Java – Why Use :: for Method References Instead of .?

javamethods

I often wonder why Java uses :: for method references instead of ., e.g. why write

  System.out::println

instead of

  System.out.println

Of course, one might simply answer: "because the designers decided so". On the other hand, I would have expected the second syntax because the dot is the usual Java syntax for accessing class members.

So is there any known special reason for introducing the new :: syntax instead of using the existing . convention for method references?

Best Answer

This is to avoid ambiguity in case if class has (static) member with the same name as method (Java allows that).

It is easy to see from code snippet in Java tutorial about method references:

Because this lambda expression invokes an existing method, you can use a method reference instead of a lambda expression:
Arrays.sort(rosterAsArray, Person::compareByAge);

If class Person in above snippet would also have member named compareByAge (of the type appropriate to pass to Arrays.sort), dot notation wouldn't allow to tell whether parameter refers to method or member.

TL;DR

In Java, the reason of public static void main(String[] args) is that

Gosling wanted
the code written by someone experienced in C (not in Java)
to be executed by someone used to running PostScript on NeWS

For C#, the reasoning is transitively similar so to speak. Language designers kept the program entry point syntax familiar for programmers coming from Java. As C# architect Anders Hejlsberg puts it,

...our approach with C# has simply been to offer an alternative... to Java programmers...

Long version

expanding above and backed up with boring references.

java Terminator Hasta la vista Baby!

VM Spec, 2.17.1 Virtual Machine Start-up

...The manner in which the initial class is specified to the Java virtual machine is beyond the scope of this specification, but it is typical, in host environments that use command lines, for the fully qualified name of the class to be specified as a command-line argument and for subsequent command-line arguments to be used as strings to be provided as the argument to the method main. For example, using Sun's Java 2 SDK for Solaris, the command line

java Terminator Hasta la vista Baby!

will start a Java virtual machine by invoking the method main of class Terminator (a class in an unnamed package) and passing it an array containing the four strings "Hasta", "la", "vista", and "Baby!"...

...see also: Appendix: I need your clothes, your boots and your motorcycle

My interpretation:
execution targeted for use like typical scripts in command line interface.

important sidestep

...that helps avoid a couple of false traces in our investigation.

VM Spec, 1.2 The Java Virtual Machine

The Java virtual machine knows nothing of the Java programming language...

^{I noticed above when studying prior chapter - 1.1 History which I thought could be helpful (but turned out useless).}

My interpretation:
execution is governed by VM spec alone, which
explicitly declares that it has nothing to do with Java language
=> OK to ignore JLS and anything Java language related at all

Gosling: a compromise between C and scripting language...

Based on above, I began searching the web for JVM history. Didn't help, too much garbage in results.

Then, I recalled legends about Gosling and narrowed down my search to Gosling JVM history.

Eureka! How The JVM Spec Came To Be

In this keynote from the JVM Languages Summit 2008, James Gosling discusses... Java's creation,... a compromise between C and scripting language...

My interpretation:
explicit declaration that at the moment of creation,
C and scripting have been considered most important influences.

Already seen nod to scripting in VM Spec 2.17.1,
command line arguments sufficiently explain String[] args
but static and main aren't there yet, need to dig further...

^{Note while typing this - connecting C, scripting and VM Spec 1.2 with its nothing-of-Java - I feel like something familiar, something... object oriented is slowly passing away. Take my hand and keep movin' Don't slow down we're nearly there now}

Keynote slides are available online: 20_Gosling_keynote.pdf, quite convenient for copying key points.

    page 3
    
        The Prehistory of Java
        * What shaped my thinking
    
    page 9
    
        NeWS
        * Networked Extensible Window System
        * A window system based on scripting....
          PostScript (!!)
    
    page 16
    
        A Big (but quiet) Goal:
          How close could I get to a
          "scripting" feel...
    
    page 19
    
        The original concept
        * Was all about building
          networks of things,
          orchestrated by a scripting
          language
        * (Unix shells, AppleScript, ...)
    
    page 20
    
        A Wolf in Sheeps Clothing
        * C syntax to make developers
          comfortable

A-ha! Let's look closer at C syntax.

The "hello, world" example...
main()
{
    printf("hello, world\n");
}
...a function named main is being defined. The main function serves a special purpose in C programs; the run-time environment calls the main function to begin program execution.

...The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments...

Are we getting closer? you bet. It is also worth following "main" link from above quote:

the main function is where a program starts execution. It is responsible for the high-level organization of the program's functionality, and typically has access to the command arguments given to the program when it was executed.

My interpretation:
To be comfortable for C developer, program entry point has to be main.
Also, since Java requires any method to be in class, Class.main is
as close as it gets: static invocation, just class name and dot,
no constructors please - C knows nothing like that.

This also transitively applies to C#, taking into account
the idea of easy migration to it from Java.

^{Readers thinking that familiar program entry point doesn't matter are kindly invited to search and check Stack Overflow questions where guys coming from Java SE are trying to write Hello World for Java ME MIDP. Note MIDP entry point has no main nor static.}

Conclusion

Based on above I would say that static, main and String[] args were at the moments of Java and C# creation most reasonable choices to define program entry point.

Appendix: I need your clothes, your boots and your motorcycle

Have to admit, reading VM Spec 2.17.1 was enormous fun.

...the command line

java Terminator Hasta la vista Baby!

will start a Java virtual machine by invoking the method main of class Terminator (a class in an unnamed package) and passing it an array containing the four strings "Hasta", "la", "vista", and "Baby!".

We now outline the steps the virtual machine may take to execute Terminator, as an example of the loading, linking, and initialization processes that are described further in later sections.

The initial attempt... discovers that the class Terminator is not loaded...

After Terminator is loaded, it must be initialized before main can be invoked, and a type (class or interface) must always be linked before it is initialized. Linking (§2.17.3) involves verification, preparation, and (optionally) resolution...

Verification (§2.17.3) checks that the loaded representation of Terminator is well formed...

Resolution (§2.17.3) is the process of checking symbolic references from class Terminator...

Symbolic references from Terminator oh yeah.

Java – Overriding – Access to Members with Reference Reassignment

If you look at the Java Language Specification (http://docs.oracle.com/javase/specs/) you will find a distinction between the static type and the dynamic type of object references.

The static type of any reference refers to the type declared in your program at development time.

The dynamic type of the reference refers to the type of object which is actually stored at the reference address at runtime.

So if you have a simple hierarchy, where A is the superclass and B is a subclass of A.

class A {
  public String a = "super";
  void m() {...}
}

class B extends A {
  public String a = "sub";
  @Override
  void m() {...}
}

A refA = new A();
A refB = new B();
B refB2 = new B();
refA.m();
refB.m();
System.out.println(refA.a);
System.out.println(refB.a);
System.out.println(refB2.a);

For example, refB's static type is A but its dynamic type is B. What does this imply at runtime?

Instance variables are statically bound, i.e. you can only access the instance variables of the static type, so the first two println() statements will print "super", since A is the static type of refA and refB. The last println() statement will in contrast print "sub", because in this case B is the static type.
Methods are dynamically bound, i.e. the invoked method is first searched in the dynamic type of the reference and if it isn't found, the class hierarchy is traversed up until a definition of the overriden method is found (this mechanism is called dynamic dispatch). This means that in case refA.m() method void m() of A is invoked and in case refB.m(), void m() of B is invoked.