Java and backend interview questions

Atomicity: all statements commit or none do.

Read uncommitted can see dirty data.

A covering index includes every column the query needs, so the engine never visits the heap or clustered table.

DISTINCT removes duplicate result rows.

It is a subquery that references columns from the outer query, so it may run once per outer row.

When a read path must join five tables on every product card and p99 latency is too high.

Synchronizing on this or on a public class object lets strangers lock the same monitor and deadlock you.

volatile makes reads and writes to that field visible across threads and prevents some instruction reordering around that field.

The consumer waits on the lock while the queue is empty, releasing the monitor.

synchronized is simpler and sufficient for most monitors.

Unbounded new Thread() under load will exhaust memory and file descriptors.

Runnable.run returns void and cannot throw checked exceptions.

Collections.synchronizedMap locks the entire map on every call.

It is the Java Memory Model rule that says memory writes by thread A become visible to thread B in a defined way.

PUT of the same body to the same URL should leave the resource in the same state no matter how many times you retry.

The client sends a unique Idempotency-Key

OFFSET skips N rows

Create: 201 with Location, or 200 if the body includes the resource.

Almost all expect versioned JSON APIs, nouns as URLs, and correct verbs and statuses.

The key identifies a client intent.