.radius + " is " + myCircle.getArea()); is commented out?

<p>The area will not be displayed, but the program will still work. (C)</p> Signup and view all the answers

In the main method, how is the initialization of yourCircle performed without parameters?

<p>By calling a default constructor. (B)</p> Signup and view all the answers

What is the purpose of the method printAreas in TestPassObject?

<p>To print a table of areas for the Circle object. (B)</p> Signup and view all the answers

What value of times is passed to the printAreas method when it is called?

<p>5 (C)</p> Signup and view all the answers

What does the line c.setRadius(c.getRadius() + 1) do?

<p>Increases the radius of the Circle object by 1. (B)</p> Signup and view all the answers

How many times will the loop in the printAreas method execute?

<p>5 (D)</p> Signup and view all the answers

What output does the call System.out.println(c.getRadius() + "\t\t" + c.getArea()); produce?

<p>It displays the radius and the area of the Circle. (A)</p> Signup and view all the answers

What is the initial radius of the Circle object myCircle when it is created?

<p>1 (A)</p> Signup and view all the answers

What would happen if the method printAreas was called with a negative value for times?

<p>It would have no output. (C)</p> Signup and view all the answers

Which statement about the Circle3 class is most likely true based on the provided code?

<p>It must have a constructor that takes a radius as an argument. (A)</p> Signup and view all the answers

What type of data does the variable n hold in the TestPassObject class?

<p>Integer (B)</p> Signup and view all the answers

What is the result of the statement System.out.println("n is " + n);?

<p>Prints the string 'n is ' followed by the value of n. (A)</p> Signup and view all the answers

Why can't the variable 'i' be accessed directly in the static main method?

<p>'i' is an instance variable. (B)</p> Signup and view all the answers

Which statement is true about the method 'm2'?

<p>It is a static method and cannot access instance variables. (C)</p> Signup and view all the answers

What is the output of the method 'm2(5, 2)'?

<p>25 (D)</p> Signup and view all the answers

In which scenario is a variable likely to be made static?

<p>When it needs to be shared across instances. (B)</p> Signup and view all the answers

What will be the effect of calling 'm1()' from 'main()'?

<p>It will cause a compile-time error. (B)</p> Signup and view all the answers

What does the statement 'k = 2' indicate about 'k'?

<p>'k' is a class variable shared by all instances. (C)</p> Signup and view all the answers

What is the role of the main method in a Java class?

<p>It serves as the starting point for program execution. (B)</p> Signup and view all the answers

How does the addition 'i + k + m2(i, k)' work in the context of 'm1()'?

<p>Both 'i' and 'k' are accessible because 'm1()' is an instance method. (B)</p> Signup and view all the answers

Flashcards

Reference Data Field

A data field that stores references to objects, not the objects themselves.

null value

A special value used to indicate that a reference data field does not refer to any object.

Default Value (Reference Type)

The initial value assigned to a reference data field if no value is explicitly set.

Local Variable

A variable declared within a method; it does not have a default value in Java.

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Instance Method (e.g., getArea)

A method associated with a specific object (instance) of a class. It is invoked by that object.

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Constructor

A special method that initializes an object when it is created. It has the same name as the class and doesn't return any value.

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Object

A self-contained entity that represents a real-world thing or concept. It has both data (attributes) and actions (methods).

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Method

A block of code within a class that performs a specific task on an object. It can take input (parameters) and produce output (return value).

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Instance Variable

A variable that belongs to an individual object. It holds data specific to that object. Often called 'fields' or 'attributes'.

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Data vs. Methods

Data (attributes) describes what an object is, while methods (actions) determine what an object can do.

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Sharing Methods

Multiple objects of the same class can share the same methods, even if they have different data. Each object will use the method on its own data.

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'this' Keyword

A special keyword used within a method to refer to the current object's instance variables. This allows access and modification to its internal data.

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Accessing Static Variables

Static variables are accessed directly using the ClassName.StaticVariable reference.

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Method vs. Constructor

A method is a block of code that performs a specific task, while a constructor is a special method that initializes an object.

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Circle() method

A method within a Circle class that would normally be used for initializing a circle object; this example does not.

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TestCircle2.java (class)

A Java program testing a Circle class by creating and using Circle objects.

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main method

The entry point of a Java program, where execution begins.

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Circle1(5.0)

Creates a Circle1 object with a radius of 5.0.

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myCircle.radius

Accesses the radius property of the "myCircle" object.

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myCircle.getArea()

Calculates and returns the area of the circle with myCircle's radius.

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Circle1()

Creates a Circle1 object using default constructor with a radius of 1.0.

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yourCircle.radius = 100

Modifies the radius of the yourCircle object to a new value of 100, changing the stored value.

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Circle1 class

A class that represents a circle, providing methods.

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Constructor (Circle1())

A special method that initializes the Circle1 object.

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Constructor (Circle1(double newRadius))

Initializes a Circle1 object with a specified radius.

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getArea() method

A method that calculates and returns the area of a circle using its radius.

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Math.PI

Mathematical constant representing the ratio of a circle's circumference to its diameter.

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Object (e.g., myCircle)

An instance of a class, containing data values associated with the class type.

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Data member (e.g. radius)

A variable within a class that stores data, associated with a class.

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Date Class

A built-in Java class that represents dates and times.

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No-Arg Constructor (Date Class)

A constructor for the Date class that takes no arguments. It creates a Date object representing the current date and time.

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toString() Method (Date Class)

A method in the Date class that returns a String representation of the date and time associated with the Date object.

