Top 100 back-end developer interview questions and answers 2023

Java uses a constructor to initialize objects. It is used when an object in a class is created. A constructor has the exact same name as its class. It is also syntactically similar to a method. However, a constructor has no explicit return type.

There are three main types:

Default Constructor: constructor with no arguments. They initialize data members with real values.

Parameterized Constructor: contains arguments in the constructor. Here we can pass more than one argument.

Copy constructor: Initializes an object using another object of the same class.

Serialization refers to a process or mechanism that converts the object's state into a byte stream. Deserialization does exactly the opposite. Serializing an object makes it possible to easily convert the byte stream into a copy of the object.

Final is an access modifier, finally is a block, and finalize is a method of an object class. The final method is executed when called. Finally is executed once the try-catch block is executed. Its execution does not depend upon the exception. Finalize method is executed before the object can be destroyed.

Wrapper classes allow using a variety of primitive data types such as int or boolean as objects. The wrapper class transforms a primitive type into an object. A primitive type is a predefined data type that the Java programming language provides.

Multiple inheritance in Java is not supported because of the ambiguity issue. To simplify the language and reduce complexity, multiple inheritance is not supported in Java.

Multiple inheritance is when one class (e.g. class B) extends beyond other classes (e.g. class A or class C), this is known as multiple inheritance. Multiple inheritance can lead to problems such as the diamond problem.

HashMap in Java works with a variety of hashing principles. It stores the entries in the map by using its static inner classes. HashMap makes use of several buckets. Each bucket of HashMap points to a Singly Linked List.

In Java 8 however, HashMap replaces Linked Lists with a binary tree once the number of bucket elements reaches a threshold. HashMap supports multiple null values, but only one null key.

A thread is a thread of execution within a program. Threads enable Java applications to be more responsive and faster by performing multiple tasks at the same time.

There are two types of threads: user threads and daemon threads. A daemon thread, which is low-priority and provides services only to user threads. A user thread, on the other hand, is a thread of high priority.

A process is the execution of a program, which allows users to execute the actions specified in the program.

A thread, on the other hand, is an execution unit. It is an execution unit and can be managed separately by a scheduler. A process can have multiple threads which execute at the same moment.

Exception Handling is a method to handle runtime errors such as ClassNotFoundException, SQLException, IOException, RemoteException, etc.

There are two main types of Java exceptions: unchecked and checked exceptions. If the compiler checks for checked exceptions, then the programmer handles them during compilation. Otherwise, a compilation error is caused. These exceptions can be handled with either a try-catch or re-throwing. It is not necessary to handle an unchecked exception.

A design pattern is a repeatable or reusable solution for common problems in software design. These patterns are similar to blueprints and often show interactions between classes or objects.

SQL (Structured Query Language) is a programming language used to communicate with databases. MySQL is a relational database management system that uses SQL as its primary language. SQL is used to create, modify, and delete databases, while MySQL is used to store and access data within databases.

Data Definition Language (DDL): This allows you to perform various operations in the database, such as CREATE, ALTER and DELETE.

Data Manipulation Language (DML): This allows you to access and modify data. It allows you to insert, update, and delete data from the database.

Data Control Language (DCL): This allows you to control data definition and data manipulation operations, such as granting and revoking user access privileges, and creating, altering, and dropping database objects, such as tables and views.

A Relational Database Management System (RDBMS) is a program used to manage a relational database. It serves as the foundation of modern database systems such as MySQL, Microsoft Access, Microsoft SQL Server, and Oracle. RDBMS uses SQL to obtain data from the database.

The SELECT statement allows you to select data from SQL databases. The result-set stores the returned data in a result table.

These SQL clauses are often used with a SELECT statement:

1. FROM: specifies the table(s) from which data is retrieved.
2. WHERE: filters the records based on certain criteria.
3. GROUP BY: groups the records based on certain criteria.
4. HAVING: filters the grouped records based on certain criteria.
5. ORDER BY: sorts the records in ascending or descending order.
6. LIMIT: limits the number of records returned from a query.
7. OFFSET: specifies the starting point from which records are returned.
8. AS: creates aliases for column names or table names.
9. JOIN: combines records from two or more tables.
10. UNION: combines the result set of two or more SELECT queries.

Online transactional processing (OLTP) allows large groups of people to perform massive database transactions in real-time, via the internet. Database transactions are when data is modified, inserted, deleted, or queried.

