What is the purpose of the peek() method in the Stream API?

The peek() method in the Stream API is used to view elements of a stream without modifying them. It is an intermediate operation that allows you to perform some action on each element as it passes through the pipeline, such as logging, debugging, or performing side effects. The peek() method is often used for troubleshooting and monitoring purposes, as it provides a way to inspect the elements of a stream at a specific point in the processing pipeline.

For example, you can use peek() to print each element of a stream to the console before applying further operations. This can help you understand how the elements are transformed or filtered by subsequent operations. The action passed to the peek() method must be a functional interface that takes an element as input and performs some operation on it, similar to the forEach() method. However, unlike forEach(), peek() does not terminate the stream; it simply allows you to observe the elements as they pass through.

It’s important to note that peek() is designed for side effects and should be used with caution, especially in parallel streams. Since it is an intermediate operation, the actions performed by peek() may not be executed in the order you expect, particularly when using parallel processing. Despite this, peek() is a valuable tool for inspecting and debugging streams, helping developers ensure that their stream operations are working as intended.