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Questions and Answers

What does the + sign indicate in the context of the TV class?

  • The method is private
  • The method has a return type
  • The method is public (correct)
  • The method is static

What is the maximum channel number that can be set for the TV object?

  • 100
  • 120 (correct)
  • 150
  • 200

Which method would you call to increase the current volume level of a TV object by one?

  • volumeUp() (correct)
  • adjustVolume()
  • setVolume()
  • increaseVolume()

If you wanted to turn off a TV object, which method would you use?

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

In the context of the TV class, how would you denote the current state of the TV being on?

<p>on: boolean (D)</p> Signup and view all the answers

What does the notation 'radius: double' represent in a UML class diagram?

<p>A data field with its type specified (D)</p> Signup and view all the answers

Which method in the Circle class is responsible for calculating the area?

<p>getArea(): double (C)</p> Signup and view all the answers

How are circle objects represented in the UML notation?

<p>With the class name followed by their properties (D)</p> Signup and view all the answers

What is demonstrated by the constructor 'Circle(newRadius: double)' in the Circle class?

<p>It initializes a new Circle object with a specified radius (B)</p> Signup and view all the answers

What is the purpose of the 'setRadius(newRadius: double): void' method in the Circle class?

<p>To change the radius of the circle to a new value (B)</p> Signup and view all the answers

How many Circle objects were created in the example above?

<p>Three Circle objects (C)</p> Signup and view all the answers

Which notation indicates that a method has a return type in the UML class diagram?

<p>methodName(parameterName: parameterType): returnType (A)</p> Signup and view all the answers

What happens when the radius of circle2 is set to 100?

<p>The area of circle2 is calculated and displayed (C)</p> Signup and view all the answers

Which method should be defined as static according to the design guide?

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

What does the method m2(int i, int j) return?

<p>i raised to the power of j (C)</p> Signup and view all the answers

Why is the radius variable in the Circle class an instance variable?

<p>Because each instance of Circle has its own radius (B)</p> Signup and view all the answers

Which statement is true regarding instance methods?

<p>Instance methods require an instance to be invoked. (C)</p> Signup and view all the answers

What is the result of invoking m1() given appropriate values for i and k?

<p>The method updates i with a complex expression (B)</p> Signup and view all the answers

What is the main reason line 6 in the first code example is incorrect?

<p>Instance variables must be accessed through an object. (B)</p> Signup and view all the answers

Which statement describes a valid way to access instance fields in a static context?

<p>Instantiate an object first to access the instance fields. (A)</p> Signup and view all the answers

What can be inferred about instance methods when called from a static method?

<p>Instance methods need to be invoked through an object. (A)</p> Signup and view all the answers

What would happen if line 7 in the first example was modified to 'A.m1();'?

<p>An error would occur since 'm1()' is an instance method. (B)</p> Signup and view all the answers

In the corrected version of the code, what is the role of the line 'A a = new A();'?

<p>It creates an object of class A to access instance members. (B)</p> Signup and view all the answers

How does the line 'int j = a.i;' access the instance variable 'i' correctly?

<p>It uses an instance of class A to refer to 'i'. (C)</p> Signup and view all the answers

What is the significance of the method 'm2' in the class A?

<p>It does not interact with any instance variables. (A)</p> Signup and view all the answers

Which statement about static fields in the provided code is true?

<p>They can be accessed without creating an instance of the class. (C)</p> Signup and view all the answers

What will the JVM do if an object is no longer referenced by any reference variable?

<p>It will automatically collect the space occupied by the object. (A)</p> Signup and view all the answers

What is the purpose of assigning null to a reference variable?

<p>To explicitly indicate that the object is no longer needed. (C)</p> Signup and view all the answers

Which of the following statements about arrays is accurate?

<p>An array can contain elements of object types. (C)</p> Signup and view all the answers

What type of exception is a NullPointerException?

<p>An exception thrown when trying to perform a method on a null object. (D)</p> Signup and view all the answers

What is an anonymous object?

<p>An object that is instantiated without being assigned to a variable. (A)</p> Signup and view all the answers

What can be inferred from the line 'ShowErrors t = new ShowErrors(5);' in program (a)?

<p>The constructor for ShowErrors does not accept any parameters. (D)</p> Signup and view all the answers

What is the likely issue in the line 'System.out.println(c.value);' from program (d)?

<p>Variable 'c' is not initialized. (B)</p> Signup and view all the answers

In program (b), which statement has an error?

<p>t.x(); (B)</p> Signup and view all the answers

What does the method distance(x: double, y: double) return?

<p>The distance between this point and the specified coordinates. (C)</p> Signup and view all the answers

Which method would you invoke to get the y-coordinate of a Point2D object?

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

In the provided program, what is printed when invoking toString() on the Point2D object p1 with coordinates (1.5, 5.5)?

<p>Point2D [x = 1.5, y = 5.5] (D)</p> Signup and view all the answers

How many Point2D objects are created in the program TestPoint2D?

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

Which line of code correctly initializes a Point2D object?

<p>Point2D p = new Point2D(0, 0); (B)</p> Signup and view all the answers

What is the output of the program if the user inputs point1 coordinates as 1.5 and 5.5 and point2 coordinates as -5.3 and -4.4?

<p>Distance is 12.010412149464313 (D)</p> Signup and view all the answers

What is the purpose of the main method in the program TestPoint2D?

