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Aporeon

Technical interview practice, from one developer to another.

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Java & Spring Backend

Java Language & Object Model

Java execution, values and references, identity and equality, object-oriented design, modern data types, exceptions, and resource ownership.

24 drillsPractice one at a timeOrdered pathConcept questionComparison questionArchitecture scenario

Recommended start

From source to JVM

Explains the path from Java source to bytecode and running code, including class loading and the JIT without turning the answer into JVM trivia.

Easy~5 minStep 1 of 24

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1From source to JVMRecommended startExplains the path from Java source to bytecode and running code, including class loading and the JIT without turning the answer into JVM trivia.Concept questionEasy~5 minJavaPractice 2What is OOP?Defines OOP directly and connects Java objects, responsibilities, state, behavior, and collaboration to a practical design.

Practice order

24 drillsOne at a time

1From source to JVMUp next / Easy / ~5 min
2What is OOP?Easy / ~5 min
3Class vs objectEasy / ~5 min
4

Concept questionEasy~5 minJava

Practice

3Class vs objectDistinguishes a Java type definition from a concrete runtime instance and keeps identity, state, and polymorphism tied to the question.Comparison questionEasy~5 minJavaPractice

4Primitive vs referenceExplains Java primitives and references through observable language behavior without relying on an inaccurate stack-versus-heap shortcut.Comparison questionEasy~5 minJavaPractice

5Java parameter passingResolves Java's pass-by-value confusion by separating mutation through a copied reference from reassignment of the caller's variable.Comparison questionMedium~7 minJavaPractice

6String immutabilitySeparates String immutability from interning and connects both to safe sharing, equality, concatenation, and practical performance.Concept questionMedium~7 minJavaPractice

7Identity vs equalityDistinguishes reference identity from logical equality and shows how nulls, strings, enums, and domain values affect the choice.Comparison questionEasy~5 minJavaPractice

8equals and hashCodeConnects Java's equality and hash contracts to real HashMap and HashSet behavior, including mutable-key failures.Concept questionMedium~7 minJavaPractice

9Encapsulation and invariantsDefines encapsulation as control over state and valid transitions, then connects it to defensive copying and persistence boundaries.Concept questionMedium~7 minJavaPractice

10What is abstraction?Defines abstraction as a deliberate contract over essential behavior and keeps Java mechanisms secondary to the design purpose.Concept questionEasy~5 minJavaPractice

11Abstraction vs encapsulationContrasts abstraction's caller-facing contract with encapsulation's control of internal state and shows how one object can use both.Comparison questionMedium~7 minJavaPractice

12Interface vs abstract classCompares Java interfaces and abstract classes through contract, shared state, implementation reuse, and evolution trade-offs.Comparison questionMedium~7 minJavaPractice

13Inheritance and substitutabilityFrames inheritance as a behavioral is-a relationship whose subtypes must remain valid substitutes, not merely a code-reuse shortcut.Concept questionMedium~7 minJavaPractice

14Composition over inheritanceExplains how delegation preserves replaceable behavior and narrower coupling while recognizing valid inheritance cases.Comparison questionMedium~7 minJavaPractice

15Overload vs overrideSeparates compile-time overload resolution from runtime overridden-method dispatch and briefly bounds static hiding.Comparison questionMedium~7 minJavaPractice

16Java access boundariesExplains private, package-private, protected, and public access through design intent rather than a visibility table alone.Concept questionMedium~7 minJavaPractice

17Static vs instanceConnects static ownership to constants and stateless operations while exposing the risks of global mutable state and initialization.Comparison questionMedium~7 minJavaPractice

18The final modifierSeparates final assignment, method overriding, and class inheritance while making clear that a final reference does not make an object immutable.Comparison questionMedium~7 minJavaPractice

19Immutable class designBuilds an immutable Java type through valid construction, final state, defensive copying, controlled exposure, and safe sharing.Architecture scenarioMedium~7 minJavaPractice

20Record vs classPositions records as transparent, shallowly immutable data carriers with generated value-oriented members and clear modeling limits.Comparison questionMedium~7 minJavaPractice

21Sealed type hierarchiesUses sealed types for deliberately closed variants and connects them to exhaustive reasoning, evolution, and open extension boundaries.Concept questionMedium~7 minJavaPractice

22Checked vs uncheckedExplains Java's catch-or-declare rule and turns exception choice into a recoverability and API-contract decision rather than a universal slogan.Comparison questionMedium~7 minJavaPractice

23Exception boundariesPlaces exception handling at boundaries that can recover, add meaningful context, or translate abstraction levels while preserving the cause.Architecture scenarioHard~10 minJavaPractice

24Try-with-resourcesExplains deterministic cleanup through AutoCloseable, reverse close order, suppressed exceptions, and clear ownership.Concept questionMedium~7 minJavaPractice

Primitive vs reference

Easy / ~5 min

5Java parameter passingMedium / ~7 min

6String immutabilityMedium / ~7 min

7Identity vs equalityEasy / ~5 min

8equals and hashCodeMedium / ~7 min

+16 more drills in this section

Java Language & Object Model - Java & Spring Backend | Aporeon