OOP

Question 1

class

Answer 1

template of blueprint from which objects are created and receive their states(properties/fields) and behaviour(methods)

Question 2

static

Answer 2

class method
belongs to class
don’t need an instance to use it
underline
only 1 instance of a static variable exists

Question 3

non static

Answer 3

instance method
belongs to object
need that instance to call it

Question 4

void

Answer 4

method doesn’t return anything

Question 5

object

Answer 5

an instance of a class
there can be multiple instances of a class in a program.
contains both the data and the function, which operates on the data.
has its own unique properties(data) and each method can be called on each object.

Question 6

abstraction

Answer 6

displaying only essential information and hiding the details.

creating new data types suited for a specific application (that the programming language does not know about.)

Question 7

encapsulation

Answer 7

hiding the code and data into a single unit to protect the inner workings of an object (concealing the implementation of the object)

makes use of information hiding (make properties private - use methods to access them.)

Question 8

information hiding

Answer 8

inner workings of a class being hidden from the class user

use access modifier private (in order to have encapsulation)

Question 9

difference between abstraction and encapsulation

Answer 9

Abstraction hides details at the design level (complexity) , while Encapsulation hides details at the implementation level (inner workings).

Question 10

public

Answer 10

Accessible everywhere
can be accessed by the class.it is declared in and by by any other class
+
no restrictions on where an
instance variable or method can be use

Question 11

private

Answer 11

-
Accessible only in its own class
instance variable or method cannot
be accessed by name outside of the class

Question 12

protected

Answer 12

Accessible in its class and any subclass and by any other class in the same package

very weak protection compared to private

Question 13

inheritance

Answer 13

one object acquires the properties and behaviours of the parent object (along w having their own unique attributes)

key word- extends use clear arrow

parent- child relationship between superclass + subclass

suitable when 2 classes have many fields + methods in common

Question 14

default constructor

Answer 14

constructor w/ no parameters
sets attributes of the object to default values

Question 15

polymorphism

Answer 15

ability of an object or method to take on multiple forms based on its current situation
overriding + overloading

Question 16

variables

Answer 16

describes the fields/Attributes/Characteristics/Properties of the object

store data for processing.

given a name (identifier), are of a specific type and have a value assigned to it.

all take up a different amount of size + has a unique address

Question 17

local variables

Answer 17

declared inside methods
do not exist when method is finished, cannot be used outside of method
used instead of global- space in RAM not wasted
Different methods can have local variables with the same names because the methods cannot see
each other’s local variables.

Question 18

global variables

Answer 18

declared in class above all methods
scope is inside the entire program
can be used by all methods

Question 19

access modifiers

Answer 19

used to tell which code has access to certain fields and methods

Question 20

types of variables

Answer 20

primitive- cant invoke methods
reference/class type.-has methods

Question 21

types of variables

Answer 21

primitive- cant invoke methods
reference/class type.-has methods

Question 22

constructor

Answer 22

special method in class called when an object is instantiated
purpose- initialise class fields

Question 23

accessor method

Answer 23

get method
have a specific data type/ return type
returns a field value
allows for visibility of data - other classes will have access to the values
no parameters
allow the programmer to obtain the value of an object’s
instance variables

Question 24

mutator method

Answer 24

set method
modify field values
void
other classes can edit data of object
has parameters
allow the programmer to change the value of an object’s instance variables in a controlled manne

Question 25

typed methods

Answer 25

return a value

Question 26

void methods

Answer 26

don’t return a value

Question 27

helper methods

Answer 27

declared as private solve some problem in the class called by public methods in the class

Question 28

instantiation

Answer 28

act of creating an object from the class

Question 29

scope of variable

Answer 29

defines where variable can be used in a programme and its lifetime depends on where it is declared in the program local or global

Question 30

method overloading

Answer 30

creation of multiple methods in a single class with the same name but with different method signature (parameters/data types ) you cannot overload based on the return type —method signature changes -done in none class can overload static,final or private method -bonded by static binding at compile time A signature consists of the name of a method together with its parameter list Differing signatures must have diff numbers and/or types of parameters

Question 31

method overriding

Answer 31

child class uses method created in parent class (same name) @override -method signature remains the same -only done in subclass -cannot override static,final + private method -subject to dynamic binding at run time (check which method to use )

Question 32

method resolution

Answer 32

When you have methods that have been overloaded, Java needs to figure out which method to call based on the parameters given when the method is called.

