immutables

Immutables is a term used in programming to describe data that cannot be changed after it is created. An immutable value object is typically stateful only at construction time; after creation, its fields remain fixed and any modification results in a new object. Immutability is a central concept in functional programming and is valued for enabling referential transparency, safer concurrent execution, and easier reasoning about code.

Key characteristics include final or read-only fields, the absence of mutator methods, and constructors or builders that produce fully initialized instances. To modify data, programmers create a new instance with the desired changes. Immutable structures often rely on techniques like persistent data structures or structural sharing to minimize performance costs associated with copying.

Benefits of immutability include simpler reasoning about program behavior, improved thread safety, easier testing, and fewer side effects. Trade-offs can involve higher memory allocation, potential performance impacts in tight loops, and the need for patterns such as builders or copy methods to express changes.

Immutables can also refer to a Java library that generates immutable value types. The Immutables library uses annotation processing to produce concrete, immutable implementations from abstract definitions. Developers annotate a class with an immutable annotation, and the tool generates a final, thread-safe implementation with a builder, fluent accessors, and proper equals, hashCode, and toString methods. It supports optional fields, default values, and integration with serialization frameworks like Jackson. This library is widely used in Java projects to create robust, immutable data carriers and DTOs with compile-time guarantees of immutability.