Java interview questions and answers

Normalization organises tables to reduce duplicate data and update anomalies.

WHERE filters rows before grouping.

DELETE removes rows and can use a WHERE clause

NULL means unknown or missing, not the integer zero or empty string.

UNION concatenates result sets and then removes duplicates, which requires a sort or hash.

A transaction is a group of SQL statements that must all commit or all roll back, so you never leave half-updated data.

REST is an architectural style for APIs over HTTP where you manipulate resources with standard verbs and URLs.

GET is safe and idempotent: it reads a resource and must not change server state beyond logging.

PUT replaces the whole resource at that URL and is idempotent.

200 OK means the request succeeded and usually includes a body.

400 is a bad request: malformed JSON or failed validation.

500 Internal Server Error means your code or an unexpected dependency failed.

A path parameter identifies a resource, like /users/17.

Repeating the same GET must not change resource state

Authentication proves who you are, for example login with password or OTP.

After login the server stores session data in memory or Redis and sends a session id in a Set-Cookie header.

A JSON Web Token is a signed string with a header, payload, and signature, often used as a bearer access token.

Hashing is one-way: even the server should not be able to recover the original password.

Spring Boot is opinionated Spring: starters, auto-configuration, and an embedded Tomcat so you can ship a fat JAR.

A bean is an object whose lifecycle Spring manages: create, inject dependencies, destroy.