What does the map() method do in a stream?

The map() method in the Stream API is used to transform each element in a stream by applying a given function to it. This method creates a new stream that contains the transformed elements, without modifying the original stream. The map() method is commonly used for tasks like converting data types, extracting fields from objects, or applying calculations to each element in a collection. It is one of the most versatile and powerful methods in the Stream API, enabling a wide range of data transformations.

The function passed to the map() method must be a functional interface that takes one input and returns one output. For example, if you have a list of strings and want to convert each string to its uppercase equivalent, you could use the map() method with a function that performs the conversion. Similarly, you can use map() to extract a specific field from a list of objects or to apply a mathematical operation to a list of numbers.

The map() method is an intermediate operation, meaning it does not produce a final result on its own. Instead, it creates a new stream that can be further processed by other stream operations. The final result is only obtained when a terminal operation, such as collect() or reduce(), is applied to the stream. This ability to transform data in a functional style makes map() an essential tool in the Stream API.