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java inheritance - static fields and methods [duplicate]

Why is it not possible to override static methods?
If possible, please use an example.

Overriding depends on having an instance of a class. The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviors for the same methods defined in the superclass (and overridden in the subclasses). A static method is not associated with any instance of a class so the concept is not applicable.
There were two considerations driving Java's design that impacted this. One was a concern with performance: there had been a lot of criticism of Smalltalk about it being too slow (garbage collection and polymorphic calls being part of that) and Java's creators were determined to avoid that. Another was the decision that the target audience for Java was C++ developers. Making static methods work the way they do had the benefit of familiarity for C++ programmers and was also very fast, because there's no need to wait until runtime to figure out which method to call.

Personally I think this is a flaw in the design of Java. Yes, yes, I understand that non-static methods are attached to an instance while static methods are attached to a class, etc etc. Still, consider the following code:
public class RegularEmployee {
private BigDecimal salary;
public void setSalary(BigDecimal salary) {
this.salary = salary;
}
public static BigDecimal getBonusMultiplier() {
return new BigDecimal(".02");
}
public BigDecimal calculateBonus() {
return salary.multiply(getBonusMultiplier());
}
/* ... presumably lots of other code ... */
}
public class SpecialEmployee extends RegularEmployee {
public static BigDecimal getBonusMultiplier() {
return new BigDecimal(".03");
}
}
This code will not work as you might expect. Namely, SpecialEmployee's get a 2% bonus just like regular employees. But if you remove the "static"s, then SpecialEmployee's get a 3% bonus.
(Admittedly, this example is poor coding style in that in real life you would likely want the bonus multiplier to be in a database somewhere rather than hard-coded. But that's just because I didn't want to bog down the example with a lot of code irrelevant to the point.)
It seems quite plausible to me that you might want to make getBonusMultiplier static. Perhaps you want to be able to display the bonus multiplier for all the categories of employees, without needing to have an instance of an employee in each category. What would be the point of searching for such example instances? What if we are creating a new category of employee and don't have any employees assigned to it yet? This is quite logically a static function.
But it doesn't work.
And yes, yes, I can think of any number of ways to rewrite the above code to make it work. My point is not that it creates an unsolvable problem, but that it creates a trap for the unwary programmer, because the language does not behave as I think a reasonable person would expect.
Perhaps if I tried to write a compiler for an OOP language, I would quickly see why implementing it so that static functions can be overriden would be difficult or impossible.
Or perhaps there is some good reason why Java behaves this way. Can anyone point out an advantage to this behavior, some category of problem that is made easier by this? I mean, don't just point me to the Java language spec and say "see, this is documented how it behaves". I know that. But is there a good reason why it SHOULD behave this way? (Besides the obvious "making it work right was too hard"...)
Update
#VicKirk: If you mean that this is "bad design" because it doesn't fit how Java handles statics, my reply is, "Well, duh, of course." As I said in my original post, it doesn't work. But if you mean that it is bad design in the sense that there would be something fundamentally wrong with a language where this worked, i.e. where statics could be overridden just like virtual functions, that this would somehow introduce an ambiguity or it would be impossible to implement efficiently or some such, I reply, "Why? What's wrong with the concept?"
I think the example I give is a very natural thing to want to do. I have a class that has a function that does not depend on any instance data, and which I might very reasonably want to call independent of an instance, as well as wanting to call from within an instance method. Why should this not work? I've run into this situation a fair number of times over the years. In practice I get around it by making the function virtual, and then creating a static method whose only purpose in life is to be a static method that passes the call on to the virtual method with a dummy instance. That seems like a very roundabout way to get there.

