When Do Two Lambdas Behave the Same?

Comparing Lambdas: A Comprehensive Overview

Introduction

Lambdas, also known as anonymous functions or closures, are widely used in modern programming languages for their brevity and expressiveness. However, there may arise scenarios where it becomes necessary to compare two lambdas, either for functional equivalency or identity.

Specification vs. Implementation Perspective

From a language specification perspective, lambda expressions are guaranteed to create an instance of a class that implements the target functional interface. However, the uniqueness or aliasing of these instances is left unspecified, providing flexibility for implementations.

Implementation Considerations

Currently, lambda expression implementation maintains a 1:1 relationship between synthetic classes (generated to implement lambdas) and capture sites in the program. Two lambdas captured at different sites, even if functionally identical, may be implemented using different classes. However, for non-capturing lambdas evaluated at the same capture site, reference equality (==) implies functional equivalence.

Alternative Approaches

• Serialization: By serializing lambdas, one can obtain their state and perform comparisons. However, this approach incurs performance and security overheads.
• Method References: Extending equality definitions for method references is being considered to improve their usability as listeners.

Limitations

Establishing functional equality for lambdas is inherently challenging. Two lambdas may be functionally identical, but if they are not evaluated at the same capture site or converted to different functional interface types, reference equality cannot be guaranteed.

toString Considerations

There have been discussions within the Java Expert Group (EG) regarding providing a more descriptive toString implementation for lambdas. However, no definitive conclusion has been reached, and it remains a topic for future consideration.

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