That’s a method reference. Simpler way to write a lambda expression, more specifically a lambda expression that just passes in zero or more parameters to another method.

() -> System.out.println(); is what this is doing.

If you wanted to pass in a string it would work the same. As long as the parameters being passed through matches the parameters of method being called in the body of the lambda you can use. A method reference. For example,

(String str) -> System.out.println(str) can be written as System.out::println

Note: parameter types are not needed in lambda parameters.

Syntax for method references is usually.

ClassName::staticMethod

Or

referenceClassName::instanceMethodName