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this(arg-list)

A special statement used within a constructor to call another constructor of the same class, usually to reuse code and prevent redundancy.

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Constructor (Java)

A special method that is called automatically when an object of a class is created. It initializes the object's data fields (attributes).

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Constructor Order in Java

In a constructor, the this(arg-list) statement must appear before any other statements in the constructor body.

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Constructor Chaining

A technique where constructors call other constructors within the same class to reuse code and avoid redundant initialization logic.

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Static Variable

A variable shared by all objects of a class. Its value is the same for every instance.

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Instance Method

A method associated with a specific object. It can access and modify that object's instance variables.

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Static Method

A method that can be invoked directly from a class without creating an object. It operates on static variables or general class-wide tasks.

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Can a Static Method Access an Instance Variable?

No, a static method cannot directly access an instance variable. Instance variables belong to specific objects, while static methods are shared by all objects.

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Can an Instance Method Access a Static Variable?

Yes, an instance method can access a static variable. Static variables are shared by all objects of a class.

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Why is i = i + k + m2(i, k) valid in an m1() method?

This code is valid because m1() is an instance method, allowing access to both the instance variable i and the static variable k. Additionally, it calls m2(), another static method.

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Why is int j = i invalid in the main() method?

This statement is invalid because main() is a static method and cannot access the non-static i variable. i belongs to an instance, and main() doesn't operate on any particular object.

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Object Creation (Circle3)

The line Circle3 myCircle = new Circle3(1); creates a new Circle3 object named myCircle and initializes its radius to 1. This line demonstrates the process of instantiation, where a blueprint (the Circle3 class) is used to create a concrete instance (the myCircle object).

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Method Invocation

The line printAreas(myCircle, n); calls the printAreas method, passing the myCircle object and the integer n as arguments. This demonstrates how methods are invoked, triggering specific actions within the program.

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Parameter Passing

When invoking printAreas(myCircle, n);, the myCircle object and the value of n are passed as arguments. These arguments provide information to the printAreas method and are used within the method's logic.

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While Loop

The while (times >= 1) loop iterates as long as the value of times is greater than or equal to 1. This structure allows the program to repeat a set of instructions until a certain condition is met.

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Print Table (printAreas)

The printAreas method generates a table of areas based on the Circle3 object and the specified number of times. It uses the object's methods (getRadius, getArea, and setRadius) to calculate and display the table content.

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Method Access (getRadius, getArea, setRadius)

The printAreas method uses methods like getRadius, getArea, and setRadius to access the Circle3 object's information. These methods allow the program to interact with the object's internal state and modify its properties as needed.

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Object State Change (setRadius)

The line c.setRadius(c.getRadius() + 1); modifies the radius property of the Circle3 object within the loop. This demonstrates how an object's state can be changed by invoking methods that modify its internal properties.

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Loop Counter (times)

The times variable serves as a counter within the while loop. Its value decreases with each iteration, controlling how many times the loop executes.

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Code Block (Loop Body)

The code within the while loop (lines 20-22) represents a code block that is repeatedly executed until the loop condition is met. This block contains the instructions to calculate and display the table content.

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Study Notes

Objects and Classes

  • Object-oriented programming uses objects to represent real-world entities.
  • Objects have unique identity, state (data fields), and behaviors (methods).
  • Data fields store object properties' current values.
  • Methods define object behaviors (actions).

Class Definition

  • A class is a blueprint or template for creating objects of the same type.
  • A class defines the data fields (properties) and methods (behaviors) for objects.
  • Objects are instances of classes.
  • You can create many instances (objects) of a class from a single class template.

Defining Classes

  • Java classes define data fields using variables and behaviors using methods.
  • Constructors are special methods that are invoked when a new object is created.
  • Constructors typically initialize data fields of the object.

Accessing Objects

  • Reference variables: Contain references to objects.
  • Use the dot operator (.) to access data fields and methods.

Reference Data Types and the null Value

  • Reference variables hold object references, not the object itself.
  • null is a special value for a reference variable when it doesn't point to any object.

Primitive vs. Reference Types

  • Primitive types (e.g., int, double) store the value directly.
  • Reference types (e.g., objects) store a reference to the value's location in memory.
  • Assigning a reference variable copies the reference, not the object itself.

Static Variables, Constants, and Methods

  • Static variables are shared among all objects of a class.
  • Static methods are called without creating an object. (directly on the class)
  • Static variables and methods are tied to the class itself, not a specific instance of it.
  • Constants (final static) are useful for storing fixed values.
  • Class name is used to access them rather than an object, and they are shared by all objects of the class.

Class Visibility

  • Public: Accessible from anywhere.
  • Private: Accessible only within the class itself.
  • Default (package-private): Accessible from classes in the same package, but not from other packages.
  • Class access is determined by how the class is declared; public, private, etc.

Passing Objects to Methods

  • Passing an object to a method passes a reference (address in memory) to the object, not a copy of the object.
  • Changes made to the object within the method will affect the original object.

Scope of Variables

  • Local variables: Defined inside a method; only accessible within that method.
  • Instance variables: Belong to a specific object; accessible from within any method of the object.
  • Static variables: Belong to the class; accessible from any method of the class or by using the class name.

The this Keyword

  • Use this to refer to the current object within a method or constructor.
  • Use this to distinguish between local variables and instance variables with the same name inside a method or constructor.

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Description

This quiz focuses on the fundamental concepts of object-oriented programming, including the definition of classes and objects, their properties, and methods. It covers how objects represent real-world entities and how classes serve as blueprints for creating these objects. Test your understanding of these key programming principles.

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