1. Knowledge of Design Patterns

It's impractical to expect all developers to know every design pattern, but familiarity with the more commonly used ones—like Singleton, Facade, Builder, Dependency Injection, and Wrapper—is beneficial. It's advisable to focus on depth rather than breadth, honing a solid understanding of a few patterns rather than skimming over many.

During my interviews, I often encountered questions about the Singleton pattern, which is relatively straightforward. What interviewers valued more was my overall awareness of design patterns and my ability to discuss their practical applications, rather than a superficial ability to rattle off definitions.

2. Practical Understanding of Classes

Classes are a fundamental topic, but you won't need exhaustive knowledge right away. It's crucial to understand the distinction between simple and complex types. You should familiarize yourself with class components—fields, constructors, and methods—as well as access modifiers.

As you advance, being aware of abstract classes and interfaces is valuable, even if you haven't yet worked on a large project. Initial familiarity with inheritance and how to access parent class content (using the keyword super) is also essential.

3. Basic SQL Knowledge

Understanding SQL is fundamental for any developer. Although I had a strong academic background in databases, I was surprised by how often I was asked to perform basic SQL operations during interviews.

Common tasks included writing simple INSERT, UPDATE, and SELECT statements. More challenging inquiries involved joining tables and understanding aggregate functions. A solid grasp of basic SQL operations is sufficient to start, but be prepared to expand your knowledge as you progress in your career.

4. Familiarity with Keywords

You can expect to encounter keyword-related questions in interviews. While memorizing all keywords is unnecessary, having practical knowledge of the most common ones is essential.

During one interview, I was asked to explain the function of various keywords within my own code, such as static, void, final, and throws. One popular question often asked is, “What’s the difference between final and finally?” If you have hands-on experience, this should pose no difficulty.

5. Understanding Access Modifiers

Most of your initial coding experience will involve simple console applications, where all code is contained within one method. As you progress to creating classes and methods, learning about access modifiers becomes crucial. This foundational knowledge is essential for internships and beyond.

I encountered a particularly insightful task during an interview that effectively tested my understanding of access modifiers.

6. Collections Knowledge

Collections are not only engaging to learn but also vital for developers. Initially, focus on distinguishing between Maps, Sets, and Lists. Familiarity with one implementation for each type of collection, such as HashMap for Maps and ArrayList for Lists, is sufficient.

7. Exception Handling

Understanding how to manage exceptions is key in programming. Be sure to know how to use a try-catch block, the purpose of each part, and the concept of multiple catch blocks to handle various error types.

Additionally, familiarize yourself with how to throw exceptions and the implications of adding throws to a method's header.

8. Comprehending Lambdas

Lambdas represent a significant advancement in JavaScript and are a frequent topic during interviews. They can be complex, so they shouldn't be one of the first concepts you tackle. If you feel confident, it's worthwhile to learn about them, but don't be discouraged if it takes time to grasp their functionality.

9. Knowledge of HTTP Codes

While I was only asked about HTTP response codes in one of my interviews, I now recognize the value of such questions. Understanding common codes like 404, 500, 400, 401, 403, and 200 is beneficial for any developer.