Why is a static method considered a method?

问题:

I'm writing an explanation for some code for a course, and have been accidentally using the words method and function interchangeably. I decided to go back over and fix the wording, but ran into a hole in my understanding.

From what I understand, a subroutine is a function if it doesn't act on an instance of a class (its effect is restricted to its explicit input/output), and is a method if it operates on an instance of a class (it may carry out side effects on the instance that make it impure).

There's a good discussion here on the topic. Note that by the accepted answer's definitions, a static method should actually be a function because an instance is never implicitly passed, and it doesn't have access to any instance's members.

With this is mind though, shouldn't static methods actually be functions?

By their definition they don't act on particular instances of a class; they're only "tied" to the class because of relation. I've seen a few good looking sites that refer to static subroutines as "methods" though (Oracle, Fredosaurus, ProgrammingSimplified), so either they're all overlooking the terminology, or I'm missing something (my guess is the latter).

I'd like to make sure I am using the correct wording.
Can anybody clear this up?

回答1:

This quote from 8.4.3.2 may help:

A method that is declared static is called a class method.

A method that is not declared static is called an instance method [...].

• Class methods: associated with a class.
• Instance methods: associated with an instance.

Java just wants you to "think object-oriented". Also, static methods have access to a surrounding scope which may include state. In a way, the class is like an object itself.

回答2:

The simple answer is that when Java decided to call everything a "method", they didn't care about the distinction between a function and a method in theoretical computer science.

回答3:

Static methods are not exactly functions, the difference is subtle, but important.

A static method using only given input parameters is essentially a function.

But static methods may access static variables and other static functions (also using static variables) so static methods may have a state which is fundamentally different to a function which are by definition stateless. (ADDENDUM: While programmers are often not so strict with using "function" as definition, a strict function in computer science can access only input parameters). So defining this case of accessing static fields it is not valid to say that static methods are always functions.

Another difference which justifies the usage of "static method" is that you can define in C derivates global functions and global variables which can be accessed everywhere. If you cannot access the class which contain static methods, the methods are inaccessible, too. So "static methods" are limited in their scope by design in contrast to global functions.

回答4:

In Java, a user-defined class is actually an instance of a subclass of java.lang.Class.

In this sense, static methods are attached to an instance of a conceptual class: they are attached to an instance of a subclass of java.lang.Class.

With this in mind, the term "class method" (an alternate name for Java's static methods) begins to make sense. And the term "class method" can be found in many places: Objective C, Smalltalk, and the JLS -- to name just a few.

回答5:

In computer science function clearly maps to a static method. But "method" of a class is a bit generic, like "member" (field member, method member). There are wordings like

Data members and method members have two separate name spaces: .x and .x() can coexist.

So the reason is, that as the philosoph Ludwig Wittgenstein said, Language is a tool with different contexts. "Method" is a nice moniker in the citation above to categorize a "member".

回答6:

Your thinking is right and it makes sense. It's just not established terminology in the Java community. Let me explain some internals that can help understand why the terminology subsists.

Java is a class based object oriented language. A method is always member of a class or instance (This is a general statement valid for other programming languages too). We think of class and instance being both objects.

Instance method (dynamic)

You cannot invoke this method from a class directly, you have to create an instance. Each instance references that method. You can overwrite a method definition with the exact same method signature (when subclassing), i.e. the reference points to a different method (which has the same signature, but can have a different method body). The method is dynamic.

Class method (static)

You only can invoke this method from the class directly, i.e. you don't need to create an instance of that class. There is only one global definition of that method in the whole program. You cannot overwrite the exact same method signature when the method is declared static, because there is only one definition valid for the whole program. Note that the method is member of the class object itself, the instances have all the same unique (and fix) reference to that method.

回答7:

Here is another take on the terminology, using Scala as a mnemonic:
In Scala you have objects, which are singleton instances of an implicitly defined class 1.

Per your definition, we can call these subroutines belonging to the object methods, as they operate on a single instance of the class.
Additionally the object will also define class A, and create all of the methods in object A as static methods on class A (for interfacing with Java) [2].

Therefore we can say that the static methods of Java class A access the same members as the Scala singleton instance, which per your definition then deserve to be called (static) methods of class A.

回答8:

Of course, the main difference is - method can use static fields, not only method parameters. But there is additional one - polymorphism! Results of evaluation Class A.doTheSameStaticMethod() and ClassB.doTheSameStaticMehod() will be depends of class. In this case function is impotent.

回答9:

Each class has an object to represent it that is an instance of a subclass of the Class class. Static methods are really instance methods on these objects that are instances of a subclass of Class. They have access to state in the form of static fields, so they are not restricted to being just (stateless) functions. They are methods.