The short answer is: it is entirely possible, but Java doesn't do it.
Here is some code which illustrates the current state of affairs in Java:
File Base.java:
package sp.trial;
public class Base {
static void printValue() {
System.out.println(" Called static Base method.");
}
void nonStatPrintValue() {
System.out.println(" Called non-static Base method.");
}
void nonLocalIndirectStatMethod() {
System.out.println(" Non-static calls overridden(?) static:");
System.out.print(" ");
this.printValue();
}
}
File Child.java:
package sp.trial;
public class Child extends Base {
static void printValue() {
System.out.println(" Called static Child method.");
}
void nonStatPrintValue() {
System.out.println(" Called non-static Child method.");
}
void localIndirectStatMethod() {
System.out.println(" Non-static calls own static:");
System.out.print(" ");
printValue();
}
public static void main(String[] args) {
System.out.println("Object: static type Base; runtime type Child:");
Base base = new Child();
base.printValue();
base.nonStatPrintValue();
System.out.println("Object: static type Child; runtime type Child:");
Child child = new Child();
child.printValue();
child.nonStatPrintValue();
System.out.println("Class: Child static call:");
Child.printValue();
System.out.println("Class: Base static call:");
Base.printValue();
System.out.println("Object: static/runtime type Child -- call static from non-static method of Child:");
child.localIndirectStatMethod();
System.out.println("Object: static/runtime type Child -- call static from non-static method of Base:");
child.nonLocalIndirectStatMethod();
}
}
If you run this (I did it on a Mac, from Eclipse, using Java 1.6) you get:
Object: static type Base; runtime type Child.
Called static Base method.
Called non-static Child method.
Object: static type Child; runtime type Child.
Called static Child method.
Called non-static Child method.
Class: Child static call.
Called static Child method.
Class: Base static call.
Called static Base method.
Object: static/runtime type Child -- call static from non-static method of Child.
Non-static calls own static.
Called static Child method.
Object: static/runtime type Child -- call static from non-static method of Base.
Non-static calls overridden(?) static.
Called static Base method.
Here, the only cases which might be a surprise (and which the question is about) appear to be the first case:
"The run-time type is not used to determine which static methods are called, even when called with an object instance (obj.staticMethod())."
and the last case:
"When calling a static method from within an object method of a class, the static method chosen is the one accessible from the class itself and not from the class defining the run-time type of the object."
Calling with an object instance
The static call is resolved at compile-time, whereas a non-static method call is resolved at run-time. Notice that although static methods are inherited (from parent) they are not overridden (by child). This could be a surprise if you expected otherwise.
Calling from within an object method
Object method calls are resolved using the run-time type, but static (class) method calls are resolved using the compile-time (declared) type.
Changing the rules
To change these rules, so that the last call in the example called Child.printValue(), static calls would have to be provided with a type at run-time, rather than the compiler resolving the call at compile-time with the declared class of the object (or context). Static calls could then use the (dynamic) type hierarchy to resolve the call, just as object method calls do today.
This would easily be doable (if we changed Java :-O), and is not at all unreasonable, however, it has some interesting considerations.
The main consideration is that we need to decide which static method calls should do this.
At the moment, Java has this "quirk" in the language whereby obj.staticMethod() calls are replaced by ObjectClass.staticMethod() calls (normally with a warning). [Note: ObjectClass is the compile-time type of obj.] These would be good candidates for overriding in this way, taking the run-time type of obj.
If we did it would make method bodies harder to read: static calls in a parent class could potentially be dynamically "re-routed". To avoid this we would have to call the static method with a class name -- and this makes the calls more obviously resolved with the compile-time type hierarchy (as now).
The other ways of invoking a static method are more tricky: this.staticMethod() should mean the same as obj.staticMethod(), taking the run-time type of this. However, this might cause some headaches with existing programs, which call (apparently local) static methods without decoration (which is arguably equivalent to this.method()).
So what about unadorned calls staticMethod()? I suggest they do the same as today, and use the local class context to decide what to do. Otherwise great confusion would ensue. Of course it means that method() would mean this.method() if method was a non-static method, and ThisClass.method() if method were a static method. This is another source of confusion.
Other considerations
If we changed this behaviour (and made static calls potentially dynamically non-local), we would probably want to revisit the meaning of final, private and protected as qualifiers on static methods of a class. We would then all have to get used to the fact that private static and public final methods are not overridden, and can therefore be safely resolved at compile-time, and are "safe" to read as local references.

Actually we were wrong.
Despite Java doesn't allow you to override static methods by default, if you look thoroughly through documentation of Class and Method classes in Java, you can still find a way to emulate static methods overriding by following workaround:
import java.lang.reflect.InvocationTargetException;
import java.math.BigDecimal;
class RegularEmployee {
private BigDecimal salary = BigDecimal.ONE;
public void setSalary(BigDecimal salary) {
this.salary = salary;
}
public static BigDecimal getBonusMultiplier() {
return new BigDecimal(".02");
}
public BigDecimal calculateBonus() {
return salary.multiply(this.getBonusMultiplier());
}
public BigDecimal calculateOverridenBonus() {
try {
// System.out.println(this.getClass().getDeclaredMethod(
// "getBonusMultiplier").toString());
try {
return salary.multiply((BigDecimal) this.getClass()
.getDeclaredMethod("getBonusMultiplier").invoke(this));
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
}
return null;
}
// ... presumably lots of other code ...
}
final class SpecialEmployee extends RegularEmployee {
public static BigDecimal getBonusMultiplier() {
return new BigDecimal(".03");
}
}
public class StaticTestCoolMain {
static public void main(String[] args) {
RegularEmployee Alan = new RegularEmployee();
System.out.println(Alan.calculateBonus());
System.out.println(Alan.calculateOverridenBonus());
SpecialEmployee Bob = new SpecialEmployee();
System.out.println(Bob.calculateBonus());
System.out.println(Bob.calculateOverridenBonus());
}
}
Resulting output:
0.02
0.02
0.02
0.03
what we were trying to achieve :)
Even if we declare third variable Carl as RegularEmployee and assign to it instance of SpecialEmployee, we will still have call of RegularEmployee method in first case and call of SpecialEmployee method in second case
RegularEmployee Carl = new SpecialEmployee();
System.out.println(Carl.calculateBonus());
System.out.println(Carl.calculateOverridenBonus());
just look at output console:
0.02
0.03
;)

