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Java Module 5

Classes, objects, records and enums interview questions

Interview questions on classes, objects, records and enums: warm-ups, then what each harder question tests and how a good answer starts.

30 questions: 10 warm-ups and 20 with model answers · Levels: Fresher to senior

Warm-up questions

Short questions with short answers, each answered in full.

  1. Fresher

    What does Java do when a class declares no constructor at all?

    Answered in the lesson Classes, objects, fields and constructors

  2. Fresher

    What does a compact source file import automatically, and what must you add when you turn it into a named class?

    Answered in the lesson Growing a compact file into classes and files

  3. Fresher

    What is the difference between package-private and protected access in Java?

    Answered in the lesson Encapsulation and access modifiers

  4. Fresher

    Can a static method use an instance field? Why or why not?

    Answered in the lesson Static members and initialisation order

  5. Fresher

    What is the class path, and what is it when you set nothing?

    Answered in the lesson Packages, imports and the class path

  6. Fresher

    What is the contract between equals and hashCode in Java?

    Answered in the lesson Java equals, hashCode and toString contracts

  7. Fresher

    What makes a Java class immutable?

    Answered in the lesson Immutable classes and defensive copies in Java

  8. Fresher

    Why can you compare Java enum constants with == instead of equals?

    Answered in the lesson Java enums with fields, methods and EnumMap

  9. Fresher

    What does the compiler generate for a Java record?

    Answered in the lesson Java records for immutable data

  10. Fresher

    What is the difference between a static nested class and an inner class in Java?

    Answered in the lesson Java nested, inner, local and anonymous classes

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

  1. Fresher to mid

    A class Account declares public void Account(String id, long balance) and assigns both fields in it. New code calling new Account("A1", 500) does not compile, and old code calling new Account() gets an account whose id is null. Explain what the compiler saw, and how you would stop this mistake reaching the main branch.

    What it tests
    Telling a constructor from a method, the default-constructor rule, reading a compiler message, and turning a silent mistake into a compile error or a failing check.
    First step
    Point at the void. A constructor has no return type, so void Account(...) is an ordinary method that happens to share the class's name. The class therefore declares no constructor, and the compiler added the default one, which explains both symptoms.
    Follow-ups

    From the lesson Classes, objects, fields and constructors

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    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  2. Fresher to mid

    Design the constructors of a Booking class: a passenger name and a seat count from 1 to 6 are required, and a coach preference is optional. How many constructors do you write, where does the validation live, and what can callers never create?

    What it tests
    Delegating with this(...), keeping every check in one constructor, class invariants, defaults that are values rather than null, and knowing when overloaded constructors stop scaling.
    First step
    Write one full constructor that takes every field and checks every rule, and make each shorter constructor delegate to it with this(...), passing a named default for what the caller left out. Then no way of creating a Booking can skip a check.
    Follow-ups

    From the lesson Classes, objects, fields and constructors

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  3. Fresher to mid

    A compact source file that began as a quick script is now 400 lines with six nested records and classes, and a second program needs its fare rules. What do you change first, and in what order, so that both programs can share the rules without changing what the script prints?

    What it tests
    What the implicit class of a compact file allows, moving nested types into top-level files, proving behaviour did not change, and choosing the moment for packages and a build tool.
    First step
    Nothing outside the file can name its implicit class, so the rules cannot be shared as they are. Record the script's output for a few fixed inputs first, then wrap the members in a named class and check the output is identical before moving anything else.
    Follow-ups

    From the lesson Growing a compact file into classes and files

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  4. Fresher to mid

    java Main.java works on your laptop, but the CI job, which compiles every .java file in the folder with javac, fails with errors in a file the program never uses. Why do the two disagree, and what do you change so that they agree?

    What it tests
    How the multi-file source launcher chooses files, how javac chooses them, and keeping local runs and CI builds consistent on purpose.
    First step
    The launcher compiles the file you name and then only the files whose classes the program uses, found by name. javac compiles every file the shell gives it, so an unused broken file only ever matters to javac.
    Follow-ups

    From the lesson Growing a compact file into classes and files

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  5. Mid to senior

    A BankAccount class exposes getBalance() and setBalance(long), and code across the service does account.setBalance(account.getBalance() - amount). What can go wrong, and how do you redesign the class and move the callers over without a big-bang change?

    What it tests
    Invariants owned by the class, lost updates from read-modify-write, the audit trail an operation gives, and migrating a widely used API safely.
    First step
    Name the rule the setter cannot protect: the balance never goes below zero and never changes without a reason. Each caller now re-implements that rule, so one forgotten check, or two callers at the same moment, is enough to break it.
    Follow-ups

    From the lesson Encapsulation and access modifiers

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  6. Mid to senior

    Your team ships a library. A helper class is public only because two of your own packages need it, and other teams have started calling it directly. How do you hide it with Java's access levels, and when do you need the module system as well?

