Chapter 9 Objects and classes

Question 1

Programming using objects is known as?

Answer 1

Object-oriented programming (OOP) involves programming using objects.

Question 2

What does an object represent?

Answer 2

An object represents an entity in the real world that can be distinctly identified.

An object has an
- identity
- state,
- behaviours.

Question 3

The state of an object consists of what?

Answer 3

The state of an object consists of a set of data fields (also known as properties) with their current values.

Question 4

The behaviour of an object is defined by what?

Answer 4

A set of methods.

Question 5

The state(also known as its properties or attributes) of an object is determined by:

Answer 5

methods

Question 6

Define classes

Answer 6

Classes are constructs that define objects of the same type.

A class is a template, blueprint, or contract that defines what an object’s data fields and methods will be.

An object is an instance of a class. You can create many instances of a class. Creating an object or an instance is referred to as instantiation.

Question 7

Constructors definition

Answer 7

A class uses variables to define data fields and methods to define actions.

A class also provides methods of a special type, known as constructors, which are invoked to create a new object.

A constructor has the same name as its class and is syntactically similar to a method. However, constructors have no explicit return type.

Question 8

Constructors syntax

Answer 8

Syntax:
class ClassName {
ClassName(parameter list) {
//constructor body
}
}

Question 9

The illustration of class templates and objects can be standardized using

Answer 9

Unified Modeling Language (UML) notation.

This notation is called a UML class diagram, or simply a class diagram.

Question 10

In the UML class diagram, the data field is denoted as (syntax)

Answer 10

dataFieldName: dataFieldType

Question 11

In the UML class diagram, the constructor is denoted as:

Answer 11

ClassName(parameterName: parameterType)

Question 12

In the UML class diagram, the method is denoted as (syntax):

Answer 12

methodName(parameterName: parameterType): returnType

Question 13

A UML Class Design consists of

Answer 13

class name
data fields
constructors and methods

Question 14

A constructor with no parameters is referred to as a

Answer 14

no-arg constructor

Question 15

Constructors must have the same name as

Answer 15

the class itself

Question 16

Constructors do not have a

Answer 16

return type—not even void

Question 17

Constructors are invoked using the

Answer 17

new operator when an object is created

ex. you have to use the new keyword when invoking a new array.

Question 18

A constructor can perform any action, but constructors are designed to perform

Answer 18

initializing actions, such as initializing the data fields of objects

Question 19

T or F. Constructors can be overloaded

Answer 19

T

Question 20

Constructors are used to construct objects. To construct an object from a class, invoke a constructor of the class using the new operator, as follows:

Answer 20

new ClassName(arguments);

or new Circle();

or new Circle(5.0);

Question 21

Define: a default constructor

Answer 21

A class may be defined without constructors.

In this case, a no-arg constructor with an empty body is implicitly defined in the class.

This constructor, called a default constructor, is provided automatically only if no constructors are explicitly defined in the class.

Question 22

A class is essentially a programmer-defined type. A class is a reference type, which means that a variable of the class type can reference an instance of the class.

Newly created objects are allocated in _____ memory and objects are accessed via the object’s _____.

Answer 22

heap

reference variables

Question 23

After an object is created, its data can be accessed and its methods can be invoked using the ___ ________, also known as the object member access operator:

Answer 23

dot operator (.)

Question 24

Referencing the object’s dataField:

Answer 24

objectRefVar.dataField

e.g., myCircle.radius

Question 25

Invoking the object’s method:

Answer 25

ObjectRefVar.methodName(arguments) e.g., myCircle.getArea()

Question 26

The data fields can be of _____ types

Answer 26

reference

Question 27

If a data field of a reference type does not reference any object, the data field holds a special literal value, ___. A ____ value means that the reference variable it is not holding a reference to an object in memory

Answer 27

null

Question 28

The default value of a data field is ____ for a reference type, ___ for a numeric type, ____ for a boolean type, and _____ for a char type. However, Java assigns no default value to a local variable inside a method

Answer 28

null - reference type 0 - numeric type false - boolean type '\u0000' - char type

Question 29

Java assigns no default value to a local variable inside a ____

Answer 29

method

Question 30

For a variable of a primitive type, the value is a _____ _____.

Answer 30

primitive literal

Question 31

Declaring object reference variables (syntax)

Answer 31

ClassName objectRefVar = new Classname(); or Circle myCircle = new Circle();

Question 32

For a variable of a reference type, the value is a reference to ___ ___ ___ ___ _____.

Answer 32

where an object is located ex. Circle myCircle = new Circle();

Question 33

Define: Garbage collection

Answer 33

The garbage collector is a program which runs on the Java Virtual Machine which gets rid of objects which are not being used by a Java application anymore. It is a form of automatic memory management.

Question 34

Define: Instance variables

Answer 34

The data field radius in the Circle class is known as an instance variable. An instance variable is tied to a specific instance of the class; it is not shared among objects of the same class.

Question 35

The object on which an instance method is invoked is called a calling object. Give the syntax: Instance methods

Answer 35

The .getArea() method in the Circle class is an instance method because you can invoke it only on a specific instance. objectRefVar.methodName(arguments) Example: Circle myCircle = new Circle(); double area = myCircle.getArea();

Question 36

If you want all the instances of a class to share data, which variable do you use?