Static methods are treated as global by the JVM, there are not bound to an object instance at all.
It could conceptually be possible if you could call static methods from class objects (like in languages like Smalltalk) but it's not the case in Java.
EDIT
You can overload static method, that's ok. But you can not override a static method, because class are no first-class object. You can use reflection to get the class of an object at run-time, but the object that you get does not parallel the class hierarchy.
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
You can reflect over the classes, but it stops there. You don't invoke a static method by using clazz1.staticMethod(), but using MyClass.staticMethod(). A static method is not bound to an object and there is hence no notion of this nor super in a static method. A static method is a global function; as a consequence there is also no notion of polymorphism and, therefore, method overriding makes no sense.
But this could be possible if MyClass was an object at run-time on which you invoke a method, as in Smalltalk (or maybe JRuby as one comment suggest, but I know nothing of JRuby).
Oh yeah... one more thing. You can invoke a static method through an object obj1.staticMethod() but that really syntactic sugar for MyClass.staticMethod() and should be avoided. It usually raises a warning in modern IDE. I don't know why they ever allowed this shortcut.

Method overriding is made possible by dynamic dispatching, meaning that the declared type of an object doesn't determine its behavior, but rather its runtime type:
Animal lassie = new Dog();
lassie.speak(); // outputs "woof!"
Animal kermit = new Frog();
kermit.speak(); // outputs "ribbit!"
Even though both lassie and kermit are declared as objects of type Animal, their behavior (method .speak()) varies because dynamic dispatching will only bind the method call .speak() to an implementation at run time - not at compile time.
Now, here's where the static keyword starts to make sense: the word "static" is an antonym for "dynamic". So the reason why you can't override static methods is because there is no dynamic dispatching on static members - because static literally means "not dynamic". If they dispatched dynamically (and thus could be overriden) the static keyword just wouldn't make sense anymore.

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.

In Java (and many OOP languages, but I cannot speak for all; and some do not have static at all) all methods have a fixed signature - the parameters and types. In a virtual method, the first parameter is implied: a reference to the object itself and when called from within the object, the compiler automatically adds this.
There is no difference for static methods - they still have a fixed signature. However, by declaring the method static you have explicitly stated that the compiler must not include the implied object parameter at the beginning of that signature. Therefore, any other code that calls this must must not attempt to put a reference to an object on the stack. If it did do that, then the method execution would not work since the parameters would be in the wrong place - shifted by one - on the stack.
Because of this difference between the two; virtual methods always have a reference to the context object (i.e. this) so then it is possible to reference anything within the heap that belong to that instance of the object. But with static methods, since there is no reference passed, that method cannot access any object variables and methods since the context is not known.
If you wish that Java would change the definition so that a object context is passed in for every method, static or virtual, then you would in essence have only virtual methods.
As someone asked in a comment to the op - what is your reason and purpose for wanting this feature?
I do not know Ruby much, as this was mentioned by the OP, I did some research. I see that in Ruby classes are really a special kind of object and one can create (even dynamically) new methods. Classes are full class objects in Ruby, they are not in Java. This is just something you will have to accept when working with Java (or C#). These are not dynamic languages, though C# is adding some forms of dynamic. In reality, Ruby does not have "static" methods as far as I could find - in that case these are methods on the singleton class object. You can then override this singleton with a new class and the methods in the previous class object will call those defined in the new class (correct?). So if you called a method in the context of the original class it still would only execute the original statics, but calling a method in the derived class, would call methods either from the parent or sub-class. Interesting and I can see some value in that. It takes a different thought pattern.
Since you are working in Java, you will need to adjust to that way of doing things. Why they did this? Well, probably to improve performance at the time based on the technology and understanding that was available. Computer languages are constantly evolving. Go back far enough and there is no such thing as OOP. In the future, there will be other new ideas.
EDIT: One other comment. Now that I see the differences and as I Java/C# developer myself, I can understand why the answers you get from Java developers may be confusing if you are coming from a language like Ruby. Java static methods are not the same as Ruby class methods. Java developers will have a hard time understanding this, as will conversely those who work mostly with a language like Ruby/Smalltalk. I can see how this would also be greatly confusing by the fact that Java also uses "class method" as another way to talk about static methods but this same term is used differently by Ruby. Java does not have Ruby style class methods (sorry); Ruby does not have Java style static methods which are really just old procedural style functions, as found in C.
By the way - thanks for the question! I learned something new for me today about class methods (Ruby style).

