为什么不可能重写静态方法?
如果可能,请举例说明。
为什么不可能重写静态方法?
如果可能,请举例说明。
当前回答
Well... the answer is NO if you think from the perspective of how an overriden method should behave in Java. But, you don't get any compiler error if you try to override a static method. That means, if you try to override, Java doesn't stop you doing that; but you certainly don't get the same effect as you get for non-static methods. Overriding in Java simply means that the particular method would be called based on the run time type of the object and not on the compile time type of it (which is the case with overriden static methods). Okay... any guesses for the reason why do they behave strangely? Because they are class methods and hence access to them is always resolved during compile time only using the compile time type information. Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it :-)
示例:让我们试着看看如果我们尝试重写一个静态方法会发生什么:-
class SuperClass {
// ......
public static void staticMethod() {
System.out.println("SuperClass: inside staticMethod");
}
// ......
}
public class SubClass extends SuperClass {
// ......
// overriding the static method
public static void staticMethod() {
System.out.println("SubClass: inside staticMethod");
}
// ......
public static void main(String[] args) {
// ......
SuperClass superClassWithSuperCons = new SuperClass();
SuperClass superClassWithSubCons = new SubClass();
SubClass subClassWithSubCons = new SubClass();
superClassWithSuperCons.staticMethod();
superClassWithSubCons.staticMethod();
subClassWithSubCons.staticMethod();
// ...
}
}
输出: SuperClass:在staticMethod内部 SuperClass:在staticMethod内部 子类:staticMethod内部
注意输出的第二行。如果staticMethod被重写,这一行应该与第三行相同,因为我们在运行时类型的对象上调用'staticMethod()'作为'子类'而不是'超类'。这证实了静态方法总是只使用它们的编译时类型信息进行解析。
其他回答
Here is a simple explanation. A static method is associated with a class while an instance method is associated with a particular object. Overrides allow calling the different implementation of the overridden methods associated with the particular object. So it is counter-intuitive to override static method which is not even associated with objects but the class itself in the first place. So static methods cannot be overridden based on what object is calling it, it will always be associated with the class where it was created.
重写是为实例成员保留的,以支持多态行为。静态类成员不属于特定实例。相反,静态成员属于类,因此不支持重写,因为子类只继承受保护和公共实例成员,而不继承静态成员。您可能希望定义一个接口,并研究工厂和/或策略设计模式,以评估替代方法。
重写静态方法有什么好处呢?不能通过实例调用静态方法。
MyClass.static1()
MySubClass.static1() // If you overrode, you have to call it through MySubClass anyway.
编辑:似乎由于语言设计中的一个不幸疏忽,您可以通过实例调用静态方法。一般没人会这么做。我的坏。
Well... the answer is NO if you think from the perspective of how an overriden method should behave in Java. But, you don't get any compiler error if you try to override a static method. That means, if you try to override, Java doesn't stop you doing that; but you certainly don't get the same effect as you get for non-static methods. Overriding in Java simply means that the particular method would be called based on the run time type of the object and not on the compile time type of it (which is the case with overriden static methods). Okay... any guesses for the reason why do they behave strangely? Because they are class methods and hence access to them is always resolved during compile time only using the compile time type information. Accessing them using object references is just an extra liberty given by the designers of Java and we should certainly not think of stopping that practice only when they restrict it :-)
示例:让我们试着看看如果我们尝试重写一个静态方法会发生什么:-
class SuperClass {
// ......
public static void staticMethod() {
System.out.println("SuperClass: inside staticMethod");
}
// ......
}
public class SubClass extends SuperClass {
// ......
// overriding the static method
public static void staticMethod() {
System.out.println("SubClass: inside staticMethod");
}
// ......
public static void main(String[] args) {
// ......
SuperClass superClassWithSuperCons = new SuperClass();
SuperClass superClassWithSubCons = new SubClass();
SubClass subClassWithSubCons = new SubClass();
superClassWithSuperCons.staticMethod();
superClassWithSubCons.staticMethod();
subClassWithSubCons.staticMethod();
// ...
}
}
输出: SuperClass:在staticMethod内部 SuperClass:在staticMethod内部 子类:staticMethod内部
注意输出的第二行。如果staticMethod被重写,这一行应该与第三行相同,因为我们在运行时类型的对象上调用'staticMethod()'作为'子类'而不是'超类'。这证实了静态方法总是只使用它们的编译时类型信息进行解析。
静态方法被JVM视为全局方法,根本不绑定到对象实例。
如果可以从类对象中调用静态方法(就像在Smalltalk等语言中那样),那么在概念上是可能的,但在Java中却不是这样。
EDIT
你可以重载静态方法,没关系。但是你不能重写静态方法,因为类不是一级对象。您可以使用反射在运行时获取对象的类,但所获得的对象并不与类层次结构并行。
class MyClass { ... }
class MySubClass extends MyClass { ... }
MyClass obj1 = new MyClass();
MySubClass obj2 = new MySubClass();
ob2 instanceof MyClass --> true
Class clazz1 = obj1.getClass();
Class clazz2 = obj2.getClass();
clazz2 instanceof clazz1 --> false
你可以对类进行反射,但它仅限于此。使用clazz1.staticMethod()不会调用静态方法,而是使用MyClass.staticMethod()。静态方法不绑定到对象,因此在静态方法中没有this或super的概念。静态方法是一个全局函数;因此,也没有多态性的概念,因此,方法重写没有意义。
但是,如果MyClass在运行时是一个调用方法的对象,这是可能的,就像在Smalltalk(或者可能是一个评论建议的JRuby,但我对JRuby一无所知)。
哦是的…还有一件事。您可以通过对象obj1.staticMethod()调用静态方法,但这实际上是MyClass.staticMethod()的语法糖,应该避免。在现代IDE中,它通常会引发一个警告。我不知道他们为什么允许走这条捷径。