Java

Question 1

What are the 3 uses of the keyword final

Answer 1

variable
method
class

Question 2

Can classes marked as final be extended?

Answer 2

no, that is the purpose of marking a class as final

Question 3

Does a final class mean the objects of that class are immutable?

Answer 3

No, you can change the fields in.,

Dog dog = new Dog()
Dog.setName(“Hamish”)

You just can’t extend it

Question 4

Can methods marked as final be overridden?

Answer 4

No

Question 5

Can variables marked as final be reassigned?

Answer 5

No - constants

ie., MAX_VALUE

Question 6

How to explain OOP to someone without a technical background

Answer 6

OOP stands for “Object-Oriented Programming,” and it’s a way for computers to understand how to do things.

Imagine you have a toy box with lots of different toys inside. Each toy is like an object, and each object has different things it can do.

For example, a toy car can move forward and backward, while a toy doll can move its arms and legs.

In OOP, we can tell the computer what each object can do and how it should behave when we interact with it.

So, it’s like teaching the computer to play with toys, but instead of just playing, the computer can use them to do all kinds of cool things!

Question 7

Can you explain the basics of Javas memory model and how it affects multi threaded programming?

Answer 7

Java’s memory model states how memory actions (reads and writes) will appear to execute to the programmer

Question 8

What is memory management?

Answer 8

Memory management is the process of removing unused objects to make space for new objects

Question 9

Where do Java objects reside?

Answer 9

In the heap

Question 10

When is the heap created?

Answer 10

The heap is created when the JVM starts up

Question 11

What are the 3 main benefits of using generics?

Answer 11

reusable

type safe

flexible

Question 12

What is a functional interface?

Answer 12

An interface that contains only a single abstract (unimplemented) method.

Question 13

Describe 2 simalarities between abstract classes and interfaces

Answer 13

• Cant instantiate either of them
• Both can contain methods

Question 14

Name 3 scenarios when an interface is a good choice?

Answer 14

1. Multiple inheritance and different class hierarchies will be used
2. Unrelated classes implementing the interface ie., Shape - rectangle, square
3. A contract, but don’t care who impmements

Question 15

Give an example of an interface?

Answer 15

interface Sender {
send ()
}

ImageSender implements Sender {
send() {
// image send code
}

Question 16

Name 4 scenarios you would use an abstract class?

Answer 16

When you want too..

• Define a template for related classes to implement
• Enforce consistency across a class hierarchy
• Provide default behaviour to inherited methods
• Encapsulate implementation details

Question 17

What is an example of an Abstract class

Answer 17

public abstract class vehicle {
public abstract void turnOn();
}

MotorCycle extends Vehicle {
public void turnOn(){ .. }
}

Car extends Vehicle {
public void turnOn(){..}
}

If we tried to use inheritance instead of an abstract class, Vehicle could be instantiated and this may mean we get behaviour we didn’t expect

Question 18

What is the difference between an interface and an abstract class in Java?

Answer 18

An abstract class can have a state, but an interface can’t

For example, if we define

protected String colour;

in an abstract class it can only be accessed by subclasses of our abstract class

Question 19

When should you use an interface instead of an abstract class

Answer 19

You should use an interface when you want multiple unrelated classes to have the same functionality

Question 20

What does immutable mean?

Answer 20

An object’s state can’t be changed after it has been created

So it will behave the same way its whole lifetime

Question 21

Is a String immutable?

Answer 21

Yes so if you checked if two Strings “Hello” were the same, they aren’t because they actually are objects with a different memory location

Question 22

Are variables mutable by default?

Answer 22

Yes we can change the value

Question 23

What are the benefits of immutable objects in Java?

Answer 23

• can share it safely between multiple threads
• are side effects free

Question 24

What is the difference between polymorphism and inheritance?

Answer 24

Inheritance allows you to create new classes based on existing classes

Polymorphism allows objects to take on multiple forms through method overriding and overloading

Question 25

What is method overriding?

Answer 25

Method overriding allows a subclass to provide its own implementation of a method that is already defined in its superclass

It must have the same name, return type, and parameter list as the method in the superclass

Question 26

What is method overloading?

Answer 26

Method overloading is a feature that allows a class to have two or more methods with the same name, but with different parameters.

The Java compiler is able to tell the difference between the methods based on the number and/or types of parameters.

Question 27

Describe inheritance?

Answer 27

Inheritance is a mechanism in object-oriented programming that allows a class to inherit properties and behaviors from a parent class.

In Java, a class uses “extends” keyword to inherit from another class

Allows you to create a new class that is a modified version of an existing class, without having to rewrite all of the code in the new class.

Question 28

Describe polymorphism?