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 :-)
Example: let's try to see what happens if we try overriding a static method:-
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();
// ...
}
}
Output:-
SuperClass: inside staticMethod
SuperClass: inside staticMethod
SubClass: inside staticMethod
Notice the second line of the output. Had the staticMethod been overriden this line should have been identical to the third line as we're invoking the 'staticMethod()' on an object of Runtime Type as 'SubClass' and not as 'SuperClass'. This confirms that the static methods are always resolved using their compile time type information only.

I like and double Jay's comment (https://stackoverflow.com/a/2223803/1517187).
I agree that this is the bad design of Java.
Many other languages support overriding static methods, as we see in previous comments.
I feel Jay has also come to Java from Delphi like me.
Delphi (Object Pascal) was one of the languages implementing OOP before Java and one of the first languages used for commercial application development.
It is obvious that many people had experience with that language since it was in the past the only language to write commercial GUI products. And - yes, we could in Delphi override static methods. Actually, static methods in Delphi are called "class methods", while Delphi had the different concept of "Delphi static methods" which were methods with early binding. To override methods you had to use late binding, declare "virtual" directive. So it was very convenient and intuitive and I would expect this in Java.

In general it doesn't make sense to allow 'overriding' of static methods as there would be no good way to determine which one to call at runtime. Taking the Employee example, if we call RegularEmployee.getBonusMultiplier() - which method is supposed to be executed?
In the case of Java, one could imagine a language definition where it is possible to 'override' static methods as long as they are called through an object instance. However, all this would do is to re-implement regular class methods, adding redundancy to the language without really adding any benefit.

overriding is reserved for instance members to support polymorphic behaviour. static class members do not belong to a particular instance. instead, static members belong to the class and as a result overriding is not supported because subclasses only inherit protected and public instance members and not static members. You may want to define an inerface and research factory and/or strategy design patterns to evaluate an alternate approach.

By overriding we can create a polymorphic nature depending on the object type. Static method has no relation with object. So java can not support static method overriding.

By overriding, you achieve dynamic polymorphism.
When you say overriding static methods, the words you are trying to use are contradictory.
Static says - compile time, overriding is used for dynamic polymorphism.
Both are opposite in nature, and hence can't be used together.
Dynamic polymorphic behavior comes when a programmer uses an object and accessing an instance method. JRE will map different instance methods of different classes based on what kind of object you are using.
When you say overriding static methods, static methods we will access by using the class name, which will be linked at compile time, so there is no concept of linking methods at runtime with static methods. So the term "overriding" static methods itself doesn't make any meaning.
Note: even if you access a class method with an object, still java compiler is intelligent enough to find it out, and will do static linking.

Overriding in Java simply means that the particular method would be called based on the runtime type
of the object and not on the compile-time type of it (which is the case with overridden static methods). As static methods are class methods they are not instance methods so they have nothing to do with the fact which reference is pointing to which Object or instance, because due to the nature of static method it belongs to a specific class. You can redeclare it in the subclass but that subclass won't know anything about the parent class' static methods because, as I said, it is specific to only that class in which it has been declared. 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
more details and example
http://faisalbhagat.blogspot.com/2014/09/method-overriding-and-method-hiding.html

What good will it do to override static methods. You cannot call static methods through an instance.
MyClass.static1()
MySubClass.static1() // If you overrode, you have to call it through MySubClass anyway.
EDIT : It appears that through an unfortunate oversight in language design, you can call static methods through an instance. Generally nobody does that. My bad.

Answer of this question is simple, the method or variable marked as static belongs to the class only, So that static method cannot be inherited in the sub class because they belong to the super class only.

