Should You Use `Function.identity()` or Lambda Expressions in Java 8?

What is the difference between using `Function.identity()` and lambda expressions like `str -> str` in Java 8 Stream API?

Arrays.asList("a", "b", "c")
          .stream()
          .map(Function.identity()) // Using Function.identity()
          .map(str -> str)          // Using lambda expression
          .collect(Collectors.toMap(
                       Function.identity(), // Using Function.identity()
                       str -> str));        // Using lambda expression

In Java 8, you can use both `Function.identity()` and lambda expressions like `str -> str` for operations within the Stream API. While both approaches can yield the same functional outcomes, there are nuanced differences primarily in terms of performance, readability, and expressiveness.

Arrays.asList("a", "b", "c")
          .stream()
          .map(Function.identity()) // Using Function.identity()
          .collect(Collectors.toMap(
                       Function.identity(),
                       str -> str)); // Using lambda expression

// Example showing performance equivalence
long count = Arrays.asList("a", "b", "c").stream()
                   .map(Function.identity())
                   .count();

// Performance testing comparison
long countLambda = Arrays.asList("a", "b", "c").stream()
                       .map(str -> str)
                       .count();

assert count == countLambda: "Both should be equal!"

Causes

• Using `Function.identity()` is a built-in method that quickly returns the input argument as output, enhancing clarity for those familiar with functional programming.
• Lambda expressions such as `str -> str` provide greater flexibility and can be more easily extended for complex transformations.

Solutions

• `Function.identity()` may be preferred when clarity and intent as a functional identity is needed, especially for those accustomed to functional paradigms.
• Lambda expressions are useful when you plan to modify or extend the functionality, adding more complexity to the transformations.

Mistake: Assuming `Function.identity()` is always faster than a lambda expression.

Solution: Performance is often comparable. Use profiling to measure specific cases.

Mistake: Using `Function.identity()` without understanding its purpose can lead to confusion for team members less familiar with functional programming.

Solution: Clearly comment your code to explain why `Function.identity()` is used.