Java OOP

Question 1

Advantages of OOP

Answer 1

easy concept - faster than procedural-oriented programming - reusable code - keep important data hidden

Question 2

Object & class

Answer 2

Object have a state (color , sex , height …)(attributes) and behaviour (swim , fly , dance…)(methods) , a class is the blueprint of object (instance of class)(example : dog , human, car).
An object is a variable of class

Note !!!! A class doesn t take any byte of memory.

Question 3

3 Steps to create an object

Answer 3

Declaration
Instantiation
Initialization

Question 4

how many public class can we have in a source file

Answer 4

we can have only one public class (the one with the same name of source file) , but multiple default classes

Question 5

“this” keyword

Answer 5

we use “this” keyword in constructor or an instance method to differentiate local variable (var) from instance variable (this.var) if they have the same name.

Call one type of constructor ( default with this() or parametrized with this(param) ) from
other in the same class. it is known as explicit constructor invocation.

Question 6

public vs static method

Answer 6

public method need a new object instantiation but static access is direct without instantiation.

Question 7

finalize() method

Answer 7

has this form :
protected void finalize(){
//code here
}

it is called before object destroyed by GC

protected is used to prevent access from outside class.This means u won t know when or if finalize() execute. if you re program ends before GC occurs , finalize() will not execute

Question 8

Method overloading

Answer 8

have the same name but different parameter ( and also number of param )

Question 9

Command line argument

Answer 9

it is the information that follows directly java program name in command line when it is executed.
($java program-name “…”)

Question 10

Java Methods Variables Arguments (var-args)

Answer 10

typeName… parameterName

In the method declaration, you specify the type followed by an ellipsis (…). Only one variable-length parameter may be specified in a method, and this parameter must be the last parameter. Any regular parameters must precede it.

Question 11

Constructor

Answer 11

it is a special type of method which initialize an object when created or do some procedures

name of constructor is same as class

java create automatically a constructor even if u don t create it , it initialize all variables to 0.

Once u created a constructor , default constructor provided by java is not invoked.

concept of creating multiple constructor in same class is : constructor overloading.

constructor don t have return type . DON T EVEN USE VOID TYPE !!!!!!!!!!!!!!!!!!!!!

access modifiers can be used to set its visibility/accessibility.

Question 12

Type of constructor

Answer 12

has 3 types :
default constructor VS No-args constructor: default is provided by compiler , but no args is defined manually by programmer and u can give edit values of variables or do more process
they both don t accept arguments
Parametrized constructor: constructor with one or more param

Question 13

Java Inheritance

Answer 13

One of java oop concepts. it is when a class acquire properties ( attributes and methods ) of another class .
A subclass inherits properties of superclass by using extends keyword :
class Sub extends Super{}

Question 14

Why we need inheritance ?

Answer 14

Code reusability
extensibility(extend functionnality of a class)
Method overriding require inheritance
Abstraction require inheritance

Question 15

“super” keyword

Answer 15

“super” keyword is like “this” keyword which differenciate between the members of a subclass from a superclass if they have same names.

for var : super.variable
for method:super.method()

It is also used to invoke superclass constructor from subclass

default constructor: super()
constructor with param: super(values)

if a class inherited properties from another class, the subclass automatically acquire default constructor of super class.

Question 16

Java inheritance (IS-A relationship) type

Answer 16

Java support only single inheritance ( doesn t support multiple inheritance) but can implements multiple interfaces.

Interface in java can extends multiple interfaces.

Question 17

What is Aggregation in java ?

Answer 17

A relationship (HAS-A) between two classes where a class contains instance of another class(like binding a class to other in one way direction)

benefits : reusable code

Question 18

Aggregation types

Answer 18

unidirectional : like above def
bidirectional : where each of class contains instance of other class

Question 19

Polymorphism

Answer 19

is the ability of an object to take on many forms , allow us to perform multiple operations using the single name of any method (interface).

method overriding is one of polymorphism concepts

any object that can pass more than one IS-A test is polymorphic.
All java objects are polymorphic, because an object can pass IS-A with its own type and the class Object.

Question 20

Polymorphism types

Answer 20

compile-time polymorphism: method overloading (static polymorphism).
runtime polymorphism: method overriding.

Question 21

method overriding

Answer 21

occur when a subclass implement a parent class method based on its requirement (overriding it ) without changing name , type , return type.

Question 22

method overloading

Answer 22

occur when a class has two or more methods with same name but different parameter (different type and number of arguments)

in method overloading return type must not change

Question 23

What is abstraction in java ?

Answer 23

Its hidden implementation details from user. providing functionnalities only without giving how it is done (code).

Question 24

How we achieve abstraction?

Answer 24

with abstract classes and interfaces

Question 25

Abstract class rules

Answer 25

*may or may not contains abstract method ( methods without body : public void get() ).

*if it contains an abstract method it must be declared abstract

*cannot be instantiated.

*to use it you must inherit it from another class.

*if a class inherit abstract class it must implement all its abstract methods.

Question 26

abstract method

Answer 26

-abstract keyword is used to declare method as abstract.
-abstract method have a method signature but no body , it ends with semi colon “;” and without curly braces “{}”.

-Any class inheriting the abstract class must either override the abstract method or declare itself as abstract.

Question 27

Encapsulation

Answer 27

hidden data (variables) and code from other classes except for method which can access and modify data.

it is also known as data hiding.

Question 28

How to achieve encapsulation ?

Answer 28

-declare variables as private .

