Which of the following is a valid method reference in Java?

All of the provided examples are valid method references in Java. Each one refers to a method using the :: syntax:

• String::length: Refers to the length instance method of the String class.
• System.out::println: Refers to the println instance method of the PrintStream class, often used for printing to the console.
• Math::max: Refers to the max static method of the Math class, which returns the greater of two values.

Method references are a concise and readable way to refer to existing methods, making them a powerful tool when working with functional programming in Java. They reduce the need for boilerplate code and make the intent of your code more explicit, leading to cleaner and more maintainable applications.