I sometimes get asked for learning resources and areas aspiring software engineers should focus on. This post will cover some of my recommendations. This is purely for software development, design, and engineering. I will share a complementary list of resources on soft skills, systems, leadership, and working within organizations in a future post.
Data structures and algorithms -- This is CS 101. Understand lists, stacks, queues, heaps, trees, graphs, and algorithms to sort, select, traverse etc. Understand big-O notation and complexity -- this is important in practice, when implementing solutions that deal with real-world data. This is foundational to the field of computer science, so unless you are working on some cutting-edge stuff, there's probably a well-known solution to your problem. You don't need to know all implementations by heart but know what applies and where to look it up when needed, be it an A* search or B-tree.
Understand your compute target -- this can be a physical machine, an operating system, a virtual machine like the JVM or .NET CLR, the browser, or the cloud. Either way you need to know what resources are available, how they are allocated, what are the performance characteristics and so on. Without understanding your compute target you won't be able to leverage it to its full capabilities and run the risk of misusing it.
Concurrent programming -- today, I believe this is unescapable. Concurrency is everywhere, regardless of whether you are building services that talk to each other, a multi-threaded native application, or a Node.JS, event loop-based application. Having a good mental model of how concurrency works, understanding deadlocks, livelocks, synchronization mechanisms, and consistency models is foundational.
Programming languages are not just how we talk to computers, they are tools for thought. They enable us to express solutions to problems. We model our solutions using the languages, and different languages are best suited to different problems. I'm a firm believer in multi-paradigm and using the best tool for a job. Saying that everything is an object, or everything is a function is an oversimplification. Many modern programming languages support multiple paradigms, so we can write object-oriented code when appropriate, and functional code when appropriate. With that said, I suggest learning:
A system programming language like C++, Rust or Swift if you need to write native, performant code. These languages are close to the machine and will help you understand OS resource management, how code gets executed, and what impacts performance.
higher-level language for writing tools and services.
Something like C#, Java or Go. These are all garbage collected and trade
off some performance for productivity.
A dynamic language for quick prototyping and experimentation. Python and Ruby come to mind. While static typing is much safer for production code, I do love thinking in Python.
A purely functional language like Haskell or Idris. This is again to understand different ways of thinking about problems. Even if you don't get to write production code in a purely functional language, you will learn alternative approaches to designing your code which you will be able to apply even when using other languages.
A language with strong support for generics, like C++, Rust, or TypeScript. Generics are a powerful way to reuse and combine code and understanding them will make you a better programmer.
From my personal experience, within the same paradigm, languages are more similar than different. In other words, once you know two, it is significantly easier to understand the third. There is value in learning new programming languages to see how they differ from existing ones and what they bring to the table.
Write code. Learning needs to be a mix of theory and practice. Here are some practice ideas:
Code katas -- These could be some good first projects when picking up a new language. For example: http://codekata.com/, https://github.com/gamontal/awesome-katas.
Programming puzzles -- These are good ways to practice problem solving, data structures, and algorithms. I really enjoy Advent of Code, which has been running every December since 2015. Facebook Hacker Cup also has some great puzzles.
Contribute to an open-source project, many have supportive communities and paths to get you started.
Work on your own project, be it a website or a game or something else you are passionate about. Scope it to fit the amount of time you must dedicate and use the technologies you want to learn.
Of course, you will do most of the learning on the job. Work on projects that interest you, projects you can learn from, and projects where you can apply what you learned.
This is a list of books on software design and craftsmanship that I highly recommend:
Code Complete -I love this one. This is The Big Book of Software Engineering, and it covers the fundamentals, like writing proper functions, using good naming in the code, testing, debugging etc.
The Pragmatic Programmer - Great book which gives general advice on what it takes to become a good programmer. From basic tips like know thy text editor and use source control to implementation and design advice.
Agile Principles, Patterns and Practices in C# - On writing software in an agile world. Principles to keep in mind (like the Open/Close Principle, the Single Responsibility Principle etc.), patterns to enable them, and agile practices. Also covers TDD, extreme programming, and all the other agile methodologies.
Design Patterns - The classic book on design patterns. It is a bit cumbersome but definitely worth reading. Patterns are basically well-known solutions to recurring design problems. Do read my previous post and don't over-index on patterns.
Refactoring - The classical book on refactoring code - re-structuring implementation without changing functionality. Talks about code smells (pieces of code that feel wrong) and how to rearchitect such code to make it right.
Emergent Design - This book talks about how design emerges as code evolves. By respecting a few principles and knowing about design patterns, you don't have to over-design and future-proof, rather keep refactoring and extending as new requirements come in.
There are a few other books which I really enjoyed, though they are a bit different than the above. I recommend these more for the aesthetics and insights (more on that below): Programming Pearls, From Mathematics to Generic Programming, Beautiful Code.
Develop a sense of aesthetics - This comes with practice. Know what good code looks like, what makes it beautiful. Don't just get code working, try to make it beautiful.
Understand your problem space -- Whatever you are working on, knowing the business domain will help inform your software design. Understand why you are doing what you are doing. Is there a better way to solve the same business problem? Do you know what will likely come up in 6 months from now or a year?
Security -- Software security is critical in today's connected world. You should understand security best practices, which hashing algorithms to use, how to properly store secrets and passwords, how trust gets established, attack vectors, how to create a threat model and so on.
AI - AI is permeating more areas of software. It is also being commoditized through libraries like scikit-learn and services like Azure Cognitive Services. While I won't quite yet put it under fundamentals, I believe using AI will soon be a must-know, much like concurrent programming. Having a good understanding of the types of problems AI can help with, when and how to apply it is very valuable.
Keep learning -- The way we build software keeps evolving. Try to keep up to date with recent developments and trends. This is one of the reasons software engineering is such an exciting field: there's always something new, there's always more to learn.