Java interviews

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Timed assessment

12 practical questions
90 minutes
Mid level

Questions are drawn from a bank of 36. Your report includes a skill breakdown and lessons to practice next.

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Practice interviews

Junior tasks cover the fundamentals. Mid and senior tasks add debugging, changing requirements and review decisions.

Position

10 interviews

Java developer
Junior
22 min

Scaled adjustment total

Moss Media stores signed whole-number adjustments. Return the sum of every adjustment multiplied by multiplier as a long. Each input is an int; the total fits in long. Do not narrow intermediate products.

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Skills and rubricNumeric promotion before aggregation

[50000,-10000,70000] at multiplier 50000 returns 5500000000. The negative item removes 500000000 from the sum.
After each item, total equals the exact sum of the processed signed long products.
An empty array returns 0; [2147483647] at multiplier 2 returns 4294967294.

Java developer
Junior
22 min

Route token validation

Alder Transit uses tokens with one uppercase ASCII letter followed by one to five decimal digits. Return the numeric suffix, allowing leading zeros. Return -1 for any other shape. Inputs are nonnull.

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Skills and rubricGuarded token parsing

R0042 returns 42 because leading zeros don't change the numeric suffix.
Before each digit, value represents exactly the validated numeric suffix prefix.
A0 returns 0, while an empty token, r12, C1x and a six-digit suffix return -1.

Java developer
Junior
22 min

Value-key shipment totals

Birch Fulfillment receives nonnull codes and parallel quantities arrays of equal length. Treat trimmed codes as value keys, keep case significant, and sum quantities by code. Return totals in first-seen key order. All totals fit in int.

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Skills and rubricValue equality and ordered aggregation

Codes [" A ","B","A"] with [2,7,4] return [6,7], retaining A before B.
Each immutable key has one accumulated total, and order records only its first encounter.
Empty arrays return []; x and X remain separate; a space-only code and an empty code merge after trimming.

Java developer
Mid
30 min

Latest accepted revision

Aster Publishing supplies parallel ids, versions and values arrays. For each id keep the value at the highest version; equal versions keep the earliest row. Return values in first-seen id order. Arrays have equal lengths; ids are nonnull.

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Skills and rubricDeterministic revision reconciliation

The saved index is the earliest row with the maximum version seen for that id.
Negative versions compare normally, repeated equal versions keep their first value, and empty input returns [].
O(n) expected time and O(k) extra space for k ids.

Java developer
Mid
30 min

Stable work ordering

Linden Repairs supplies rows [priority, arrival, id]. Return ids ordered by higher priority first, then earlier arrival, then smaller id. Each row has exactly three ints, with priority and arrival in -1000..1000 and unique ids.

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Skills and rubricComparator direction and total ordering

The comparator resolves the first field that differs and returns zero only for equal ordering keys.
Empty input returns []; negative priorities remain valid; equal priority changes the comparison to arrival.
O(n log n) sorting time and O(n) storage for the copied outer array and output, plus sorting workspace.

Java developer
Mid
30 min

Polymorphic handling rules

Brook Parcel applies a handling rule to each nonnegative weight. code 0 returns weight, code 1 returns weight+3, and code 2 returns weight*2. Unknown codes contribute -1. Return all results; codes and weights are parallel arrays of equal length.

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Skills and rubricInterface dispatch and substitutable behavior

Each valid code selects exactly one Rule, whose runtime implementation computes that row's result.
An unknown negative code returns -1, code 2 at weight 0 returns 0, and empty arrays return [].
O(n) time and O(n) returned space; the three rule objects use constant extra storage.

Java developer
Mid
30 min

Bounded reservation policy

Cove Theater has capacity seats. Process requested party sizes in order. A positive party is accepted only if it fits remaining seats; invalid or oversized requests leave state unchanged. Return [accepted parties, occupied seats, rejected parties]. capacity is nonnegative.

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Skills and rubricEncapsulated capacity invariant

After each party, occupancy is between zero and capacity; each request increments exactly one outcome counter.
A party exactly filling capacity succeeds; zero capacity rejects positive parties; an empty request list returns all zeros.
O(n) time and O(1) extra state, with a constant-size returned report.

Java developer
Senior
38 min

Idempotent adjustment ledger

Elm Subscriptions receives parallel event ids and signed deltas. Apply only the first occurrence of each id, even if a later duplicate carries a different delta. Return [final balance, applied event count, duplicate count] as long[]. opening and every total fit in long; ids are nonnull.

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Skills and rubricIdempotent effects and event identity

The balance contains exactly one signed delta per first-seen id, and seen records precisely those applied identities.
O(n) expected time and O(k) stored identities; the state grows with the number of distinct events.
The id claim and balance change must commit together in one transaction, or recovery can permanently skip an effect or apply it twice. A unique id constraint prevents competing claims but doesn't make a separate balance write atomic. I would also decide whether a duplicate with conflicting payload is rejected or audited; the current contract ignores all later payloads. Retaining every id is an unbounded-memory policy that needs a documented durable retention strategy.

Java developer
Senior
38 min

Atomic budget batches

Hawthorn Events starts with nonnegative opening credits. Each batch is an ordered array of signed adjustments. Accept a batch only if every running balance within it stays nonnegative. A rejected batch leaves the balance unchanged. Return [final balance, committed batches, rejected batches]. Arithmetic fits in int.

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Skills and rubricTransactional validation and commit

Before a batch starts, balance contains only committed adjustments; tentative state is discarded on any invalid prefix.
O(total visited adjustments) time and O(1) extra state, without copying all batches.
No. An external side effect isn't undone by discarding local state. I would validate before sending and, for a durable service, commit state with an outbox record in one database transaction. A separate dispatcher can retry delivery with an idempotency key. That provides a recoverable workflow rather than claiming one atomic transaction spans an arbitrary external service.

Java developer
Senior
38 min

Defensive snapshot boundary

Maple Telemetry builds a Snapshot from values. Then add sourceDelta to every element of the caller's original array. Read an exported snapshot array and add exportDelta to each exported element. Return [stored snapshot, changed export]. The stored snapshot must retain the original values; totals fit in int.

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Skills and rubricDefensive copying across ownership boundaries

The private snapshot array is never exposed and preserves the values present at construction.
O(n) time and O(n) memory per copy; the returned evidence contains two independent rows.
No. Array clone copies element references, so callers can still mutate the shared objects. I need immutable element types or element copies deep enough to cross the ownership boundary. If the objects share cycles or identity relationships, the copy policy must preserve that graph explicitly. A final array field prevents reassignment of the field, not mutation of its elements.

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Java interviews

Timed assessment

Practice interviews

Scaled adjustment total

Route token validation

Value-key shipment totals

Latest accepted revision

Stable work ordering

Polymorphic handling rules

Bounded reservation policy

Idempotent adjustment ledger

Atomic budget batches

Defensive snapshot boundary

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