Java and backend interview questions

Lazy is the safe default for associations so you do not pull an entire object graph on every find.

It is the first-level cache: a map of managed entities for the current unit of work, usually the transaction.

persist inserts a new entity and fails if the id already exists in some setups.

You pass PageRequest.of(page, size, sort) into a repository method that returns Page or Slice.

EntityManager is the JPA API for the persistence context.

It means persist, merge, remove, and refresh cascade to children.

INNER keeps only customers who have orders.

The engine walks a balanced tree of page pointers instead of reading every row.

Functions on the column, implicit type conversion, leading wildcards like LIKE '%foo', or low selectivity can force a sequential scan.

1NF: no repeating mark columns like mark1, mark2

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.