What does the peek method do in Stream API?

The peek method in Java Stream API is designed to perform some action on each element of the stream without consuming it, that is, without modifying the stream itself. It returns the same stream, allowing for debugging or logging actions during processing.

Example usage:

// Example of using peek for logging
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

List<String> processedNames = names.stream()
    .filter(name -> name.length() > 3) // Filter names longer than 3 characters
    .peek(name -> System.out.println("Processing name: " + name)) // Log each name after filtering
    .map(String::toUpperCase) // Convert to uppercase
    .collect(Collectors.toList()); // Collect into a list

// Output:
// Processing name: Alice
// Processing name: Charlie

Important points:

• peek is an intermediate operation.
• It is mainly used for debugging or performing side effects.
• Do not rely on peek to change the state of stream elements or external variables, as the order and number of calls can be unpredictable in parallel streams.
• If no terminal operation follows peek, the action specified in peek will not be executed.