When to Choose `Function.identity()` Over `t -> t` in Java 8 Lambdas?

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Function.identity() vs. t->t in Java 8 Lambdas

In Java 8, the Function.identity() method is a static factory method that returns an identity function. This means that a function, when applied to any argument, returns the argument itself.

Now, the question arises: why would one use Function.identity() instead of the more readable lambda expression, t->t? While both options achieve the same functionality, there are subtle differences between them.

Identity vs. New Instance

In the current JRE implementation, each occurrence of t->t creates a distinct lambda implementation class. On the other hand, Function.identity() always returns the same instance. This distinction can have implications in memory usage, especially if identity functions are used frequently.

Debugging Considerations

When debug information is enabled, the synthetic method generated for t->t lambda expressions contains a line debug attribute that points to the source code where the lambda is defined. This can aid in debugging during an operation, as it allows developers to trace the origin of a particular Function instance. In contrast, debugging Function.identity() instances during an operation provides no such source code information.

Choice of Usage

While Function.identity() may offer slight performance benefits, its usage should not solely be driven by memory optimization. The readability of code is paramount. If the developer believes that t->t enhances code clarity, then it should be preferred over Function.identity().

Conclusion

Function.identity() and t->t both serve as viable options for defining identity functions in Java 8. While each has its unique characteristics, their choice depends on the specific requirements of the code and the developer's preferences.

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