-provide public getter and setter to view and modify variables values.

Question 29

Benefits of encapsulation

Answer 29

-the field of a class can be made read-only (only getter) or write-only (only setter);

(or also fully encapsulated getter+setter)

-a class have total control over what is stored in its fields.

Question 30

What is interface ?

Answer 30

We use interface to achieve abstraction.
Interface is also a reference type and is similar to the class.
However, a class describes states and behaviors of object . Interface contains behaviors that a class implements.
Interface contains constants , abstract methods(without body) ; and static and default methods ( the only ones with body).
Interface is like a contract

Question 31

Class and interface similarities

Answer 31

-contains any number of methods
-Writting in .java extension , and have the same name of file
-byte code appears in .class file
-appears in packages, and their byte code appears in a directory structure that matches package name

Question 32

Class and interface differences

Answer 32

-interface cannot be instantiated
-interface doesn t have a constructor
-method in interface is by default abstract , and field must be both static and final
-A class can implements( don’t extends) multiple interfaces
- an interface can extends multiple interfaces.

Question 33

Properties of interface

Answer 33

-Interface is implicitly abstract, you don t need to declare abstract keyword , same thing for its methods.

-its methods are implicitly public.

Question 34

What is tagging interface

Answer 34

Also known as marker interface , is an interface with no method and field on it
=> it’s an interface with empty body

Question 35

What is tagging interface purpose

Answer 35

-Create a common parent(among a group of interfaces).
-Adds a data type to a class: class become interface type through polymorphism.

Question 36

Packages

Answer 36

is a grouping of related types ( classes , interfaces , enumerations and annotations ) providing access protection and namespace management

Question 37

Package types

Answer 37

Built-in packages
user-defined packages

Question 38

Examples of Built-in packages

Answer 38

java.lang:bundles/gathers the fundamental classes
java.io: bundles classes for input and output functions (like System.in and System.out).

Question 39

Why we use “Import” keyword Java ?

Answer 39

To make use of package we need to import it with import keyword.

Question 40

Inner , outer and nested classes

Answer 40

Inner class is a class that is defined in another class , it works with nested classes where both inner and outer class exists . An outer class is the main class in which all inner classes are defined

Question 41

static class

Answer 41

can be defined only inside outer class ( if it is a main/outer class or not a nested class it’s impossible to use static keyword)

we can access its members without instantiation we need only class name.

Question 42

Anonymous class (classe anonyme)

Answer 42

Is a nameless inner class , and because it’s nameless ,it doesn t have a constructor because constructor have same name as its class.

Question 43

Use of anonymous class

Answer 43

we mainly use it to instantiate interface (or an abstract class)

Question 44

Type of anonymous class

Answer 44

*anonymous inner class that extends class
__________________________
*anonymous inner class that implements interface
for example :

public interface Soft{
public void dev();
}

=> Soft s=new Soft(){
//code here
public void dev(){
sout(“hi”);
}
}
s.dev();
______________________________
*anonymous inner class as argument

Question 45

Singleton

Answer 45

is a design pattern , it’s a creational pattern.
This pattern involves only a single class which is responsible to create only one Object.this class provides a way to access it’s only object directly without the need to instantiate its object

Question 46

Advantages of Singleton

Answer 46

it saves memory because only one object instance is created and it also provides global access to its instance

Question 47

Use of Singleton

Answer 47

create classes related to database connection

Question 48

Rules to create Singleton

Answer 48

private constructor(to defeat instantiation)
*instance attribute is private static + SingletonType
*method to return instance :getInstance() is public + SingletonType

Question 49

3 common types of Singleton

Answer 49

*Eager Initialization: instance is already instantiated to new ClassName()
*Lazy Initialization: instance attribute is null at the beginning and we initialize instance on getInstance() after check it’s still null then we return it
*Thread-safe Singleton (syncronized method) : Lazy initialization + we use sychronized on getInstance();

Question 50

Wrapper classes

Answer 50

are does whose object wraps a primitive data type within them (java.lang package contain them).
All wrapper classes (Integer,Float,Short,Byte, Double,Long) are subclasses of the abstract class number

Question 51

Enum

Answer 51

is a special class which contains a group of pre-defined constant values and can be used in switch expressions for comparison.
By default an enum doesn’t require a constructor.

Question 52

enum constructor+attribute scope

Answer 52

as enum is a special class, it can only have a private or package-private(without access modifier) constructor , if it’s public , it will trigger a compile time error

+ it’s attribute should be private final

Question 53

some enum methods

Answer 53

toString(), name()

Question 54

What is immutable class ?

Answer 54

Immutable class is a class whose objects cannot be modified after their creation

Question 55

What is immutable class rules ?

Answer 55

*Final Class
*Final Fields
*No setters
*Deep copies : the class ensures making copy of object rather than storing reference to avoid external modification

Question 56

Association vs Aggregation vs Composition ?

Answer 56

*Association is the most general form of relationship, where objects interact with each other but have no strong dependency.
*Aggregation is a “whole-part” relationship, but the parts can exist independently of the whole.
*Composition is a more tightly coupled form of aggregation, where the contained objects cannot exist without the whole object.

Question 57

What is difference between composition and inheritance ?

Answer 57

*Composition is better for modeling “has-a” relationships where the lifecycle of the contained objects is tied to the lifecycle of the container. It promotes flexibility and loose coupling.
*Inheritance is better for “is-a” relationships where a subclass needs to extend or modify the behavior of a parent class. It promotes code reuse but can lead to tight coupling.