Question library
Questions from real interviews. Drill any of them by voice in a couple of minutes, with an interviewer, a score and feedback.
How is reflow different from repaint, and why does frequent reflow hurt performance?JuniorWhy are strings immutable in Java, and why shouldn't you concatenate strings with plus in a loop?JuniorYour Python script runs slowly. How will you figure out what exactly is slowing it down?JuniorA load test showed that at 500 requests per second, response time grew from 200 milliseconds to three seconds. How would you track down the cause, and what would you put in the report?Mid-levelA nightly data processing script used to run for ten minutes, and now it takes two hours. How would you find out where it's spending its time?Mid-levelA service running on G1 periodically has pauses of one to two seconds, and it's hurting latency. What would you do, and when is it worth looking at a different garbage collector?Mid-levelThe LCP on the home page is around four seconds. How would you figure out the causes, and what would you do to bring it down?Mid-levelUsers complain that the page stutters and lags when they scroll. How would you look for the cause, and what role might reflow and repaint play here?Mid-levelYour app's bundle has grown to several megabytes, and the first load has become slow. How would you shrink it?Mid-levelA Spring Boot service mostly calls other services and the database. Load is growing to tens of thousands of requests per second, and Tomcat runs out of threads. Which concurrency model would you choose: increase the pool, move to virtual threads, or go with a reactive stack, and why?Mid-levelAfter a release, Core Web Vitals dropped in production, primarily INP and LCP. How would you find the cause, and what do you usually end up fixing?SeniorHow do you track down a performance bottleneck in Python code? What's the most telling case from your own experience?SeniorHow does Stream API execution work under the hood, and when would you use a parallel stream and when wouldn't you?SeniorWhat happens in the browser between a change to the DOM or styles and pixels appearing on screen? How do you use that to keep animations and scrolling from janking?SeniorВ день зарплаты эндпоинт истории операций начинает отвечать по несколько секунд, а база упирается в сто процентов CPU. Как вы будете разбираться и что делать?SeniorDesign a Java service that handles thousands of requests per second and, for each one, calls several slow external systems. How would you organize threads, pools, and overload protection?SeniorYour Java service has to respond with a p99 of no more than ten milliseconds. How would you design it with the JVM's characteristics in mind, and how would you find the source of latency if the target isn't being met?Senior