Question 33

constants

Answer 33

cannot be assigned another value later in the programme public static final NAME

Question 34

formal vs actual parameters

Answer 34

actual- when you call a method and pass real values in it formal-when you create a method and pass values as parameters

Question 35

method signature

Answer 35

first line that describes whether or not the method is typed or void, what its name is, what parameters it contains, modifier,return type,name,parameters

Question 36

reasons protected access does not offer secure protection

Answer 36

1. Anyone can create a subclass from an existing class and could therefore access and change values in fields which could have dire consequences on applications using those classes. 2. In Java, all classes in the same package have access to protected fields, whether or not they are subclasses.

Question 37

default access

Answer 37

nothing specified Accessible by any class or subclass in the same package. This is sometimes called package accessibility

Question 38

array

Answer 38

data structure collection of variables of same type elements ordered + each has a specific + constant position (index)

Question 39

method

Answer 39

collection of statements that are grouped together to perform an operation defines behaviour

Question 40

toString method

Answer 40

create String representation of an object. every class has default toString method

Question 41

for loops

Answer 41

allows you to efficiently write a loop that needs to execute the same code a specific number of times. for (initialisation,condition,change) { action}

Question 42

while loops

Answer 42

repeatedly executes a target statement as long as a given condition is true. initialisation while (condition is true) { action change}

Question 43

advantages of inheritance

Answer 43

Reusability of code: we can use our existing code in our subclass from the super class • Subclasses can take on existing functionality while creating their unique methods for their access only • information hiding

Question 44

advantages of encapsulation

Answer 44

guarantees integrity of an object’s data as it is securely hidden restricted data access from outside sources

Question 45

call method from parent class (override)

Answer 45

super.methodName() for constructor - super(field,field)

Question 46

composition

Answer 46

The has-a relationship between two Java objects. One object is a field of another object. uml diagram - arrowhead is diamond

Question 47

casting

Answer 47

Provided code compiles correctly, upcast will never generate run time errors Downcasting casts a parent object into one of its descendant classes

Question 48

copy constructor

Answer 48

a type of constructor that creates an object using another object of the same class. The process of initializing members of an object through a copy constructor is known as copy initialization. It is also called member-wise initialization because the copy constructor initializes one object with the existing object, both belonging to the same class on a member-by-member copy basis. single argument of same type as class created object should be separate & independent copies instance variables, use setters for primitive variables, else make new object for class variables (class must have its own copy constructor)

Question 49

static binding

Answer 49

aka early binding decision of which definition of a method to use (method definition is associated with its invocation) is made at compile time made based on the type of the variable naming the bject uses static, not late, binding with private, final, and static methods

Question 50

final

Answer 50

method marked final indicates that it cannot be overridden with a new definition in a derived class ▪ If final, the compiler can use early binding with the method public final void someMethod() { . . . } ▪ A class marked final indicates that it cannot be used as a base class from which to derive any other classe

Question 51

upcasting

Answer 51

taking a child instance to a parent type. object of a derived class is assigned to a variable of a base class (or any ancestor class)

Question 52

downcasting

Answer 52

Downcasting is when a type cast is performed from a base class to a derived class (or from any ancestor class to any descendent class) ▪ Downcasting has to be done very carefully ▪ In many cases it doesn't make sense, or is illegal Downcasting to a specific type is only sensible if the object being cast is an instance of that typ discountVariable (DiscountSale)saleVariable; = //will produce run-time error discountVariable = saleVariable //will produce compiler error

Question 53

clone method

Answer 53

Every object inherits a method named clone from the class Object ▪ The method clone has no parameters ▪ It is supposed to return a deep copy of the calling object ▪ However, the inherited version of the method was not designed to be used as is ▪ Instead, each class is expected to override it with a more appropriate version protected Object clone() If a class has a copy constructor, the clone method for that class can use the copy constructor to create the copy returned by the clone method Overriding the clone( ) method is better than using a copy constructor, because dynamic binding ensures that the copy is always of the right type; a copy constructor cannot do so.

