Returning to Java after 7 years (Java 8 to Java 21). What should I relearn first?
Meenal Deshpande@meenal_restarts
Good morning all. It is 5.30 am here, the kids are not awake yet, so I am posting before the chaos starts 🙂
I was a Java developer at a service company till 2018: mostly Java 8, Spring 4 with XML configuration, and Struts on one old project (kindly do not laugh). After a seven year break I am preparing to return to work. Now I see Java 21 everywhere: records, var, sealed classes, pattern matching, virtual threads.
What I have already done:
- Installed JDK 21 with SDKMAN
- Built a small REST API with Spring Boot 3 following a tutorial (it worked, I was very happy)
- Opened the list of JEPs from Java 9 to 21. It is very long, and I closed it again
Kindly guide: in what order should I learn the new features, for interviews and for actual work? And what can I safely ignore for now? I am having a query also about whether people still use Optional the way we did, or if that changed.
Thanking you in advance.
Accepted answer
Welcome back Meenal! You already did the hardest part (actually starting). My order, from what I use daily on client projects:
1. var (Java 10). 10 minutes. Local variables only.
2. Records (16). Your DTOs become one line: record AccountDto(String iban, long balance) {}
3. Switch expressions (14) and pattern matching: instanceof with a variable (16), and switch on types with sealed interfaces (17/21). Example:
sealed interface Payment permits Upi, Card, NetBanking {}
record Upi(String vpa) implements Payment {}
record Card(String last4) implements Payment {}
record NetBanking(String bank) implements Payment {}
String describe(Payment p) {
return switch (p) {
case Upi u -> "UPI " + u.vpa();
case Card c -> "Card ending " + c.last4();
case NetBanking n -> "Netbanking " + n.bank();
};
}
No default needed, the compiler knows all cases. Add a new Payment type and every switch that misses it stops compiling.
4. Text blocks (15) for SQL and JSON in tests.
5. Small collection things: List.of, Map.of (9), stream.toList() (16).
6. Optional: same API, but use it only as a return type. Not for fields, not for parameters. That is what changed in practice.
7. Virtual threads (21): understand the idea. In Spring Boot 3.2+ it is one property, spring.threads.virtual.enabled=true.
Safe to ignore for now: modules (JPMS), almost nobody uses them in app code. Nashorn was removed anyway.
Outside the language: Spring Boot 3 with Java config only, JUnit 5, Testcontainers, basic Docker. That covers 90% of interviews I have taken on the client side.
Get the best of #compiling weekly
One email each Monday with the most useful posts. No spam; unsubscribe in one click.
Comments
Sign in to join the conversation.
Sign in to commentRohit Verma@rohit_codes
Meenal ji welcome back 🙏 Struts pe koi nahi hasega, aadhe banks aaj bhi usi pe chal rahe hain 😂 @omkar_java ye tumhara area hai, dekho zara
Accepted answer
Omkar Joshi@omkar_java
Welcome back Meenal! You already did the hardest part (actually starting). My order, from what I use daily on client projects:
1. var (Java 10). 10 minutes. Local variables only.
2. Records (16). Your DTOs become one line: record AccountDto(String iban, long balance) {}
3. Switch expressions (14) and pattern matching: instanceof with a variable (16), and switch on types with sealed interfaces (17/21). Example:
sealed interface Payment permits Upi, Card, NetBanking {}
record Upi(String vpa) implements Payment {}
record Card(String last4) implements Payment {}
record NetBanking(String bank) implements Payment {}
String describe(Payment p) {
return switch (p) {
case Upi u -> "UPI " + u.vpa();
case Card c -> "Card ending " + c.last4();
case NetBanking n -> "Netbanking " + n.bank();
};
}
No default needed, the compiler knows all cases. Add a new Payment type and every switch that misses it stops compiling.
4. Text blocks (15) for SQL and JSON in tests.
5. Small collection things: List.of, Map.of (9), stream.toList() (16).
6. Optional: same API, but use it only as a return type. Not for fields, not for parameters. That is what changed in practice.
7. Virtual threads (21): understand the idea. In Spring Boot 3.2+ it is one property, spring.threads.virtual.enabled=true.
Safe to ignore for now: modules (JPMS), almost nobody uses them in app code. Nashorn was removed anyway.
Outside the language: Spring Boot 3 with Java config only, JUnit 5, Testcontainers, basic Docker. That covers 90% of interviews I have taken on the client side.
Meenal Deshpande@meenal_restarts
Omkar, thank you so much. This is exactly the order I needed. The sealed + switch example made pattern matching clear in one look. Saved. I will do records and switch this week after the kids sleep 🙂
Ankit Khurana@faang_or_bust
7 year gap in 2026 lol. market is brutal, freshers with 500 leetcode are jobless. good luck i guess
Meenal Deshpande@meenal_restarts
Thank you for the wishes. Seven years ago I was also the person in the office answering questions at 9 pm. In these seven years I raised two children and cared for my father, which taught me more about priorities and deadlines than any sprint did. I am not asking if the market is easy. I am asking what to learn. If you have a suggestion on that, I will be grateful 🙂
Sneha Kulkarni@laidoff_not_out
meenal 👏 the most polite destruction i have seen on this site. also our mock on saturday is still on, i'll ask you records questions now
Tomasz Kowalski@tomasz_k
Good list from Omkar. I add one: learn to read Spring Boot 3 stack traces, they are longer than in your time. And ignore modules, everybody ignores them, including Spring.
Kunal Bhatia@kunal_dsa
for interviews: records + streams + one virtual threads question is what I saw in 4 of 5 Java rounds this year. nobody asked about modules. you will be fine 🙏
Anitha Joseph@anitha_miss
Very well asked, Meenal. You already did more than most of my students: installed, built something, then asked. One small suggestion: write the same small program in Java 8 style and Java 21 style side by side. The difference will stay in your memory. All the best.