Interview Prep · Phase 2
Functional Java as an interviewer probes it — not "name three functional interfaces" but why a lambda is not an anonymous class, how a stream pipeline actually executes, when parallel streams make things slower, and what Java 9 through 21 added on top.
Every Java 8 feature an interviewer asks about, why each was added, and the one-sentence answer for each — lambdas, streams, Optional, default methods, java.time, CompletableFuture and Metaspace.
Read tutorialWhy a lambda is not an anonymous class, what invokedynamic and LambdaMetafactory do at first call, the allocation difference between capturing and non-capturing lambdas, and what this means.
Read tutorialThe four core shapes and how to derive the other thirty-nine, why primitive specialisations exist, compose versus andThen, and the checked-exception problem with a clean workaround.
Read tutorialHow a stream pipeline actually executes: why nothing runs until the terminal operation, what short-circuiting really means, stateful versus stateless operations, and why a stream is single-use.
Read tutorialThe collector API in depth: multi-level groupingBy, downstream collectors, the toMap duplicate-key exception, the null-value trap, teeing and flatMapping, and writing a Collector by hand.
Read tutorialWhy every parallel stream in your JVM shares one pool, which sources split well, the N times Q rule for deciding, and why a blocking call inside a parallel stream can stall the whole application.
Read tutorialWhat Optional was designed for and what it was not, the orElse versus orElseGet trap that evaluates the fallback every time, chaining with map and flatMap, and why Optional fields are a mistake.
Read tutorialWhy default methods were added, the three resolution rules when a class inherits conflicting defaults, calling a specific supertype with X.super.method(), and private interface methods.
Read tutorialChoosing between Instant, LocalDateTime and ZonedDateTime, Period versus Duration, what happens at a daylight-saving gap, and how to store timestamps so they survive a zone change.
Read tutorialWhat actually changed after Java 8 and why it matters in an interview: var, records, sealed interfaces, pattern matching for switch, text blocks, the module system and virtual threads.
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