Answer 28

Polymorphism is the ability of an object to take on multiple forms. In Java, polymorphism is achieved through method overriding and method overloading.

Question 29

What are 3 cache problems?

Answer 29

Cache penetration
Cache avalanche
Cache breakdown

Question 30

What is a load balancer?

Answer 30

A load balancer is an algorithm that routes client requests (ie., database write, cache queries) across all servers

Like a traffic warden directing traffic

Question 31

What is the Java Memory Model made up of?

Answer 31

Thread Stacks and a heap area

Question 32

What is a thread stack?

Answer 32

An area of memory that contains variables, method calls etc

Can be fixed or variable size

If stack size exceeds the limit it will throw a Stack Overflow error

Question 33

What is the heap?

Answer 33

Contains objects created during the application lifecycle

Question 34

Will the heap change in size?

Answer 34

The heap size might change when the application runs

Question 35

What happens when the heap is full?

Answer 35

Garbage is collected

Question 36

What are key features of Java?

Answer 36

• Complied (converted to bytecode - run on platform with jvm)
• developed for class based and object oriented paradigms
• time effort to keep code readable
• strongly typed
• faster types assigned during compilation

Question 37

Give an example of an interface

Answer 37

Ie., interface Shape
double area
Rectangle implements Shape
implementation of double area

Question 38

What are 6 key concepts in Java?

Answer 38

Classes
Objects
Abstraction

Polymorphism
Inheritance
Encapsulation

Question 39

What is a class?

Answer 39

a template for creating objects

has a variable, method, constructor

Question 40

What are objects?

Answer 40

An object is an instance of a class that has a state and behaviours

Question 41

What is abstraction

Answer 41

Abstraction allows us to hide the implementation from the user (in abstract classes / interfaces) and only show the user information needed to use the implementation

Question 42

What is polymorphism?

Answer 42

Polymorphism means many forms. It lets us process data differently depending on the input.

Question 43

What is inheritance?

Answer 43

Inheritance allows a new created class (child) to acquire the properties of an existing class (parent)
IS A relationship

Question 44

What is encapsulation?

Answer 44

Encapsulation lets us hide the data and functions of a class . This helps protect it from being misused

Question 45

Describe 4 benefits of using Inheritance?

Answer 45

Reduce the amount of duplicate code

Only need to change code in one place

Can override methods of the parent class

Keeps data private - derived class can’t alter it

Question 46

Give 2 examples of what interfaces are useful for?

Answer 46

decoupling code
implementing polymorphism

Question 47

Describe generics in Java

Answer 47

Generics allows you to write code that can work with a range of different data types in a type-safe manner

Classes, interfaces, and methods can operate on a parameterized type which is determined at compile time.

Generics use type parameters, which are enclosed in angle brackets (“<” and “>”).

Question 48

Give an example of generics in Java

Question 49

Why do we overwrite public boolean equals(Object o)

Answer 49

To define what it means to be equal

Question 50

Describe 4 things an inherited class will have?

Answer 50

extends

super() constructor

override toString()

protected attributes

Question 51

What are object arrays initally

Answer 51

null

Question 52

What is a variable array initially?

Answer 52

0

Question 53

What is instanceof used for?

Answer 53

instanceof is used to test if an object is of a given type

public class Round {}

public class Ring extends Round {}

@Test
void checkInstance(){
Ring ring = new Ring();
assertTrue(ring instanceof Round);
}

Question 54

How does the instanceof operator work?

Answer 54

The instanceof operator works on the principle of the is-a relationship.

The concept of an is-a relationship is based on class inheritance or interface implementation.

Question 55

Give an example of a violation of encapsulation?

Answer 55

public variables which allow anyone to change them directly

Question 56

What happens if we try to override a final variable, method or class?

Answer 56

We’ll see a compiler error

… is final and cannot be overwritten

Question 57

How can an abstract class in Java have a state?

Answer 57

An abstract class can have a state if it has variables and a constructor

Ie.,

public abstract class Shape {

protected String color;

public Shape(String color) {
this.color = color;
}

public abstract double getArea();

public abstract double getPerimeter();
}

public class Circle extends Shape {
private double radius;

public Circle(String color, double radius) {
super(color);
this.radius = radius;
}
}

Question 58

What contributes to the state of an object in Java?

Answer 58

Both the constructor and the instance variables contribute to the state of a Java object. The constructor is responsible for initializing the instance variables to their initial state, while the instance variables themselves hold the values that define the object’s state at any given time.

Question 59

What does state mean in OOP?

Answer 59

“state” typically refers to the set of values held in an object’s instance variables at any given time. An object’s state is defined by the values of its instance variables, and these values can change over time as the object interacts with the rest of the program.

Question 60

What is memoization?