Java Programming - MCQ Practice Questions

This is vulnerable to SQL injection as user input is directly concatenated. Using PreparedStatement with parameters prevents this: "SELECT * FROM users WHERE id=?"

addBatch() adds SQL command to batch, executeBatch() executes all batched commands at once, improving performance significantly for multiple operations.

execute() returns boolean (true if ResultSet available), executeQuery() returns ResultSet for SELECT, executeUpdate() returns row count for INSERT/UPDATE/DELETE.

CallableStatement is used to invoke stored procedures and functions. It extends PreparedStatement and supports IN, OUT, and INOUT parameters.

TYPE_SCROLL_INSENSITIVE creates scrollable ResultSet (can move forward/backward) but changes made to database after ResultSet creation are not reflected.

update*() methods (like updateInt(), updateString()) modify individual column values in the current row, while updateRow() commits all these changes to the database.

PreparedStatement with parameterized queries is the most secure approach as it separates SQL logic from data. The database treats parameters as data, not executable code, preventing SQL injection.

The default isolation level depends on the specific database and JDBC driver configuration. Different databases have different defaults (MySQL is REPEATABLE_READ, Oracle is READ_COMMITTED).

The wasNull() method checks if the last value retrieved from a CallableStatement was NULL. To get the return value, you typically use getInt(), getString(), etc., and then check wasNull().

executeBatch() returns an int array where each element represents the update count for the corresponding statement. A value of -3 (Statement.EXECUTE_FAILED) indicates failure for that statement.

A SavePoint allows you to roll back part of a transaction without rolling back the entire transaction. You can create nested transactions by setting multiple savepoints and rolling back to specific ones.

Unclosed connections remain allocated, eventually exhausting the connection pool and causing application failures.

Closing a Statement typically invalidates its associated ResultSet in most JDBC drivers, though behavior may vary.

Try-with-resources (try-with-resources statement) automatically closes AutoCloseable resources, preventing memory leaks.

JDBC Connections are not thread-safe. Each thread must have its own Connection to prevent concurrent access issues and data corruption.

Direct concatenation of user input in SQL queries exposes the application to SQL injection attacks. PreparedStatement should be used instead.

When rollback() is called without savepoints, all changes made since the last commit are undone and rolled back.

executeBatch() with addBatch() reduces network round-trips and improves performance for bulk operations.

FetchSize controls how many rows are fetched from the database at once, optimizing performance for large datasets.

JdbcTemplate in Spring Framework automatically manages JDBC resources and is the recommended approach in modern applications.