    What it tests
    What package access can and cannot do, designing packages around access, and module exports, including qualified exports and reflection.
    First step
    Ask why two packages need it. If they can become one package, the helper becomes package-private and the problem is gone. If not, public is the only access level that crosses packages, so only a module can stop outside code.
    Follow-ups

    From the lesson Encapsulation and access modifiers

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  7. Mid to senior

    A class keeps a static HashMap that caches results by their input. In production, memory grows until the service is restarted; in tests, results from one test leak into the next. Explain both symptoms, and redesign the cache.

    What it tests
    The lifetime of static state, unbounded caches, thread safety of a shared map, test isolation, and giving a cache an owner, a size limit and an eviction rule.
    First step
    A static field lives as long as its class, which in a service means as long as the JVM. Nothing ever removes an entry, so the map grows with every new input, and every test in the same run shares the same map.
    Follow-ups

    From the lesson Static members and initialisation order

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  8. Mid to senior

    A base class used by four teams calls an overridable init() hook from its constructor. One team's subclass reads its own endpoint field in init() and sends it to a server, which receives null. Explain the failure, and redesign the base class without breaking the other three subclasses.

    What it tests
    The order of construction across a class hierarchy, dynamic dispatch while an object is being built, and evolving a base class that other teams extend.
    First step
    Follow the creation order: the subclass constructor begins with super(), so the base constructor runs before the subclass's field initialisers. Its call to init() is dispatched to the override, which reads the field while it still holds its default value, null.
    Follow-ups

    From the lesson Static members and initialisation order

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  9. Fresher to mid

    Your program compiles, but at run time it stops with NoClassDefFoundError for a class from a library. What is different between compile time and run time, and how do you find and fix the cause?

    What it tests
    How javac and the JVM each find classes, telling NoClassDefFoundError from ClassNotFoundException, and checking the class path a program really runs with.
    First step
    Compile time and run time use separate class paths. javac found the library through its own -cp, but the java command that runs the program does not list that JAR, or lists another version of it. Look at the exact run command before changing any code.
    Follow-ups

    From the lesson Packages, imports and the class path

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  10. Mid to senior

    Two JAR files on the class path both contain com.acme.util.Strings, in different versions. What does the JVM do, how would you notice, and how do you fix it?

    What it tests
    The search order of the class path, the errors a version clash produces, finding which JAR a class came from, and resolving conflicting dependencies.
    First step
    The class path is searched in order and the first Strings found is loaded; the other copy is never used and nothing warns about it. The symptom appears later, as an error such as NoSuchMethodError where code expects the other version.
    Follow-ups

    From the lesson Packages, imports and the class path

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  11. Mid to senior

    A service keeps Order objects in a HashSet to drop duplicates. The set keeps growing, and contains() returns false for orders you can see in it when you print the set. What could cause this, and how do you find out which cause it is?

    What it tests
    How hash collections use hashCode and equals, telling an overload, a missing hashCode and a changed key apart by their symptoms, and fixing the class instead of its callers.
    First step
    Name the three causes that fit: equals(Order) overloading equals instead of overriding it, equals overridden without hashCode, and a field read by hashCode that changes after the order is added. Then read Order for @Override, for a hashCode method and for setters on the fields that equals compares.
    Follow-ups

    From the lesson Java equals, hashCode and toString contracts

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  12. Mid to senior

    A key class returns 42 from hashCode "to be safe". Every answer is still right, but a cache keyed by it got slower and slower as it grew. Explain why, estimate the work for 10,000 lookups, and say what changes if the key class implements Comparable.

    What it tests
    The difference between a legal and a useful hash code, how HashMap buckets and tree bins behave under total collision, and estimating cost from the number of keys.
    First step
    The contract only demands equal hash codes for equal objects, so 42 is legal. But every key now lands in one bucket, and a lookup has to compare its key with the entries there one by one, so the cost of one lookup grows with the size of the cache instead of staying constant.
    Follow-ups

    From the lesson Java equals, hashCode and toString contracts

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  13. Fresher to mid

    In a code review, a Config class has private final Map<String, String> settings, assigned from the constructor argument, and a getSettings() that returns the field. The author says it is immutable because the field is final and there are no setters. Do you agree? What would you change, and what does each change cost?

    What it tests
    Telling a final reference from an immutable object, defensive copies on the way in and out, views against copies, and weighing copy cost against safety.
    First step
    Disagree: final only fixes which map the field refers to. The caller who passed the map can still change it, and so can every caller of getSettings(), so the object changes without a setter. Show it with a three-line test before proposing the fix.
    Follow-ups

    From the lesson Immutable classes and defensive copies in Java

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  14. Mid to senior

    A pricing service builds a new immutable PriceTable every minute and stores it in a plain static field; request threads read the field without locks. What exactly does the Java memory model guarantee the readers, what does it not guarantee, and what would you change?

