Nidhi Tiwari

Hello, I share real scenarios I deal with as a backend engineer. Mostly around: • Java • System Design • AKS / Cloud • Production edge cases Lately I post question-based scenarios, the kind interviewers ask and the kind that actually break in production. If you enjoy solving these, consider following. New questions every day.

Assume this implementation is used by ~100 concurrent threads. Would you consider it safe for production use? If not, what specific issues would you expect to see? #Java #Concurrency #SystemDesign

@javarevisited B. Exception. Streams in Java are single-use. After the first forEach, the stream is consumed/closed. Calling forEach again will lead to IllegalStateException (stream has already been operated upon or closed).

Java Stream Interview ☕ What will happen here? Stream<Integer> stream = Stream.of(1,2,3); stream.forEach(System.out::println); stream.forEach(System.out::println); A. Works B. Exception Why?

@0xlelouch_ Total downtime is 50 minutes, exceeding the 43-minute SLA, so yes it’s a breach and the customer is eligible for compensation.

An interesting question if you are into tech infra and observability! Your service has 99.9% uptime SLA (43 minutes downtime/month). You had 2 outages: 20 minutes and 30 minutes. Customer demands refund. Do they get it?

@_jaydeepkarale ORDER BY created_at, default is ASC (ascending) That means oldest records first, not latest. So this query returns, first 10 payments ever created, not the newest ones.

A dashboard query looks like this: SELECT * FROM payments ORDER BY created_at LIMIT 10; The team says this returns the latest payments. Are they correct?

@SumitM_X Optimistic Locking + Atomic DB Update

Two users hit "Book Now" at the exact same millisecond on Booking and Expedia. Only 1 seat left. Both requests reach your system. How do you make sure ONLY ONE ticket gets booked?

Case 1: a == b is true "Ja" + "va" is a compile-time constant (two string literals joined), so Java automatically interns it. Both a and b point to the same object in the String Pool. Case 2: c == b is false part + "va" involves a variable (part), so concatenation happens at runtime, creating a new String object on the heap, different reference than b.

What prints? String a = "Ja" + "va"; String b = "Java"; System.out.println(a == b); Now try this: String part = "Ja"; String c = part + "va"; System.out.println(c == b); Why different results?

@GuidesJava Ans: String Why? When you pass null, Java chooses the most specific method. So m1(String s) is more specific than the others, it gets called.

Most asked Java coding program: What is the output of this program? Check out the answer and explanation: youtu.be/rwgmCundLL0?si…

@javarevisited Scaling databases is hard because data must stay consistent and synchronized across nodes, unlike stateless app servers.

Interviewer: If horizontal scaling is easy with cloud platforms, why is scaling databases still hard?

@javarevisited @override If equals() is overridden without hashCode(), HashSet allows duplicates because equal objects may have different hash buckets. PS: I post similar content. Please check out my page and follow if it interests you.

Spot the issue: class User { private String email; @Override public boolean equals(Object o) { return email.equals(((User) o).email); } } What breaks when this object is used in a HashSet?

This is how it works: Pre-stored popular searches : Google already has common queries saved Prefix matching : As you type “java”, it finds all searches starting with that Fast systems (caching) : Keeps frequent results ready in memory AI + your data :Uses your history, location, and trends

Interviewer: You start typing a search query. And Google suggests results instantly. How?

@javarevisited 2

How many objects are created? String s1 = "Hello"; String s2 = "Hello"; String s3 = new String("Hello"); 1? 2? 3? 👀

@devops_nk ls, cd, cp, rm, mkdir

What are the your most 10 commonly use Linux commands ?

@Math_files This is a geometric series with first term a = 1/2 and common ratio r = 1/2.

1/2 + 1/4 + 1/8 + ... = 1

@Cipher_twt Lol

Vibe coder writing books in future

Java 26 just dropped and it’s LOADED! HTTP/3, Faster JVM, Primitive Patterns & finally… Applets are DEAD. 1. Primitive Types in Patterns No more boxing/unboxing headaches use primitives directly in switch switch (age) { case int i when i < 18 -> "Minor"; case int i when i < 60 -> "Adult"; default -> "Senior"; } 2. HTTP/3 Support Native HTTP/3 support ,no external libraries needed. HttpClient client = HttpClient.newBuilder() .version(HttpClient.Version.HTTP_3) .build(); 3. Lazy Constants Initialize only when needed, better performance. static final lazy String CONFIG = loadConfig(); 4. Structured Concurrency Cleaner, safer parallel execution try (var scope = new StructuredTaskScope.ShutdownOnFailure()) { var user = scope.fork(() -> fetchUser(id)); var order = scope.fork(() -> fetchOrder(id)); scope.join().throwIfFailed(); return new Response(user.get(), order.get()); } 5. Goodbye Applets This won’t even compile now: class MyApplet extends Applet {} RIP: 1995 → 2026 Follow for more such daily Java content. #Java26 #JDK26 #JavaDeveloper #Programming #Tech

Java just made it official: Applets are DEAD in Java 26 Remember college days? Writing your first tag, drawing circles, rectangles, and smiley faces using Graphics, making that bouncing ball animation (that never bounced smoothly), building a basic calculator UI, changing background colors on button click. It felt like magic back then, “Java can run inside a browser?!” And then, Waiting forever for it to load, clicking “Allow / Run” on scary security popups, debugging why nothing is showing on screen. Good times. But slowly, Browsers dropped support, Security issues piled up, Better technologies took over. And now in Java 26, Applets are gone for good. Not just a removal, it’s pure nostalgia. From: College applet programs to Desktop apps to Spring Boot & microservices Java evolved. And so did we. If you ever made a bouncing ball or drew a smiley using Applet, you’re not old, you’re OG Java dev! #Java #Java26 #Nostalgia #TechTwitterÇevir

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Nidhi Tiwari@NidhiDevNotes·2d

@Kekius_Sage The "nobody's in control" scenario Not one villain just AI systems running critical infrastructure, finance, and decisions, and slowly humans just stop being the ones actually in charge. Death by a thousand automations

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Kekius Maximus@Kekius_Sage·3d

What is the worst-case scenario for AI?

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Nidhi Tiwari@NidhiDevNotes·2d

Since SSH just hangs with no output at all from local terminal too, this is almost certainly a network/firewall issue, not the instance itself. Check this, Go to AWS Console → your EC2 instance → Security Groups → Inbound Rules → check if port 22 is open. If not, add a rule: Type: SSH, Port: 22, Source: your IP (or 0.0.0.0/0 to test). Also quickly check: Does the instance have a public IP assigned? If no public IP, SSH will never reach it. Are you using the correct username? (ubuntu for Ubuntu, ec2-user for Amazon Linux)

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Dhanush N@Dhanush_Nehru·2d

@itsrajputamit when i try ssh from local terminal it just doesn't output any, no response at all

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Dhanush N@Dhanush_Nehru·2d

SSH to an EC2 instance is not working. How to fix this?

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Nidhi Tiwari@NidhiDevNotes·2d

@thefarmersheep “QR” in QR code stands for Quick Response. It was named that because it can be scanned and read very fast, much quicker than traditional barcodes.

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sheep@thefarmersheep·3d

what did “QR “ in QR code mean

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