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Java & Spring Backend

Java Collections, Generics & Streams

Collection choice, hashing, generics, wildcards, lambdas, stream execution, Optional, and practical data transformation.

24 drillsPractice one at a timeOrdered pathComparison questionConcept questionDebugging scenario

Recommended start

List vs Set vs Map

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

Easy~5 minStep 1 of 24

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24 of 24 shown

1List vs Set vs MapRecommended startChooses a Java collection by the behavior the code needs: ordering, duplicates, membership, lookup keys, and iteration.Comparison questionEasy~5 minJavaPractice 2ArrayList vs LinkedListCompares Java list implementations using actual access patterns, traversal cost, memory locality, and insertion location.

Practice order

24 drillsOne at a time

1List vs Set vs MapUp next / Easy / ~5 min
2ArrayList vs LinkedListMedium / ~7 min
3How HashMap worksMedium / ~7 min
4

Comparison questionMedium~7 minJava

Practice

3How HashMap worksExplains Java HashMap lookup through hashing, buckets, equality, collisions, resizing, and stable keys without implementation trivia.Concept questionMedium~7 minJavaPractice

4Java map implementationsChooses among common Java map implementations by lookup behavior, iteration order, sorting, range operations, and key contracts.Comparison questionMedium~7 minJavaPractice

5Immutable vs unmodifiableSeparates an unmodifiable API surface from a stable snapshot and from deep immutability of contained objects.Comparison questionMedium~7 minJavaPractice

6Safe collection iterationExplains fail-fast iteration, safe removal patterns, structural modification, and why the exception is not a concurrency guarantee.Debugging scenarioMedium~7 minJavaPractice

7Comparable vs ComparatorDistinguishes a type's natural order from external, reusable sort strategies and connects comparison consistency to sorted collections.Comparison questionEasy~5 minJavaPractice

8Why use generics?Explains generics as compile-time type-safe reuse that improves API contracts and removes scattered casts.Concept questionEasy~5 minJavaPractice

9Generic invarianceExplains Java generic invariance through the unsafe write that subtype assignment would otherwise permit.Concept questionMedium~7 minJavaPractice

10Bounded type parametersUses bounds to express capabilities required by generic code and distinguishes them from wildcard flexibility at call sites.Concept questionMedium~7 minJavaPractice

11Wildcards and PECSApplies upper and lower bounded wildcards to producer and consumer APIs while explaining their limits.Concept questionMedium~7 minJavaPractice

12Type erasureExplains how Java compiles generic code and connects erasure to runtime checks, reflection limits, overloads, arrays, and bridge methods.Concept questionMedium~7 minJavaPractice

13Raw types and warningsTreats raw types as a legacy compatibility boundary that can introduce heap pollution and delayed runtime failures.Concept questionMedium~7 minJavaPractice

14Arrays vs generics varianceContrasts runtime-checked array covariance with compile-time-safe generic invariance and explains why generic arrays are restricted.Comparison questionMedium~7 minJavaPractice

15Functional interfacesExplains Java lambdas as implementations of single-abstract-method contracts and connects standard interfaces to API design.Concept questionEasy~5 minJavaPractice

16Lambda capturesExplains Java lambda capture through copied local values, mutable referenced objects, lifetime, and safe state handling.Concept questionMedium~7 minJavaPractice

17Stream executionExplains stream sources, lazy intermediate operations, terminal execution, short-circuiting, encounter order, and single use.Concept questionMedium~7 minJavaPractice

18map vs flatMapDistinguishes one-to-one transformation from transformation plus flattening for nested collections, streams, and optionals.Comparison questionMedium~7 minJavaPractice

19Streams vs loopsChooses streams or loops by transformation shape, control flow, side effects, debuggability, and measured performance.Comparison questionMedium~7 minJavaPractice

20Grouping and collectorsExplains collectors as terminal reduction strategies and composes grouping, mapping, counting, and summary operations.Concept questionMedium~7 minJavaPractice

21toMap duplicate keysMakes duplicate-key behavior explicit through validation, merge policies, grouping, and deterministic map selection.Debugging scenarioMedium~7 minJavaPractice

22Stream side effectsExplains interference, stateful lambdas, execution-order assumptions, safe collection, and visibility of unavoidable effects.Concept questionMedium~7 minJavaPractice

23Parallel streamsEvaluates parallel streams through workload size, splittability, CPU cost, ordering, common-pool contention, state, and measurement.Concept questionHard~10 minJavaPractice

24Using Optional wellUses Optional to model absence at return boundaries and avoids eager defaults, null Optionals, unsafe get calls, and awkward field or parameter usage.Concept questionMedium~7 minJavaPractice

Java map implementations

Medium / ~7 min

5Immutable vs unmodifiableMedium / ~7 min

6Safe collection iterationMedium / ~7 min

7Comparable vs ComparatorEasy / ~5 min

8Why use generics?Easy / ~5 min

+16 more drills in this section

Java Collections, Generics & Streams - Java & Spring Backend | Aporeon