    What it tests
    Final-field semantics, the difference between the safety of an object and the visibility of the reference to it, and safe publication with volatile or an atomic reference.
    First step
    Separate the two questions. Because PriceTable is immutable with final fields, a reader that sees a reference to it sees a fully built table. But the plain field gives no promise about when, or whether, a reader sees the newest reference at all.
    Follow-ups

    From the lesson Immutable classes and defensive copies in Java

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  15. Fresher to mid

    An orders table stores the status as the enum's ordinal(). A developer adds ON_HOLD between PLACED and PACKED and deploys. What breaks, how would you repair the existing rows, and what should the table store instead?

    What it tests
    What ordinal() is, why persisted values must not depend on declaration order, planning a data migration, and choosing a stable representation.
    First step
    ordinal() is the constant's position. Inserting ON_HOLD at position 1 moves PACKED, SHIPPED and the rest up by one, so every stored row from position 1 on is now read as the state before it, with no error anywhere.
    Follow-ups

    From the lesson Java enums with fields, methods and EnumMap

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  16. Mid to senior

    A shared library adds RETURNED to its public Status enum in a minor release. Services that upgraded the library without recompiling start failing with MatchException in code that has not changed. Explain the failure, and design how the library and its users should handle new constants.

    What it tests
    Exhaustive switch expressions, separate compilation and binary compatibility, and evolving a published enum without surprising its users.
    First step
    The services' switch expressions were checked as exhaustive against the old four constants. Code compiled then is not checked again, so when RETURNED reaches such a switch at run time no label applies, and the specification says that throws MatchException.
    Follow-ups

    From the lesson Java enums with fields, methods and EnumMap

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  17. Mid to senior

    Your team wants to turn a widely used value class, with getters and hand-written equals and hashCode, into a record. Which behaviours can change for existing callers, and how would you check them before merging?

    What it tests
    Exactly what a record generates, which of those members differ from a typical hand-written class, and planning a change that many callers and stored data depend on.
    First step
    List what the record will generate and compare it member by member with the class: accessor names, which fields equals and hashCode read, the toString format, finality, constructors and setters, and how the object is serialized. Each difference is a possible break for somebody.
    Follow-ups

    From the lesson Java records for immutable data

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  18. Mid to senior

    record Percentage(double value) rejects values below 0 or above 100 in its compact constructor. Tests pass, but in production a HashSet holds both Percentage[value=0.0] and Percentage[value=-0.0], and a NaN slipped through validation. Explain both, and fix the record.

    What it tests
    How a record compares primitive components, IEEE 754 signed zero and NaN, why comparisons with NaN are always false, and validating floating-point input.
    First step
    Look at the two comparisons separately. value < 0 is false for -0.0 and for NaN, so both pass the check. The generated equals compares a double component with Double.compare, which treats -0.0 and 0.0 as different, so the set keeps both.
    Follow-ups

    From the lesson Java records for immutable data

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

  19. Mid to senior

    A desktop app's screens each register an inner-class listener with an event bus that lives as long as the app. After an hour of opening and closing screens, a heap dump shows hundreds of closed screens still in memory. Explain the leak, how you would confirm it, and two ways to fix it.

    What it tests
    The hidden reference from an inner object to its outer object, reading a reference chain in a heap dump, and fixing a leak through lifecycle or through the type of reference.
    First step
    Follow the chain from a garbage-collection root: the long-lived bus holds the listener, the listener is an inner object whose hidden field this$0 refers to its screen, and the screen holds its whole view tree. Nothing ever unregisters the listener, so every closed screen stays reachable.
    Follow-ups

    From the lesson Java nested, inner, local and anonymous classes

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  20. Fresher to mid

    You are writing a RingBuffer class and its iterator. Would you make the iterator an inner class or a static nested class, and how would you make it fail fast when the buffer changes while someone is iterating?

    What it tests
    When a nested class needs its enclosing object, iterator state per iteration, and the fail-fast pattern with a modification counter and ConcurrentModificationException.
    First step
    The iterator needs the buffer's array, head and size, and every iterator needs its own position, so an inner class that reads the outer fields and holds only its position is the natural fit. For fail-fast behaviour, count structural changes in the buffer and compare.
    Follow-ups

    From the lesson Java nested, inner, local and anonymous classes

    In the answer:

    • Model answer
    • Follow-up answers
    • Common mistakes
    • Rubric
    • Variants
    • about 3 minutes

All Java interview questions · The Java track