Easy solution: Use singleton instance. It will allow overrides and inheritance.
In my system, I have SingletonsRegistry class, which returns instance for passed Class. If instance is not found, it is created.
Haxe language class:
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!";
}
}

A Static method, variable, block or nested class belongs to the entire class rather than an object.
A Method in Java is used to expose the behaviour of an Object / Class. Here, as the method is static (i.e, static method is used to represent the behaviour of a class only.) changing/ overriding the behaviour of entire class will violate the phenomenon of one of the fundamental pillar of Object oriented programming i.e, high cohesion. (remember a constructor is a special kind of method in Java.)
High Cohesion - One class should have only one role. For example: A car class should produce only car objects and not bike, trucks, planes etc. But the Car class may have some features(behaviour) that belongs to itself only.
Therefore, while designing the java programming language. The language designers thought to allow developers to keep some behaviours of a class to itself only by making a method static in nature.
The below piece code tries to override the static method, but will not encounter any compilation error.
public class Vehicle {
static int VIN;
public static int getVehileNumber() {
return VIN;
}}
class Car extends Vehicle {
static int carNumber;
public static int getVehileNumber() {
return carNumber;
}}
This is because, here we are not overriding a method but we are just re-declaring it. Java allows re-declaration of a method (static/non-static).
Removing the static keyword from getVehileNumber() method of Car class will result into compilation error, Since, we are trying to change the functionality of static method which belongs to Vehicle class only.
Also, If the getVehileNumber() is declared as final then the code will not compile, Since the final keyword restricts the programmer from re-declaring the method.
public static final int getVehileNumber() {
return VIN; }
Overall, this is upto software designers for where to use the static methods.
I personally prefer to use static methods to perform some actions without creating any instance of a class. Secondly, to hide the behaviour of a class from outside world.

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.

Now seeing above answers everyone knows that we can't override static methods, but one should not misunderstood about the concept of accessing static methods from subclass.
We can access static methods of super class with subclass reference if this static method has not been hidden by new static method defined in sub class.
For Example, see below code:-
public class StaticMethodsHiding {
public static void main(String[] args) {
SubClass.hello();
}
}
class SuperClass {
static void hello(){
System.out.println("SuperClass saying Hello");
}
}
class SubClass extends SuperClass {
// static void hello() {
// System.out.println("SubClass Hello");
// }
}
Output:-
SuperClass saying Hello
See Java oracle docs and search for What You Can Do in a Subclass for details about hiding of static methods in sub class.
Thanks

The following code shows that it is possible:
class OverridenStaticMeth {
static void printValue() {
System.out.println("Overriden Meth");
}
}
public class OverrideStaticMeth extends OverridenStaticMeth {
static void printValue() {
System.out.println("Overriding Meth");
}
public static void main(String[] args) {
OverridenStaticMeth osm = new OverrideStaticMeth();
osm.printValue();
System.out.println("now, from main");
printValue();
}
}

Base method hiding in C# and Java

I am coming from Java background and currently learning C#. I just had a big surprise regarding (what I perceive as ) a difference in a way that an object accesses methods from base/derived class. Here is what I mean:
In Java if I do something like this
class InheritanceTesting
{
public void InheritanceOne()
{
System.out.println("InheritanceOne");
}
}
class NewInherit extends InheritanceTesting
{
public void InheritanceOne()
{
System.out.println("InheritanceTwo");
}
}
then run the following:
public static void main(String[] args) {
InheritanceTesting inh = new NewInherit();
inh.InheritanceOne();
}
I get the result:
InheritanceTwo
If I do exactly the same in C#:
class InheritanceTesting
{
public void InheritanceOne()
{
Console.WriteLine("InheritanceOne");
}
}
class NewInherit : InheritanceTesting
{
public new void InheritanceOne()
{
Console.WriteLine("InheritanceTwo");
}
}
Then:
InheritanceTesting inh = new NewInherit();
inh.InheritanceOne();
result is
InheritanceOne
I remember being taught in Java that "object knows what type it is instantiated to", therefore, no surprises when I call the overridden method. Does this mean that the situation is the opposite in C#? Object only "knows" its declared type? If so, what is the logic/advantage in that? It seems to me that Java treats base classes like interfaces - here is your type and here is your actual implementation. I am new to C# and maybe I am missing something obvious here?