OLTP stands for online transaction processing and OLAP stands for online analytical processing. OLTP can be described as an online database modification and OLAP is an online database query response system.

SQL comments are used to clarify SQL statements and prevent SQL statements from being executed. Comments are essential in many programming languages.

Single-line Comments: It begins with two consecutive hyphens (-).

Multi-line Comments: It begins with /*, and ends with //.

PEAR is a repository and framework for PHP components that can be reused. PEAR stands for PHP Extension and Application Repository. It includes all types of PHP code and libraries. You can also use the command line interface to automatically install "packages".

Static websites have fixed content, meaning that it is not updated regularly. The content is coded directly into the HTML and requires manual updates when changes are needed. Dynamic websites are those which have content that is updated regularly and automatically.

To execute PHP scripts, use the PHP Command Line Interface and then add the file name in the following way.

php script.php

PHP is partly case-sensitive. Variable names are case-sensitive, but function names aren't. It will still work if you use lowercase for the function name and call them in uppercase. While user-defined functions can be used in any case, the rest of the language is case-sensitive.

PHP parsing engine requires a way for PHP code to be distinguished from other elements of the page. The method for doing this is called "escaping PHP".

Python is an interpreted and object-oriented programming language that supports various programming models, including procedural and functional programming. It contains modules, exceptions, dynamic typing, high-level data types, classes, and dynamic typing.

Yes. Python is a case-sensitive language.

The acronym PEP stands for Python Enhancement Proposal. It's a set of rules that defines how Python code should be formatted to ensure maximum readability.

Decorators in Python are functions that modify the behavior of other functions. A decorator is a function that takes another function as an argument, adds some kind of functionality, and returns the modified function without changing its definition. Decorators provide a simple syntax for calling higher-order functions, allowing developers to modify existing functions without changing their source code

The .py files contain the python source codes files. The bytecode of the Python files can be found in the .pyc files. When the code is imported from another source,.pyc files are created. The interpreter converts source .py files to .pyc files which reduces time.

Python memory management involves a private heap that contains all Python data structures and objects. The Python memory manager ensures that this private heap is managed internally.

Python Namespaces represent collections of objects which are associated with a unique name and whose lifespan is determined by the scope of a variable.

Python modules contain Python code. These codes can be either function classes or variables. A Python module is a .py file that contains executable code.

Python packages are namespaces that contain multiple modules.

Accessibility refers to the user's ability to access the system. If the user is unable to access the application, it will be assumed that the application is unavailable. High availability means that the application will always be available without interruption.

Clustering with redundant server nodes is a common way of achieving higher availability for web applications. Availability is usually expressed as a percentage of uptime in a given period of time.

Microservices architecture offers many benefits. Here are some of the pros:

• Microservices are easily adaptable to other frameworks and technologies.
• One process failure does not necessarily affect the whole system.
• Supports both small and large companies.
• It can be used independently and takes relatively little time.

A containerization is a form of virtualization that was developed as an alternative to hypervisor-based virtualization.

Containerization means that the operating system is shared between the containers, rather than being cloned for each machine. Docker, for example, is a container virtualization platform that can be used as an alternative to hypervisor-based arrangements.

Functional testing is a verification test to determine if we have built an appropriate product. Does the software satisfy the requirements of the business? Functional tests confirm that the program operates according to what the developer believes it should.

Acceptance Testing is an activity to verify that we create the correct product. Does the customer really want it? Tests of acceptance confirm that the product can solve the problem it was designed to handle. This is best done with the help of the user such as doing the tasks the software helps in.

Denormalization is a technique that is used to access data from higher to lower forms of a database. It assists database managers to improve the efficiency of the whole infrastructure by introducing redundant data into tables. It incorporates redundant data into a table by integrating database queries that integrate data from multiple tables into one table.

An index is an approach to tuning the performance and making it easier to retrieve records in the database. The index generates an entry for each value. Hence, it is faster to access information.

This DISTINCT query is used in conjunction with the SELECT query to get rid of duplicates and return only unique records. Sometimes, an entire table is filled with duplicate records. The DISTINCT clause in SQL is used to remove duplicates in a SELECT statement's results set.

In SQL, a Trigger is a particular kind of stored procedure that is defined to execute automatically after data modifications. It lets you execute a series of codes whenever an update, insert, or another query is performed against a particular table.

The cross join produces the cross product or Cartesian product from two tables. The natural join is built on all columns that have the same name as well as data types within both tables.

