Question 1

What are the four fundamental pillars of object-oriented programming, and can you give a simple example for each?

Accepted Answer

The four fundamental pillars of object-oriented programming are encapsulation, inheritance, polymorphism, and abstraction.

First, encapsulation. This is the principle of bundling data and the methods that manipulate that data into a single unit called a class. Internal data is protected by making it private, and public methods are exposed to access it. For example, a BankAccount class might have a private balance attribute and public deposit and withdraw methods.

Second, inheritance. This allows a class to inherit properties and methods from another class. The child class extends the par

Question 2

What is the difference between an array and a linked list? When would you use one over the other?

Accepted Answer

An array and a linked list are two fundamental data structures with different characteristics.

An array stores elements contiguously in memory. Each element has an index and can be accessed directly in constant time, meaning O of one. The size is generally fixed at creation. Insertion or deletion in the middle of the array is costly because all subsequent elements must be shifted, giving a complexity of O of n.

A linked list stores elements in nodes scattered throughout memory. Each node contains a value and a pointer to the next node. Accessing a specific element requires traversing the

Question 3

What are the main HTTP methods used in a REST API and what is each one used for?

Accepted Answer

The main HTTP methods used in a REST API are GET, POST, PUT, PATCH, and DELETE. Each corresponds to a specific operation on resources.

GET is used to retrieve data. It is a read method that does not modify server state. For example, GET /users returns the list of users, and GET /users/123 returns the user with identifier 123.

POST is used to create a new resource. The request body contains the data for the resource to be created. For example, POST /users with a JSON body creates a new user.

PUT is used to completely replace an existing resource. If the resource does not exist, it may

Question 4

Explain the different categories of HTTP status codes and give common examples for each category.

Accepted Answer

HTTP status codes are organized into five categories, identified by their first digit.

1xx codes are informational. They indicate that the request was received and processing continues. Code 100 Continue means the server is waiting for the rest of the request. These codes are rarely used directly in APIs.

2xx codes indicate success. Code 200 OK is the standard response for a successful request. Code 201 Created indicates a resource was created successfully, typically after a POST. Code 204 No Content means success but no content to return, often used after a DELETE.

3xx codes concern

Question 5

What are the main differences between SQL and NoSQL databases? When would you choose one over the other?

Accepted Answer

SQL and NoSQL databases have fundamentally different characteristics.

SQL databases, also called relational databases, store data in tables with rows and columns. They use a fixed schema defined in advance. Relationships between tables are established through foreign keys. They use the SQL language for queries and guarantee ACID properties for transactions. Common examples are PostgreSQL, MySQL, and SQL Server.

NoSQL databases store data in different ways. Document stores like MongoDB use JSON documents. Key-value stores like Redis use simple pairs. Column-family stores like Cassandra or

Question 6

What is a primary key and a foreign key? What role do they play in a relational database?

Accepted Answer

Primary keys and foreign keys are essential for structuring data in a relational database.

A primary key uniquely identifies each record in a table. It cannot be null and must be unique. For example, in a users table, the id column would be the primary key. Each user has a different id, which allows retrieving them without ambiguity.

A primary key can be simple, using a single column, or composite, using multiple columns. For example, in an order items table, you might have a composite key of order_id and product_id.

A foreign key is a column that references the primary key of another

Question 7

What is the difference between authentication and authorization?

Accepted Answer

Authentication and authorization are two distinct but complementary security concepts.

Authentication answers the question "Who are you?" It is the process of verifying a user's identity. When you enter your email and password to log in, you are authenticating. The system verifies that you are indeed the person you claim to be.

Authorization answers the question "What are you allowed to do?" It is the process that determines which resources and actions an authenticated user can access. After logging in, the system checks whether you have permission to access an admin page or delete an ar

Question 8

Why should you never store passwords in plain text? How do you secure them properly?

Accepted Answer

Storing passwords in plain text is a serious security flaw. If the database is compromised, all passwords are exposed. Since users often reuse the same passwords, this can compromise their accounts on other services.

To secure passwords, we use hashing. A hashing algorithm transforms the password into a fixed-length string that cannot be reversed. Even with the hash, you cannot recover the original password.

However, a simple hash is not enough. Attackers use rainbow tables, precomputed lists of hashes for common passwords. To counter this, we add a salt, a random value unique to each us

Question 9

What is a unit test and why is it important in backend development?

Accepted Answer

A unit test is an automated test that verifies the correct behavior of a small, isolated unit of code, typically a function or method. The goal is to validate that a unit produces the expected result for given inputs.

The characteristics of a good unit test are as follows. It is isolated and does not depend on other components like databases or networks. It is fast to execute, usually a few milliseconds. It is repeatable and always gives the same result. It is self-validating, meaning it passes or fails without human intervention.

The importance of unit tests in backend development is si

Question 10

What are the different types of tests and what is the difference between unit tests, integration tests, and end-to-end tests?

Accepted Answer

There are several levels of tests, often represented by the test pyramid.

Unit tests form the base of the pyramid. They test an isolated unit of code, like a function or class, without external dependencies. They are very fast, very numerous, and inexpensive to maintain. For example, testing that a tax calculation function returns the correct amount.

Integration tests are in the middle of the pyramid. They verify that multiple components work correctly together. You test the interaction between code and a real database, an external API, or a file system. They are slower and more complex

Backend Developer

Topics

Programming Fundamentals

API Design (REST)

Database Design (SQL/NoSQL)

Authentication & Security

Testing Basics

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