A slightly more interesting case is the following
class InheritanceTesting
{
public void InheritanceOne()
// Java equivalent would be
// public final void InheritanceA()
{
Console.WriteLine("InheritanceA - One");
}
public virtual void InheritanceB()
// Java equivalent would be
// public void InheritanceB() // note the removing of final
{
Console.WriteLine("InheritanceB - One");
}
}
class NewInherit : InheritanceTesting
{
public new void InheritanceOne()
// There is no Java equivalent to this statement
{
Console.WriteLine("InheritanceA - Two");
}
public override void InheritanceB()
// Java equivalent would be
// public void InheritanceB()
{
Console.WriteLine("InheritanceB - Two");
}
}
What you are seeing are some of the difference between C# and Java, you can get C# to behave like Java as the method InheritanceB will show.
C# methods are final by default, so you need to take a positive action to make it possible to override a method by marking it as virtual. So the virtual method InheratanceB will behave like you expect methods to behave, with method dispatch based on the object type, not the reference type. e.g.
NewInherit example = new NewInherit();
InheritanceTesting secondReference = example;
example.InheritanceB();
secondreference.InheritanceB();
Will both produce InheritanceB - Two as the method InheritanceB was virtual (able to be overriden) and overridden (with the override method).
What you where seeing is called method hiding, where the method can not be overriden (non-virtual) but can be hidden, hidden methods are only hidden when the reference (not the object) is of the derived type so
NewInherit example = new NewInherit();
InheritanceTesting secondReference = example;
example.InheritanceA();
secondreference.InheritanceA();
Will produce InheritanceB - Two first and InheritanceB - One second. this is because (at least in the simple cases) invocation of final methods is bound at compile time based on the reference type. This has a performance benifit. Binding of virtual methods needs to be differed to runtime as the compiler may not be aware of the instances class.
In practice method hiding is not widely used, and some organisation have coding standards forbidding it. the normal practice is to mark the methods you expect a sub-class to be able to override as virtual and in the sub-class use the keyword override.
More directly answering your questions
Does this mean that the situation is the opposite in C#? Object only
"knows" its declared type?
No, c# knows both the constructed type (instance) and the declared type (reference). It uses the instance type for overridden methods and the declared type for final methods even if the instance has hidden the method.
If so, what is the logic/advantage in that?
No the case, I believe there are performance benefits in binding at compile time where possible, e.g. allow in-lining of methods. Also as explained there is not a loss of flexibility as you can have the same behaviour as Java by using the virtual and override keywords.

Java treats methods as virtual by default, C# methods are non-virtual by default. If you want the same behavior in C# use the virtual keyword. In Java you can use final to ensure an inherited class doesn't override a method.
The reason C# methods are not virtual by default is most likely to prevent people from being able to change every inherited functions behavior on the fly in ways the base class designer didn't intend. This gives more control to the base class designer and ensures that inheritance is carefully and willfully planned for rather than just done on the fly.

Java makes methods virtual by default, while in C# you have to explicitly enable virtual inheritance.
That's why you added the new modifier right? Because you got a warning about it? That's because without a method being virtual, you replace it statically if in a derived class you redefine it. If instead of calling InheritanceOne() through a base pointer you call it through a derived pointer you'll get the result you expect -- the compiler chooses non-virtual methods at compile time, based on compile time only information.
TL;DR: Anytime you want to use inheritance for a method, make it virtual in C#. new is one of the worst things in the language for methods, it has no real use and only adds gotchas to your code.

You might think that you have written the same thing (same words), though you did not (you used new keyword in c# version) but C# equivalent of what you wrote in Java is this.
class InheritanceTesting
{
public virtual void InheritanceOne()
{
Console.WriteLine("InheritanceOne");
}
}
class NewInherit : InheritanceTesting
{
public override void InheritanceOne()
{
Console.WriteLine("InheritanceTwo");
}
}
In Java, by default all methods, except privates and statics of course, are virtual and any method has same signature with a super class method is an override.

By default, every method in Java can be overridable by its sub classes(unless private/static etc).
In C#, you have to make a method virtual if it has to be overriden by sub classes.
In your C# example, its not a overriden method so the behaviour is expected.

Reason for not supporting dynamic polymorphism for static methods in Java

Why does not Java support dynamic polymorphism for static methods?
If the answer is "static methods are not supposed to be called on instances and hence method call is not needed to be resolved at runtime", then further question is 'Why does Java allow me to call static methods on instances?'. Why doesn't it simply block the user from calling the method on instances directly by giving some compile time error.
Otherway round, What would have gone wrong if Java would have supported Runtime Polymorphism for static methods?

Why does Java allow me to call static methods on instances?
Your assumption is wrong. It never calls on the instance of the class. It always called on class.
Try below sample code and you will never get NullPointerException
class ABC {
public static void hello() {
System.out.println("Hello");
}
}
ABC abc = null;
abc.hello();
What would have gone wrong if Java would have supported Runtime Polymorphism for static methods?
Polymorphism comes into picture when you override the method in subclass. since static method belong to class hence there is no meaning of overriding static methods. Hence Polymorphism always works for instance methods only that belongs to the instances of the class.

Static methods are resolved based on a variable's type and not the instance's class. This allows for some optimisations, since the exact method to be called is always known at compile time. Allowing polymorphic static methods would prevent this.
A result of allowing static methods to be called on instances is the following.
class A {
static void func() {}
}
class B extends A {
static void func() {}
}
B b = new B();
A a = b;
b.func(); // calls B.func()
a.func(); // same instance, but calls A.func()
Highly confusing, and counter intuitive. Based on how static methods were implemented allowing static methods to be called on instances was a major design flaw and should always be avoided.
By definition static methods do not require an instance to be called. By allowing polymorphic calls you require an instance, and if you require an instance to determine which method to call then why is the method static?

