Why Does `Comparator.reversed()` Fail with Lambda Expressions in Java Sorting?

Why Comparator.reversed() Fails with Lambdas

When sorting a list of User objects using Comparator.comparing with a lambda expression, an error may occur:

userList.sort(Comparator.comparing(u -> u.getName()).reversed()); // Compiler error

This is due to a limitation in the compiler's type inference mechanism. The compiler struggles to determine the type of the lambda argument 'u'.

In the following example, using a method reference allows the compiler to infer the target type and avoid the error:

userList.sort(Comparator.comparing(User::getName).reversed()); // works

The method reference provides additional type information, which the compiler uses to infer the type of 'u' as User.

To avoid the error when using a lambda, you can explicitly specify the type of the lambda argument:

userList.sort(Comparator.comparing((User u) -> u.getName()).reversed());

In future compiler releases, this issue may be addressed and the error may no longer occur.

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