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
方法重写可以通过动态调度实现,这意味着对象的声明类型不决定其行为,而是决定其运行时类型:
Animal lassie = new Dog();
lassie.speak(); // outputs "woof!"
Animal kermit = new Frog();
kermit.speak(); // outputs "ribbit!"
尽管lassie和kermit都声明为Animal类型的对象,但它们的行为(method .speak())会有所不同,因为动态调度只会在运行时将方法调用.speak()绑定到实现,而不是在编译时。
现在,这里是静态关键字开始有意义的地方:单词“静态”是“动态”的反义词。所以你不能重写静态方法的原因是因为静态成员上没有动态分派——因为静态字面上的意思是“非动态的”。如果它们是动态分派的(因此可以被重写),静态关键字就没有意义了。
其他回答
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。
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。
Yes. Practically Java allows overriding static method, and No theoretically if you Override a static method in Java then it will compile and run smoothly but it will lose Polymorphism which is the basic property of Java. You will Read Everywhere that it is not possible to try yourself compiling and running. you will get your answer. e.g. If you Have Class Animal and a static method eat() and you Override that static method in its Subclass lets called it Dog. Then when wherever you Assign a Dog object to an Animal Reference and call eat() according to Java Dog's eat() should have been called but in static Overriding Animals' eat() will Be Called.
class Animal {
public static void eat() {
System.out.println("Animal Eating");
}
}
class Dog extends Animal{
public static void eat() {
System.out.println("Dog Eating");
}
}
class Test {
public static void main(String args[]) {
Animal obj= new Dog();//Dog object in animal
obj.eat(); //should call dog's eat but it didn't
}
}
Output Animal Eating
According to Polymorphism Principle of Java, the Output Should be Dog Eating. But the result was different because to support Polymorphism Java uses Late Binding that means methods are called only at the run-time but not in the case of static methods. In static methods compiler calls methods at the compile time rather than the run-time, so we get methods according to the reference and not according to the object a reference a containing that's why You can say Practically it supports static overring but theoretically, it doesn't.
通过重写,我们可以根据对象类型创建一个多态性质。静态方法与对象无关。因此java不支持静态方法重写。
简单的解决方案:使用单例实例。它将允许重写和继承。
在我的系统中,我有SingletonsRegistry类,它为传递的class返回实例。如果没有找到instance,则创建它。
Haxe语言类:
package rflib.common.utils;
import haxe.ds.ObjectMap;
class SingletonsRegistry
{
public static var instances:Map<Class<Dynamic>, Dynamic>;
static function __init__()
{
StaticsInitializer.addCallback(SingletonsRegistry, function()
{
instances = null;
});
}
public static function getInstance(cls:Class<Dynamic>, ?args:Array<Dynamic>)
{
if (instances == null) {
instances = untyped new ObjectMap<Dynamic, Dynamic>();
}
if (!instances.exists(cls))
{
if (args == null) args = [];
instances.set(cls, Type.createInstance(cls, args));
}
return instances.get(cls);
}
public static function validate(inst:Dynamic, cls:Class<Dynamic>)
{
if (instances == null) return;
var inst2 = instances[cls];
if (inst2 != null && inst != inst2) throw "Can\'t create multiple instances of " + Type.getClassName(cls) + " - it's singleton!";
}
}