Mistakes to Avoid in a Machine Coding Round

A Machine Coding Round is not just about writing code that works.

You are expected to demonstrate how you approach a real-world software problem — including code structure, object-oriented design, maintainability, extensibility, in-memory storage, error handling, and design principles.

After conducting 100+ machine coding interviews, here are some of the most common mistakes candidates should avoid.

1. Not Researching the Round Requirements ¶

One common mistake is preparing for Machine Coding like a DSA round.

Machine Coding is not just about solving a problem. You are expected to design and build a working system while writing clean, maintainable, and extensible code.

Before preparing, understand what the company expects from the round:

What type of problems are asked?
How much time is given?
Which language is allowed?
Are requirements added during the round?
Is LLD or system design discussed?
How is the solution evaluated?

For example, a company may ask you to build a Parking Lot, Splitwise, Elevator, or Movie Booking System and then introduce additional requirements during the interview.

The key idea: Don't prepare only for solving the problem. Prepare for designing, implementing, and evolving the solution within the given time.

2. Writing Everything in a Single File ¶

A common mistake, especially among freshers and junior engineers, is putting the entire implementation into one file.

This makes the code:

Difficult to read
Difficult to debug
Difficult to test
Difficult to extend

Instead, separate your code into logical components such as:

models/
services/
repositories/
exceptions/
strategies/

The exact structure can vary depending on the problem, but the goal is to keep responsibilities separated.

3. Poor Class and Model Design ¶

A common mistake is to put data and too much business logic into the same model class.

For example:

class Movie {
    int totalSeats;
    int bookedSeats;

    void bookSeat() {
        if (bookedSeats < totalSeats) {
            bookedSeats++;
        }
    }
}

Here, Movie is responsible for both holding movie data and handling the booking process.

As the requirements grow, more business logic can get added to Movie, making the class harder to understand and maintain.

Improved Design ¶

Keep the model focused on representing data and move the business logic to the appropriate class:

class Movie {
    int totalSeats;
    int bookedSeats;
}

class BookingService {

    void bookSeat(Movie movie) {
        if (movie.bookedSeats < movie.totalSeats) {
            movie.bookedSeats++;
        }
    }
}

Now:

Movie          → Holds movie data
BookingService → Handles booking logic

This separation makes the code easier to understand, modify, and extend.

The key idea: Give each class a clear responsibility. A model should primarily represent the data, while business logic should be handled by the appropriate class.

4. Using the Wrong In-Memory Data Structure ¶

Machine coding problems often require you to implement storage in memory.

Using a List for everything can lead to unnecessary O(n) lookups.

For example:

List<User> users;

If you frequently search for a user by ID, you may have to iterate through the entire list.

A better option is often:

Map<Integer, User> userMap;

This allows key-based lookup in approximately O(1) average time.

Choose Data Structures Based on Operations ¶

Before selecting a data structure, ask:

Do I need fast lookup by ID?
Do I need ordering?
Do I need uniqueness?
Do I need priority-based access?
Do I need to support frequent insertion/deletion?

The right data structure should follow the requirements.

5. Tightly Coupling Storage and Business Logic ¶

Avoid directly coupling your business logic to a specific storage implementation.

Poor design:

class UserService {
    private UserDao userDao = new UserDao();
}

Here, UserService is directly dependent on UserDao. If you later want to replace the database with in-memory storage, another database, or a mock for testing, you need to change UserService.

Better design:

interface UserStorage {
    void save(User user);
    User findById(String userId);
}

class InMemoryUserStorage implements UserStorage {
    // implementation
}

Now UserService depends on the UserStorage interface instead of a specific implementation.

UserService
     ↓
UserStorage
     ↑
InMemoryUserStorage

This is good because the business logic doesn't care how the data is stored.

You can easily:

Change the storage implementation
Write unit tests using a mock storage
Add a new storage implementation later

This is a practical application of the Dependency Inversion Principle (DIP).

6. Ignoring Proper Error Handling ¶

Another common mistake is relying on generic exceptions or simply logging errors.

Instead, define meaningful exceptions where appropriate.

For example:

class UserNotFoundException extends Exception {

    public UserNotFoundException(String message) {
        super(message);
    }
}

Then your service can communicate the actual failure clearly:

if (!userMap.containsKey(id)) {
    throw new UserNotFoundException(
        "User with ID " + id + " not found."
    );
}

Good error handling makes your implementation easier to understand and maintain.

7. Ignoring Basic Programming Concepts ¶

Strong fundamentals are extremely important in machine coding rounds.

Before your interview, make sure you are comfortable with concepts such as:

OOP/SOLID principles
Interfaces and abstract classes
Composition vs inheritance
Shallow copy vs deep copy
String mutability
Concurrency and locking
Basic time and space complexity

These concepts directly affect the quality of the code you write during the round.

8. Focusing Only on Making the Code Work ¶

Getting the happy path working is important, but it shouldn't be the end goal.

Interviewers may also look at:

How easily the code can be extended
Whether responsibilities are separated
Whether classes have clear responsibilities
Whether the implementation handles invalid inputs
Whether the code is readable
Whether the chosen data structures make sense

A working solution is the starting point.

A clean, maintainable, and extensible solution is what can differentiate you.

9. Not Leaving Time for Testing ¶

One of the easiest mistakes to avoid is spending the entire interview writing implementation and having no time left to test it.

Reserve time at the end to test:

Happy Paths ¶

Test the expected flow first.

Edge Cases ¶

Think about:

Empty input
Invalid IDs
Duplicate operations
Maximum/minimum values
Missing entities
Repeated operations

Failure Cases ¶

Make sure invalid operations fail gracefully instead of producing unexpected behavior.

A Simple Checklist Before You Submit ¶

Before finishing your machine coding round, ask yourself:

Final Thoughts ¶

The biggest mistake in a machine coding round is treating it like a DSA problem with more code.

The goal is not simply to finish as many lines of code as possible.

Think like a software engineer:

Understand → Design → Implement → Test → Improve

Focus on clean code, good object-oriented design, separation of concerns, appropriate data structures, extensibility, and maintainability.

These practices not only help you perform better in machine coding interviews — they also build the habits required to write better production software.

Mistakes to Avoid in a Machine Coding Round

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