是“继承”正确的术语来解释超类的静态方法可以通过子类访问吗？

Question

Clarification: this question is not about access modifier

Confirmed that Bm() and bm() statements both works in the following code:

class A {
  static void m() { //some code }
}

class B extends A {
}

class Example {
  public static void main (String [] args) {
    B.m(); // running A's m() static method
  }

  public void try() {
    B b = new B();
    b.m(); // running A's m() static method
  }
}

My question is can we said "static method is inherited"?

if "inherited" is the correct term, if we add a method to B class we same signature of the static class:

class A {
  static void m() { //some code }
}

class B extends A {
  static void m() { //some code }
}

class Example {
  public static void main (String [] args) {
    B.m(); // running B's m() static method
  }

  public void try() {
    B b = new B();
    b.m(); // running B's m() static method

    A a = new B();
    a.m(); // running A's m() static method, no polymorphism
  }
}

In this case, notice that we have no polymorphism, is it the correct term to said that "static method is inherited but not overridden, subclass static method hide superclass static method"?

Last doubt, for these 2 terms, "inherited" and "overriden", which one is directly tied to the term "polymorphism" ?

Answer 1

Yes, I think "inherit" is the correct term here, even if it's not as descriptive as it might be. From section 8.4.8 of the Java Language Specification :

A class C inherits from its direct superclass and direct superinterfaces all non-private methods (whether abstract or not) of the superclass and superinterfaces that are public, protected or declared with default access in the same package as C and are neither overridden (§8.4.8.1) nor hidden (§8.4.8.2) by a declaration in the class.

That doesn't specify instance methods, but there are specific restrictions on what is allowed to hide or override what, which wouldn't make sense if static methods were not inherited.

Really though, I would simply view static methods as "accessible without qualification" rather than anything else, given that they don't take part in polymorphism etc. I think it's worth being very clear about that - for example, one static method can hide an inherited one, but it can't override it.

In other words, while I think "inherit" is technically correct, I would try to avoid using it without any further explanation.

For your second question, I'd say that based on the above, overriding is tied to polymorphism, but inheriting isn't necessarily.

(I would also strongly advise you to avoid calling static methods "via" variables, and also to use the name of the class which declares the static method wherever you specify the name at all. I know that's not part of the question, but I thought I'd just add it anyway...)

Answer 2

I think trying to apply words like 'inherited' and 'overridden' to this sort of thing is not productive. It's misleading because it gives the impression there is something comparable of what goes on with virtual instance methods, and you point out there isn't.

(But as Jon Skeet points out, the Java language spec doesn't agree with me, it groups these together in the same section.)

Answer 3

Guys I would like to share my knowledge in java with all java lovers out there!

First of all let me tell you that static members are those members which can be accessed directly without creating an object of that particular class, when static members of a class is used in some other class then it should be used by specifying the class name .(dot) static member's name(eg Ai in the following example), and also if any subclass class is getting inherited from a super class which has static members and if both subclass and super class are in the same package then that static members can be accessed from the base class without even using the class name of the super class. Please go through the Example:

package myProg; 
class A 
{ 
static int i = 10; 
A() 
{ 
System.out.println("Inside A()"); 
} 
} 

class B extends A 
{ 
public static void main(String[] args) 
{ 
System.out.println("i = " + i); //accessing 'i' form superclass without specifying class name 
System.out.println("A.i = " + A.i); //again accessing 'i' with the class name .(dot) static member 'i' 

/* 
we can also use the super class' object in order to access the static member compiler 
will not show any error but it is not the exact way of using static members. static members are created 
so that it could be used without creating the class object. see below the usage of object to use the 
static member i. 
*/ 
A obj = new A(); //creating class A object and A() also gets called 
obj.i = 20; 
System.out.println("obj.i = " + obj.i); 
} 
} 

/* 
This program was to show the usage of static member. Now, lets discuss on the topic of static member inheritance. 
SO GUYS I WOULD LIKE TO TELL YOU THAT STATIC MEMBERS ARE NOT INHERITED. This undermentioned program is 
to show the inheritance of static members. 
*/ 
class A 
{ 
static int i = 10; //initially the value of i is 10 
static int j = 20; //lets use one more static member int j, just to see the value of it through class A, and B 
static 
{ 
/* 
This is static initialization block(SIB) of class A, 
SIB gets executed first in the order of top to bottom only one time 
when the class is loaded. 
*/ 

System.out.println("A-SIB"); 
} 
} 
class B extends A 
{ 
static 
{ 
i = 12;//trying to modify the i's value to 12 
System.out.println("B-SIB"); 
} 
} 
class D extends A 
{ 
static int k = 15; 
static 
{ 
System.out.println("D-SIB"); 
} 
} 
class C 
{ 
public static void main(String [] arhs) 
{ 
System.out.println("D.k: " + D.k); 
/*here we are trying to access the static member k from class D, 
it will try to search this class and then that class 
will be loaded first i.e. SIB of class D will be loaded and SOP 
statement of class D will be printed first. When the class loading 
is done then the compiler will search for the static int k in class 
D and if found SOP statement will be executed with the value of k. 
*/ 
/* 
ONE GROUND RULE: as soon as the compiler see this statement the compiler will load 
class D into the memory, loading of class into memory is nothing but 
storing all the static members (i.e. static constant & SIBs) into the 
memory. 
*/ 


System.out.println("B.i: " + B.i); 
/*Now, this is what we are talking about... we think that static int i's 
value is there in class B and when we write B.i it compiles and executes 
successfully BUT... there compiler is playing a major role at this statement... 
Try to understand now... we know that class B is the subclass of class A 
BUT when the compiler sees this statement it will first try to search 
the static int i inside class B and it is not found there, then since it is 
the subclass of A, the compiler will search in class A and it is found 
there. NOW, WHEN STATIC INT I IS FOUND IN CLASS A THE COMPILER WILL CHANGE 
THIS STATEMENT B.i TO A.i and the class B WILL NOT AT ALL BE LOADED INTO 
THE MEMORY BECAUSE STATIC I IS ONLY PRESENT IN CLASS A. But in the previous 
statement static int k is present inside class D so when we try to access k's value 
the class D will be loaded into memory i.e. SIB will be executed and then 
the SOP statement of the value of k. USING B.i IS NOT WRONG BUT COMPILER 
ASSUMES THAT THE PROGRAMMER HAS MADE A MISTAKE AND IT REPLACES THE CLASS NAME 
B.i TO A.i. Thus this show that static members are not inherited to the subclass.And 
therefore the vaue of i will NOT BE CHANGE TO 12 AS YOU CAN SEE IN THE SIB OF 
CLASS B, it will be 10 only.. 
*/ 



System.out.println("A.j: " + A.j);//this statement will be executed as it is, compiler will not make 
System.out.println("A.i: " + A.i);//any modifications to these statements.  


System.out.println("B.j: " + B.j);//again compiler will modify this statement from B.j to A.j 

} 
} 

Guys if you still have any confusion mail me at this email-id: pradeep_y2k@yahoo.co.in

Regards Pradeep Kumar Tiwari

Answer 4

好的静态方法不能被覆盖，但可以重新定义，换句话说，它被称为隐藏检查这个http://www.coderanch.com/how-to/java/OverridingVsHiding他们解释得很好