PyChecker is an analysis tool for static code that helps to identify the flaws within Python the source code. PyChecker warns you about the style and severity of the problem. Additionally, Pylint is another tool that examines whether the module conforms to the standard for coding.

Python modules contain Python code. These codes can be either function classes or variables. A Python module is a .py file that contains executable code.

These are some of the most popular built-in modules:

• os
• sys
• JSON
• math
• random
• data time

Global variables

Global variables are declared outside of a function or in global spaces. These variables can be accessed from any function within the program.

Local variables

A local variable is any variable declared within a function. This variable is only present in the local space.

It will throw an error if you attempt to access the local variable from outside the function add().

When naming a PHP variable, you should follow the following guidelines:

The name of a variable must start with a letter or underscore character. Variable names can contain numbers, letters, or underscores. However, You cannot use characters like +, -, and %.

Python requires indentation. You can use it to identify a block of code. An indented block contains all code that is contained within loops, classes, and functions. This is typically done with four space characters. Indenting your code incorrectly will throw errors and cause failure in execution.

Python arrays are similar to lists in that they are both used to store multiple values. However, unlike lists, arrays are limited to a single data type and are fixed in size. Additionally, arrays are much more memory efficient than lists since they only store values of a single type.

A function is a block that executes code only when it's called. The def keyword in Python is used to define Python functions.

Output: Hi, Welcome to Turing.com

init can be described as a Python constructor or method. This method is used to automatically allocate when a new class instance or object is created. All classes have the "init" method.

Output:

ABC

25

50000

The NULL data type is a special one-value data type. A variable of datatype NULL is a non-valued variable. It can be assigned using the following: $var = NULL; or $var = null;

The UNIQUE constraint prevents identical column values from appearing in more than one record. The UNIQUE constraint assures that every value within a column is unique.

You can define a constant using the define() function. To retrieve its value, you simply need to specify its name. A constant that contains a $ is not required. A valid constant name begins with a letter or underscore.

You can generate HTML using PHP scripts. Additionally, you can pass information from HTML to PHP. PHP executes on the server and returns its results in strings, arrays, and objects. Then, we display their values in HTML.

The self is an instance or an object of a class. This parameter is always included in Python. Java does not have this option. It can be used to distinguish between attributes and methods in a class with local variables.

In the init method, the self variable refers to a newly created object. In other methods, it refers to the object whose method was called.

The Random Module is the standard module used to generate random numbers. This is how the method works:

Pickling and unpickling are processes related to serializing and deserializing objects in Python. Pickling is the process of transforming an object into a byte stream, while unpickling is the inverse operation, i.e. transforming a byte stream back into an object hierarchy. Pickling allows complex objects to be stored and transmitted as a single entity, and unpickling allows the byte stream to be converted back into the original object hierarchy.

There are three types of Arrays available in PHP,

Indexed Array – An array that has a numeric index is called the indexed array. Values are stored in linear order and can be accessed by linear means.

Associative Array - An array that includes strings as an index is also known as an associative array. This stores element values in association with key values, rather than in strict linear index order.

Multidimensional Array - A multidimensional array contains one or more arrays. Multiple indices are used to access the values.

The set_time_limit(0) attribute is added to the script's beginning to infinite the time it takes to execute. This prevents PHP from displaying the error message 'maximum execution time exceeded.

Black Box Testing is a method of testing functions of the software, that involves not knowing their internal code structure, implementation details, or internal routes. It focuses solely on the input and output of software applications. It is completely driven by specifications and software requirements. Another name for it is behavioral testing.

Data Abstraction provides only the necessary information and hides the implementation from the world. It is possible to achieve this by using Python through abstract classes and interfaces.

The Help() and dir() functions are available from the Python interpreter and are used to view a consolidated listing of the built-in features.

Help() function: The help() function is used to display the documentation string. It can also help you see the assistance related to modules, attributes, keywords, etc.

Dir() function: The dir() function displays specified symbols.

It's utilized to calculate the length of a string, list, array, etc.

Output: 3

FOREIGN KEY is a field (or set of fields) in one table which is linked to the PRIMARY KEY in another table. The table that has this foreign key is known as the child table. The table that has a primary key is known as the parent table.

Typo hinting is used to define the expected data type for an argument in a function declaration. When you call it, PHP determines whether the arguments are of the specified type. If they are not, the run-time shows an error, and the execution will stop.

In SQL there is a built-in function called GetDate() that can provide the current date and time in a 'YYYY-MM-DD hh:mm:ss.mmm' format.