Explain the expression System.out::println.

The expression System.out::println is a method reference in Java 8 and above. It represents a lambda expression that calls the println method of the System.out object.

A method reference is used for concise lambda expressions when the lambda simply calls an existing method. In this case, it is equivalent to the lambda expression (value) -> System.out.println(value).

It is most commonly used in streams (Stream API) to perform an action on each element of the stream, for example:

// Example of using a method reference to println in a stream
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
names.stream().forEach(System.out::println);

This code will print each name from the list on a new line, using the println method of the System.out object for each stream element.