Dynamic Polymorphism can be supported for method overriding. But for static method overriding is not possible because it leads us to method hiding. Hence Dynamic polymorphism is not supported for static methods.

I'm glad I can call a static method through an instance of a class. It lets me call the method from the instance without qualification. And my IDE warns me if I try to (legally but irrationally) call it through an instance.
public static void foo() {
//yadda yadda
}
public void bar() {
foo(); // this is legal
MyClass.foo() // this is also legal, but would be necessary if I couldn't call it through the instance
}
// in some other class:
new MyClass().foo() // this is legal but silly and my IDE warns me about it

Java - Class Variable

I have a variable: Abstract a. It is a class that extends the Abstract class. However, I need to cast this Abstract variable into a more specific class variable that extends the Abstract class.
My situation: I have two classes, Class1 and Class2 that both extend the Abstract class with methods implemented in each one. I now have an Abstract class variable to work with. I do not know if it is Class1 or Class2, so I cannot simply do a (Class1) a or a (Class2) a (casting).
So how would I successfully cast this variable so that I can use the inner methods?
I was thinking along the lines of using a.getClass().getName() to determine how to cast it, but I am stuck from here on out.

Your new question appears to be asking how to dynamically cast a variable to an arbitrary type unknown at runtime. This is probably a duplicate of java: how can i do dynamic casting of a variable from one type to another? but to summarize, this is not (easily) possible, isn't recommended, and speaks to other issues in your code.
Think about it this way, what variable would you possibly be able to use to store your newly cast object? Imagine if we had a (child) cast operation in Java, that took a variable defined as a parent class, and cast it down to its child (e.g. List -> LinkedList):
public static void func(Abstract a){
???? var = (child)a;
// Do something with var?
}
Notice that 1) there's no way you could ever specify a type for var, since we don't know at runtime what type it will be; and 2) there's nothing we'd be able to do with var beyond the behavior defined in Abstract anyways, because the compiler can't predict which methods will be availible to var other than what's available to Abstract.
If you need to implement class-specific behavior, you should do so inside the class. Have an abstract method which each class has to implement, and which can do whatever you need them to do. Or, if you cannot ensure that, don't define a function that takes an Abstract as an argument; instead define however many functions that take Class1, Class2, etc. objects as parameters, like so:
Abstract method to require all child classes behave similarly
public abstract class Abstract{
/** Do the class-specific behavior you want to do currently in func */
public abstract void operation();
public static void func(Abstract a){
a.operation();
}
}
Functions only for classes that can actually handle what you want
public static void func(Class1 a){
// do something
}
public static void func(Class2 a){
// do something
}
Again, if neither of these options are viable for you (and of course, blocks of instanceof calls aren't acceptable) then I'd be willing to bet money there's something structural in the way you're using Java that's fundamentally incorrect. If you want to post a code sample of exactly what you're trying to accomplish by child-casting, perhaps we can shed some light as to what the issue is.
Leaving this here for posterity - OP's original question asked about creating new instances of an object cast as its abstract parent.
Pretty straightforward, get the object's class object, and create a new instance. For more complex constructors, see the Java documentation on creating new instances dynamically.
public class ClassVar
{
public static abstract class Abstract
{
}
public static class Class1 extends Abstract
{
}
public static class Class2 extends Abstract
{
}
/**
* Given an instance of a child of Abstract, returns a new instance
* of the same class
*/
public static Abstract newInstance(Abstract obj) throws InstantiationException, IllegalAccessException
{
return obj.getClass().newInstance();
}
public static void main(String[] args) throws InstantiationException, IllegalAccessException
{
System.out.println(newInstance(new Class1()).getClass());
System.out.println(newInstance(new Class2()).getClass());
}
}
Result:
class ClassVar$Class1
class ClassVar$Class2

Basically you can use reflection by using
Class cl = ...
cl.newInstance()
The more 'expanded' answer you can find here

Since you edited your question again 3h ago at the time of writing here's my second answer to a problem I thought was solved. It's obvious nobody got what you're really asking for in the first place. Try to improve how you're asking questions.
However, the answer is simple:
From the point of view of object orientation you simply shouldn't have to (Liskov Substitution principle). Even if you have exact knowledge about exactly two possible instances, you should look for a better approach for the problem you are trying to model.
If you have to, determine the class name and check for equality or carry an extra identifier and compare that one. Implementation couldn't be simpler.

Can I use methods of a class without instantiating this class?

