Java interview questions for freshers

A fail-fast iterator, like ArrayList's, throws ConcurrentModificationException if the collection is structurally changed during iteration, except through the iterator itself.

LinkedHashMap keeps insertion order, or access order if you enable it.

HashMap and HashSet place objects into buckets using hashCode, then confirm equality with equals.

A HashMap lookup hashes the search key and only scans that bucket.

Checked exceptions extend Exception but not RuntimeException and must be declared or caught, like IOException.

throw actually raises an exception object from a method body.

final means a variable cannot be reassigned, a method cannot be overridden, or a class cannot be subclassed.

It auto-closes AutoCloseable resources such as streams and JDBC connections when the block exits, even on exception.

You swallow bugs like NullPointerException and hide them behind a generic 500, which makes debugging painful.

A process is an OS-isolated program with its own virtual memory.

The OS pauses one thread or process, saves registers and scheduler state, and resumes another.

A race happens when two threads read and write shared state with no happens-before, so the result depends on timing.

It acquires an intrinsic lock on an object before entering the block or method and releases it on exit.

sleep pauses the current thread for a time and does not release a synchronized lock you already hold.

Thread A holds lock L1 and waits for L2, while thread B holds L2 and waits for L1, so both wait forever.

A primary key uniquely identifies a row, cannot be NULL, and a table has only one.

A foreign key is a column, or set of columns, that must match a unique key in another table.

INNER JOIN returns only rows that match on both sides.

An index is an extra data structure, usually a B-tree, that lets the engine find rows without scanning the whole table.