<p>To perform user input and demonstrate the Point2D functionality. (B)</p> Signup and view all the answers

Which one of the following methods is NOT a part of the Point2D class?

<p>calculateDistance(): double (D)</p> Signup and view all the answers

Flashcards

Class Diagram Notation

UML (Unified Modeling Language) diagrams used to represent classes, their attributes (data fields), methods, and relationships. They visualize the structure and behavior of a program.

Data Field

A variable of class structure that stores data related to an object or class. Data fields define characteristics.

Data Field Notation

In class diagrams, data fields are written with the field name followed by its type (e.g., radius: double).

Constructor

A special method used to initialize an object when it's created. It sets initial values for data fields.

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Constructor Notation

In class diagrams, constructors use the class name and parameters. (e.g., Circle(radius: double)).

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Class

A blueprint for creating objects; it specifies data fields (attributes) and behaviors (methods) of an object.

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Object

An instance of a class. An object is created from a class.

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Method Notation

In class diagrams, methods are written as methodName(parameterName: parameterType): returnType (e.g., getArea(): double).

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Instance Variable

A variable that belongs to a specific object of a class.

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Static Variable

A variable that belongs to the class itself, not to any specific object.

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Instance Method

A method that operates on the data of a specific object.

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Static Method

A method that belongs to the class, not to any specific object.

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Accessing Instance Variable

Using object name + dot + instance variable name to access variable of an object.

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Accessing Static Variable

Using class name + dot + static variable name to access a class-level variable.

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Calling Instance Method

Using object name + dot + instance method name to execute a method specific to an object.

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Calling Static Method

Using class name + dot + static method name to execute a class-level method.

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Main method in a class

A method within a class that provides the entry point for running the class.

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Object-Oriented Modeling

Using classes to represent real-world entities with their attributes (states) and actions (behaviors).

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TV Class Attributes

Properties describing a TV object, such as channel, volume, and power status.

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TV Class Behaviors

Actions that a TV object can perform, like changing channels or adjusting volume.

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UML Diagram

A visual representation of a class's structure, including attributes (variables) and methods (actions).

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Instance Method

A method that operates on the data of a specific object.

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Static Method

A method that operates on the class itself, not on any specific object.

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Instance Variable

A variable belonging to a specific object of a class.

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Static Variable

A variable belonging to the class, not to any specific object.

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When to use Static?

Use static methods and variables when the operation or data doesn't depend on a specific object, but rather on the class itself.

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Garbage Collection

Automatic process where the JVM reclaims memory occupied by objects that are no longer in use.

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NullPointerException

Error that occurs when you try to access a member of an object that has been assigned the null value.

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Accessing Object Members

Using the dot operator ('.') to access data fields or invoke methods of an object.

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Anonymous Object

An object created without a reference variable, used temporarily; immediately used.

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Array vs. Object

Arrays are objects in Java, containing ordered elements of a particular type, while primitive types are not objects.

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Array Element Types

Arrays can contain elements of object or primitive types.

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Array Default Value

Elements in arrays have default values based on the data type (e.g., 0 for integers, 0.0 for doubles).

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Null Variable Assignment

Explicitly setting a reference variable to null to release an object's memory.

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Point2D object

Represents a point in a 2D plane with x and y coordinates.

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distance method

Calculates the distance between two Point2D objects.

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getX() method

Returns the x-coordinate of a Point2D object.

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getY() method

Returns the y-coordinate of a Point2D object.

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toString() method

Returns a string representation of a Point2D object.

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Point2D creation

Creating a Point2D object involves specifying its x and y coordinates.

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Point coordinates

x and y values defining a point's position.

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Distance calculation

Method to find the distance between two points based on their coordinates.

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Study Notes

Introduction

  • Object-oriented programming (OOP) allows you to develop large-scale software and GUIs effectively.
  • Java features like selections, loops, methods, and arrays are insufficient for GUI and large-scale software.

Defining Classes for Objects

  • A class defines the properties (attributes) and behaviors (actions) for objects.
  • Objects represent real-world entities (student, desk, circle, button, loan).
  • Object state is represented by data fields (variables) with current values (e.g., a circle's radius, a rectangle's width and height).
  • Object behavior is defined by methods (actions). Methods can be used to get information from an object, change its data fields, or perform another action.

Example: Circle Class

  • A class template defines an object's data fields and methods.
  • A class is a blueprint or contract. An object is an instance of a class.
  • Data fields define the characteristics of an object (eg a circle’s radius),
  • Methods define the actions an object can perform.

Defining Classes and Creating Objects

  • Classes define objects.
  • Objects are created from classes.
  • Example code creates three circles with different radii and shows their area and perimeter.
  • The code changes the radius of one circle and shows the new area and perimeter.

Constructing Objects Using Constructors

  • A constructor is a special kind of method used to create objects.
  • It has the same name as the class.
  • It does not have a return type.
  • It is invoked using the new operator.

Accessing Objects via Reference Variables

  • Objects are accessed via reference variables.
  • Reference variables hold references to objects.
  • To access object data and methods use the dot operator with the reference variable (e.g. objectRefVar.dataField).
  • Reference variables that appear to hold objects actually hold references to them.

Reference Data Fields and the null Value

  • Data fields can use reference types (e.g. String).
  • The null value represents the absence of reference to an object of a reference type.
  • Data fields have default values (null for reference types, 0 for numeric types, false for booleans).

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