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Java developer skills: what to learn first, and what can wait

Knowing the language is the entry ticket, not the job. Hiring managers for Java roles are mostly buying someone who can build and run Spring services against a real database. Learn in this order and you won't waste months on things nobody asks about yet.

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Step one

Gets you the interview

Core Java: collections, generics, exceptions, streamsScreeners test these in the first call. If you can't explain when to use a HashMap versus a TreeMap, or what a checked exception is for, you won't reach the second round.

Spring BootIt's the name recruiters search for most. Build at least one service with controllers, a service layer and Spring Data repositories so you can talk through how it's wired.

SQL and a relational databaseAlmost every Java service reads and writes PostgreSQL, Oracle or MySQL. Write joins and indexes by hand, not just through an ORM.

Git, Maven or Gradle, and IntelliJ IDEAYou're expected to clone a repo, build it, run the tests and debug with breakpoints on day one without someone walking you through it.

Step two

Gets you the offer

Hibernate and JPA, including their trapsInterviewers love lazy loading, the N plus one problem and transaction boundaries, because they separate people who've shipped JPA code from people who've only read about it.

Testing with JUnit, Mockito and TestcontainersWriting tests during the coding round, unprompted, is one of the clearest signals you'll work well on a team.

Kafka or another message brokerMany Java shops connect services through events. Knowing consumer groups, offsets and what happens when a message fails puts you ahead of a lot of candidates.

Docker and a CI pipelineBeing able to containerise your service and read a failing Jenkins or GitHub Actions build shows you can get code to production, not just to your laptop.

Step three

Gets you promoted

JVM internals and performance tuningReading heap dumps, thread dumps and garbage collection logs is what lets you fix the incident everyone else is stuck on, and that's how seniors get noticed.

Concurrency in depthExecutors, CompletableFuture, virtual threads and lock-free structures matter once you own services under real load.

System design for distributed servicesDeciding service boundaries, caching with Redis, retries, idempotency and observability with Prometheus and Grafana is most of a senior's design work.

Kubernetes and a cloud platformSeniors are expected to understand how their service is deployed, scaled and secured on AWS, Azure or Google Cloud, not hand that off entirely.

Certificates worth your time

Certificate	Best for	Effort	Worth it?
Oracle Certified Professional: Java SE 21 Developer	Early-career developers and career changers who need proof they know the language	A couple of months of steady evening study	Worth it if you lack Java work experience. The exam is tough and covers corners of the language you'll rarely use, and experienced developers usually skip it.
Spring Certified Professional	Developers moving into Spring-heavy roles who want structured study	A few weekends if you already use Spring at work	Nice to have. It forces you to learn Spring's internals properly, but few postings require it.
AWS Certified Developer - Associate	Java developers whose services run on AWS	A few weeks of study alongside hands-on practice	Useful for cloud-heavy teams, and it shows up in postings more than the Java-specific certificates do.

Exam names, versions and requirements change, so check the details directly with Oracle, Broadcom or AWS before you book.

Put it on your résumé like this

Weak

Worked on Java backend services using Spring.

Strong

Rebuilt the order-status API in Spring Boot and fixed JPA fetch issues, cutting p95 response time from 1.8s to 240ms across 3 services handling 2M requests a day.

Questions people ask

Should I learn Java or Kotlin first?

Java. Most job postings and most existing code are Java, and Kotlin is easy to add once you know the JVM and Spring. Learning Kotlin first can leave gaps when you're dropped into an older Java codebase.

Do I need to know algorithms and data structures for Java jobs?

Some, yes. You should be comfortable with lists, maps, sets, trees and basic complexity. Many Java employers lean toward practical coding tasks over hard puzzles, but larger tech companies still ask them.

Is Spring Boot enough, or do I need to learn plain Spring too?

Start with Spring Boot, since that's what you'll use. As you move up, learn what Boot is doing under the hood, like auto-configuration, bean scopes and proxies, because that's what you need when something breaks.

Which Java version should I learn?

Learn a recent long-term support release and its features, such as records, switch expressions and virtual threads. Also expect to meet older code, so understand what came before those features.

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