The :: in Java is known as the method reference operator. It is used to refer to methods without invoking them. These are can be useful in contexts that expect a functional interface*, such as with the Java Streams API or other places where lambda expressions are used.

:: references to a method. It doesnt execute it immmediately. () calls a method. It is executed immediately.

"To Reference": When you use ClassName::methodName (in case of static methods) or instance::methodName, you are creating a reference to the method. This means you are specifying which method to use, but you are not actually executing (calling) it at that point. System.out::println is a method reference. It tells the forEach method to use System.out.println for each element in the list. The println method is called by the forEach method, not directly by us.

"To Call": When you invoke (call) a method, you execute it immediately. For example, System.out.println("Hello") calls the println method and executes it, printing "Hello" to the console.