I have a class with several methods and there is no constructor among these methods.
So, I am wondering if it is possible to call a method of a class without a creation of an instance of the class.
For example, I can do something like that:
NameOfClass.doMethod(x1,x2,...,xn)
In general I do not see why it should be impossible. I just call a function which does something (or return some values). If it is possible, what will happen if the method sets a value for a private variable of the class. How can I reach this value? In the same way?
NameOfClass.nameOfVariable

It's called static variables and static methods. Just try it and see that it compiles.

If the methods are static, yes.
But you won't be able to access non-static members.

1) YES, you can use the methods of a class without creating an instance or object of that class through the use of the Keyword "Static".
2) If you declare the method as "Static" then you can call this method by :
*ClassName.MethodName()*
3) E.g.
class Hello {
public static void print()
{
System.out.println("HelloStatic");
}
}
class MainMethod {
public static void main(String args[])
{
// calling static method
Hello.print();
}
}
4) The output of the above program would be : HelloStatic

As many have pointed out: This is only possible if the method is static. Maybe some OOP background is in order: A method should always belong to a class. So what is the use of calling a method without an instance of a class? In a perfect OO world there shouldn't be any reason to do that. A lot of use cases that have to do with static methods talk about assigning some kind of identity to your class. While this is perfectly reasonable in a programming world it isn't very convincing when it comes to object oriented design.
As we program in an imperfect world there is often a use case for a "free function" (The way Java or C++ implement sort() for example). As Java has no direct support for free functions classes with only static "methods" are used to express those semantics with the added benefit of the class wrapper providing a "namespace". What you think of this workaround and if you see it as a flaw in language design is IMO a matter of opinion.

In most languages you can do it only if method is static. And static methods can change only static variables.

I have a class with several methods and there is no constructor among these methods.
If you don't explicitly define a constructor then you get a default constructor provided by the compiler. So if those methods aren't static, try this:
NameOfClass x = new NameOfClass();
x.doMethod(x1,x2,...,xn);

A method on a class operates in the context an instance; it has access to the instance's member variables. You understand this, because you ask about what happens if the method accesses one of those variables.
You can understand why it doesn't work by asking yourself the question: "Where is the data?" If you don't have an instance, where is the instance variable? Where is the data? And the answer is that it doesn't have a place and therefore doesn't work.
The difference with a static function and static member variables is that you can answer the question about the location of the data. The static variables available regardless of whether there is a specific instance or not. The instance specific vs. class specific decision is one that you must make considering what you actually want to do.

That would be static methods.

I have a class with several methods
and there is no constructor among
these methods.
Do you mean you have something like:
public class X
{
public void foo()
{
}
}
or do you mean you have something like:
public class X
{
private X()
{
}
public void foo()
{
}
}
If it is the fist way then, yes, there is a constructor and it will look like this:
public X()
{
super();
}
if it is the second way then there is probably a method like:
public static X createInstance()
{
return (new X());
}
If you really mean can classes have methods that do things without ever creating an instance, then yes you can, just make all of the methods and variables static (usually this is not a good idea, but for some things it is perfect).

Since qre is a static method and doesn't have an access to instances of the enclosing class you'll have first to create an instance and then access it. For example:
public class Foo {
private int bar;
public static void qre() {
Foo foo = new Foo();
foo.bar = 5;
System.out.println("next bar: " + (++5));
}
}

In proper encapsulation, you should not "see" what is happening upon instanciation. To rely on a class's lack of a constructor is breaking this form. The designed of the class my have in mind to add formal state initialization in the constructor at a later date. Your "contract" with the class is only that you can use the methods as they are currently designed.
If you desire to use the functionality of that method without the class overhead, maybe it best for you to include that method in your existing "client" class (of course this is just "copy and paste" coding and is considered an anti-pattern of of software design.

If you are using lombok, here is what you can do:
package util;
import lombok.experimental.UtilityClass;
#UtilityClass
public class UtilFunctions {
public static int SumOfTwoNumber(int a, int b){
return a+b;
}
}
In UtilFunctions class here with #UtilityClass annotation, you can keep all the functions which you want to call anywhere in your project. For the sake of simplicity, I have created a function SumOfTwoNumber which I want to use in my main class. Here is how I can call it in the main class:
import util.UtilFunctions;
public class HeadFirstJavaApplication {
public static void main(String[] args) {
int a = 10, b = 20;
int sum = UtilFunctions.SumOfTwoNumber(a,b);
System.out.println(sum);
}
}
P.S: Don't forget to add lombok dependency to be able to use it. In case of gradle, here is how to add the lombok dependency:
dependencies {
compileOnly 'org.projectlombok:lombok:1.18.16'
annotationProcessor 'org.projectlombok:lombok:1.18.16'
testCompileOnly 'org.projectlombok:lombok:1.18.16'
testAnnotationProcessor 'org.projectlombok:lombok:1.18.16'
}