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Aporeon

Technical interview practice, from one developer to another.

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Concept questionMedium~7 min

How do collectors support grouping and aggregation in Java streams?

Explains collectors as terminal reduction strategies and composes grouping, mapping, counting, and summary operations.

Concept questionMedium~7 min

How does a Java Stream pipeline execute?

Explains stream sources, lazy intermediate operations, terminal execution, short-circuiting, encounter order, and single use.

Java

Practice drill

Concept questionMedium~7 min

How does HashMap lookup work in Java?

Explains Java HashMap lookup through hashing, buckets, equality, collisions, resizing, and stable keys without implementation trivia.

Concept questionMedium~7 min

How do Java wildcards and PECS guide generic API design?

Applies upper and lower bounded wildcards to producer and consumer APIs while explaining their limits.

Comparison questionEasy~5 min

How do you choose between List, Set, and Map in Java?

Chooses a Java collection by the behavior the code needs: ordering, duplicates, membership, lookup keys, and iteration.

Debugging scenarioMedium~7 min

How should Collectors.toMap handle duplicate keys?

Makes duplicate-key behavior explicit through validation, merge policies, grouping, and deterministic map selection.

Concept questionMedium~7 min

How should Optional be used in Java, and what mistakes should you avoid?

Uses Optional to model absence at return boundaries and avoids eager defaults, null Optionals, unsafe get calls, and awkward field or parameter usage.

Concept questionEasy~5 min

What is a functional interface, and how do Java lambdas use it?

Explains Java lambdas as implementations of single-abstract-method contracts and connects standard interfaces to API design.

Comparison questionEasy~5 min

What is the difference between Comparable and Comparator in Java?

Distinguishes a type's natural order from external, reusable sort strategies and connects comparison consistency to sorted collections.

Comparison questionMedium~7 min

What is the difference between immutable and unmodifiable collections in Java?

Separates an unmodifiable API surface from a stable snapshot and from deep immutability of contained objects.

Comparison questionMedium~7 min

What is the difference between map and flatMap in Java streams?

Distinguishes one-to-one transformation from transformation plus flattening for nested collections, streams, and optionals.

Concept questionMedium~7 min

What is type erasure in Java, and what are its practical consequences?

Explains how Java compiles generic code and connects erasure to runtime checks, reflection limits, overloads, arrays, and bridge methods.

Comparison questionMedium~7 min

When are Java streams clearer than loops, and when are they not?

Chooses streams or loops by transformation shape, control flow, side effects, debuggability, and measured performance.

Concept questionMedium~7 min

When do you use bounded type parameters in Java?

Uses bounds to express capabilities required by generic code and distinguishes them from wildcard flexibility at call sites.

Concept questionHard~10 min

When should you use a parallel stream in Java?

Evaluates parallel streams through workload size, splittability, CPU cost, ordering, common-pool contention, state, and measurement.

Comparison questionMedium~7 min

When would you choose ArrayList or LinkedList in Java?

Compares Java list implementations using actual access patterns, traversal cost, memory locality, and insertion location.

Comparison questionMedium~7 min

When would you use HashMap, LinkedHashMap, or TreeMap?

Chooses among common Java map implementations by lookup behavior, iteration order, sorting, range operations, and key contracts.

Comparison questionMedium~7 min

Why are Java arrays covariant while generics are invariant?

Contrasts runtime-checked array covariance with compile-time-safe generic invariance and explains why generic arrays are restricted.

Concept questionMedium~7 min

Why are raw types and unchecked warnings risky in Java?

Treats raw types as a legacy compatibility boundary that can introduce heap pollution and delayed runtime failures.

Concept questionMedium~7 min

Why are side effects risky in Java stream operations?

Explains interference, stateful lambdas, execution-order assumptions, safe collection, and visibility of unavoidable effects.

Debugging scenarioMedium~7 min

Why does ConcurrentModificationException happen, and how do you modify a collection safely?

Explains fail-fast iteration, safe removal patterns, structural modification, and why the exception is not a concurrency guarantee.

Concept questionEasy~5 min

Why does Java use generics?

Explains generics as compile-time type-safe reuse that improves API contracts and removes scattered casts.

Concept questionMedium~7 min

Why is List<Integer> not a subtype of List<Number> in Java?

Explains Java generic invariance through the unsafe write that subtype assignment would otherwise permit.

Concept questionMedium~7 min

Why must local variables captured by a Java lambda be effectively final?

Explains Java lambda capture through copied local values, mutable referenced objects, lifetime, and safe state handling.