Answer 36

static variables in this context static does not mean not changing this just means shared among the whole class

Question 37

Define: Static variables

Answer 37

Static variables store values for the variables in a common memory location. Because of this common location, if one object changes the value of a static variable, all objects of the same class are affected. Java supports static methods as well as static variables

Question 38

Static methods can be called without creating an instance of the class Static variables are shared by all the instances of the class. Static methods are not tied to a specific object. Static constants are final variables shared by all the instances of the class. To declare static variables, constants, and methods, use the ___ ___.

Answer 38

static modifier.

Question 39

To declare a static variable or define a static method, put the modifier static in the variable or method declaration as follows:

Answer 39

static int numberOfObjects; //static variable static int getNumberObjects() { //static method return numberOfObjects; }

Question 40

Constants in a class are shared by all objects of the class. Thus, constants should be declared as final static. For example, the constant PI in the Math class is defined as follows:

Answer 40

final static double PI = 3.14159265358979323846;

Question 41

You are given a class named Clock that has one int instance variable called hours. Write a constructor with no parameters for the class Clock. The constructor should set hours to 12.

Answer 41

Clock(){ hours = 12; }

Question 42

Invoking the object’s method:

Answer 42

ObjectRefVar.methodName(arguments) e.g., myCircle.getArea()

Question 43

Copying variables assign circle c1 with a radius of 5 assign circle c2 with a radius of 9 copy the value of c1 into c2

Answer 43

Circle c1 = new Circle(5); Circle c2 = new Circle(9); c1 = c2; The initial radius 5 of object c1 has gone into the JVM "garbage collection"

Question 44

An instance method can invoke an _____ or _____ method and access an ____ or _____ data field

Answer 44

instance static

Question 45

A static method can invoke a static method and access a static data field. However, a static method cannot invoke an ___ ___ or access an ____ ___ ___, since static methods and static data fields don’t belong to a particular object.

Answer 45

Instance Method instance data field

Question 46

Answer 46

Question 47

Answer 47

Question 48

A variable or a method that is dependent on a specific instance of the class should be an

Answer 48

instance variable or method.

Question 49

A variable or a method that is not dependent on a specific instance of the class should be a

Answer 49

static variable or method.

Question 50

For example, every circle has its own radius, so the radius is dependent on a specific circle. Therefore, radius is an ____ ____ of the Circle class. Since the getArea method is dependent on a specific circle, it is an _____ ______.

Answer 50

instance variable instance method

Question 51

None of the methods in the Math class, such as random, pow, sin, and cos, is dependent on a specific instance. Therefore, these methods are _____ _____. The main method is ____ and can be invoked directly from a ____.

Answer 51

static methods static class

Question 52

A package in Java is used to _____ _____ _____. Think of it as a folder in a file directory. We use packages to avoid name conflicts, and to write a better maintainable code.

Answer 52

group related classes

Question 53

Packages are divided into two categories:

Answer 53

Built-in Packages (packages from the Java API) User-defined Packages (create your own packages)

Question 54

Using a Built-In Package

Answer 54

Syntax: import package.name.Class; // Import a single class import package.name.*; // Import the whole package

Question 55

Using a Built-In Package Examples:

Answer 55

import java.util.Scanner; //Import the Scanner class from the java.util package import java.util.*; //Import ALL the classes in the java.util package:

Question 56

Visibility Modifiers - No keyword (default access)

Answer 56

The class, data, or method is visible to any class in the same package

Question 57

Visibility Modifiers - Public

Answer 57

The class, data, or method, is visible to any class in any package

Question 58

Visibility Modifiers - Private

Answer 58

The data or methods can be accessed only by declaring the class

Question 59

To prevent direct modifications of data fields, you should declare the data fields _____, using the _____ modifier This is known as ____ _____ ____________.

Answer 59

private data field encapsulation

Question 60

The data fields in the Circle class can be modified directly (e.g., c1.radius = 5 or Circle.numberOfObjects = 10). This is not a good practice—for two reasons:

Answer 60

Data may be tampered with. The class becomes difficult to maintain and vulnerable to bugs.

Question 61

A private data field cannot be accessed by an object from outside the class that defines the private field. However, we often need to get or modify a data field. To make a private data field accessible, provide _____ method to return its value. To enable a private data field to be updated, provide a _____ method to set a new value.

Answer 61

a getter (accessor) setter (mutator)

Question 62

A getter (accessor) method has the following signature: Example: get the radius

Answer 62

public returnType getPropertyName() public double getRadius()

Question 63

A setter (mutator) method has the following signature: Example: set radius

Answer 63

public void setPropertyName(dataType propertyValue) public void setRadius(double radius)

Question 64

(Socrative) Variables that are shared by every instance of a class are:

Answer 64

static variables

Question 65

A method that is associated with an individual object is called:

Answer 65

an instance method

Question 66

Encapsulation means:

Answer 66

that data fields should be hidden to prevent direct access to a client program