How Can I Efficiently Query Java Object Collections Using SQL-Like Syntax?

Querying Object Collections with SQL-Like Queries in Java

Overview

Programmers frequently encounter situations where they must query in-memory object collections based on complex criteria resembling SQL or Criteria queries. A typical scenario involves filtering a collection of Car objects to retrieve cars manufactured in the 1960s, with license plates beginning with "AZ," and ordering the results by car model name.

Filtering's Limitations

Filtering, as described in other responses, is a suitable approach but suffers from scalability issues. Its time complexity is O(n t), where n is the collection size and t is the number of tests applied to each object. Performance degrades as more objects are added and/or query complexity increases.

Indexing and Set Theory

An alternative method involves building indexes for fields within objects and utilizing set theory. For example, creating an index on the Car.color field allows fast retrieval of blue cars in O(1) time complexity.

Standing Query Index

Another approach is a "standing query index," where queries are registered with a "smart" collection. As objects are added or removed, the collection evaluates each object against the registered queries. Matching objects are automatically added to or removed from query-specific sets. Subsequently, objects matching any registered query can be retrieved in O(1) time.

CQEngine

CQEngine (Collection Query Engine) embodies these concepts. It provides a NoSQL query engine for retrieving objects from Java collections using SQL-like queries, eliminating the overhead of iterating through the collection. It harnesses indexing and standing query indexes, among other techniques. CQEngine is open-source and available in Maven Central.

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