HSQLDB (HyperSQL Database) offers several key features that make it a popular choice for developers working with Java applications. Here are some of its notable features:

Embedded and Server Modes : HSQLDB can operate in both embedded mode, where it runs within the same JVM as the application, and server mode, where it operates as a standalone database server accessible over a network. This flexibility allows developers to choose the deployment mode that best fits their application architecture.

In-Memory and Persistent Storage : HSQLDB supports both in-memory and persistent storage options for data tables. In-memory tables reside entirely in RAM, providing extremely fast read and write access, while persistent tables are stored on disk, ensuring data durability and persistence across database restarts.

Transactional Support : HSQLDB provides full support for transactions, ensuring data consistency and integrity. It adheres to the ACID (Atomicity, Consistency, Isolation, Durability) properties, allowing developers to perform complex operations with confidence.

SQL Compatibility : HSQLDB supports a wide range of SQL standards, making it compatible with existing SQL-based applications and tools. It implements most SQL-92 and SQL:2008 features, along with additional extensions and optimizations for improved performance and usability.
Cross-Platform : Being written in Java, HSQLDB is platform-independent and can run on any platform that supports the Java Virtual Machine (JVM). This makes it a versatile choice for developing applications across different operating systems and environments.

High Performance : Despite its small footprint, HSQLDB offers high-performance capabilities, especially for read-heavy workloads and in-memory operations. It employs various optimization techniques, such as caching, indexing, and query optimization, to deliver efficient data processing and retrieval.

Ease of Use : HSQLDB is designed to be easy to set up, configure, and use. Its simple deployment model, along with comprehensive documentation and user-friendly tools like HSQLDB Manager, simplifies the development and management of databases for developers of all skill levels.

Scalability : While HSQLDB is well-suited for small to medium-sized applications, it also offers scalability features for handling larger datasets and higher loads. By deploying it in server mode and leveraging clustering and replication capabilities, developers can scale their database infrastructure to meet growing demands.