Question 54

covariant return types

Answer 54

overriding a method, if it is a class type then the returned type can be changed to a descendant class of the returned type when Changing the return type from Object to the type of the class being cloned is allowed because every class is a descendent class of the class Object method is one that can be replaced by a "narrower" (derived) type when the method is overridden in a subclass.

Question 55

precondition of a method

Answer 55

states what is assumed to be true when the method is called

Question 56

postcondition of a method

Answer 56

states what will be true after the method is executed, as long as the precondition hold

Question 57

access permission overriden method

Answer 57

access permission of an overridden method can be changed from private in base class to public (or other more permissive access) in the derived class access permission of an overridden method can't be changed from public in base class to more restricted access permission in derived class

Question 58

superclass

Answer 58

base class more general and inclusive, less complex

Question 59

subclass

Answer 59

derived class is more specialized, less inclusive, more complex private method from parent is completely unavailable, unless invoked indirectly

Question 60

package/default

Answer 60

aka friendky access instance variable or method definition that is not preceded w/ a modifier can be accessed by name inside definition of any class in same package more restricted than protected If a class in the current directory is not in any other package, then it is in the default package

Question 61

binding

Answer 61

associating a method definition with a method invocation is called binding

Question 62

late binding

Answer 62

aka dynamic binding method definition is associated with its invocation when method is invoked (at run time) for all methods (except private, final, static) allows for method to be written in base class to perform a task, even if portions of that task aren't yet defined

Question 63

Animal myPet = new og() Dog extends Animal variable and object

Answer 63

variable type: animal object type: dog The type of a class variable determines which method names can be used with the variable ▪ However, the object/argument named by the variable determines which definition with the same method name is used

Question 64

order of visibility

Answer 64

private → none (default/package-private) → protected → public private - Only accessible within the same class (most restrictive) none/default - Accessible within the same package protected - Accessible within the same package AND by subclasses in other packages public - Accessible everywhere (least restrictive)

Question 65

dynamic binding can apply to instance methods

Answer 65

true

Question 66

boolean default

Answer 66

false

Question 67

primitives default

Answer 67

0 / 0.0 (zero of their type)

Question 68

class type default

Answer 68

null

Question 69

boxing

Answer 69

process of going from a value of a primitive type to an object of its wrapper class create an object of wrapper class using the primitive value as an argument new object will contain an instance variable that stores a copy of the primitive value

Question 70

unboxing

Answer 70

process of going from an object of a wrapper class to the corresponding value of a primitive type no parameters Object.objectValue() eg : int i = integerObject.intValue();

Question 71

references

Answer 71

Every variable is implemented as a location in computer memory. When the variable is primitive, its value is stored in the memory location assigned to the variable. Each primitive type always require the same amount of memory to store its value. When the variable is a class type, only the memory address (or reference) where its object is located is stored in the memory location assigned to the variable. The object named by the variable is stored in some other location in memory the value of a class variable is a memory address or reference

Question 72

call by values

Answer 72

A method cannot change the value of a primitive variable that is an argument to the method A method can change the values of the instance variables of a class type that is an argument to the method

Question 73

immutable

Answer 73

A class that contains no methods (other than constructors) that change any of the data in an object of the class is called an immutable class ▪ Objects of such a class are called immutable objects ▪ It is perfectly safe to return a reference to an immutable object because the object cannot be changed in any way ▪ The String class is an immutable class

Question 74

mutable

Answer 74

A class that contains public mutator methods or other public methods that can change the data in its objects is called a mutable class, and its objects are called mutable objects ▪ Never write a method that returns a mutable object ▪ Instead, use a copy constructor to return a reference to a completely independent copy of the mutable objec

Question 75

deep copy

Answer 75

copy that, with one exception, has no references in common with the original ▪ Exception: References to immutable objects are allowed to be shared

Question 76

package access

Answer 76

aka default access package access is more restricted than protected can be accessed by name inside the definition of any class in the same package cant be accessed outside the package Package access gives more control to the programmer defining the classes Whoever controls the package directory (or folder) controls the package access All classes in the current directory (not belonging to some other package) belong to an unnamed package called the default package