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Require subclass to implement a static method [duplicate]

This question already has answers here:
Is there a way to make sure classes implementing an Interface implement static methods?
(9 answers)
Closed 2 years ago.
Coming from Python and Objective-C land, I may not fully understand what static methods are in Java, I think of them as "methods that operate on all members of the class" or "class-specific methods that are available when you don't have an instance of that class."
But is there a syntax for saying: "This abstract superclass requires each concrete subclass to implement this static method"? I know that static abstract isn't permitted, but it would conceptually be something like this:
public abstract class Lander {
#RequireImplmentationInSubclass // clearly my made-up name...
static abstract boolean probe(Radio radio);
}
public class MarsLander extends Lander {
static boolean probe(Radio radio) {
// ... some MarsLander specific implementation
}
}
public class LunarLander extends Lander {
static boolean probe(Radio radio) {
// ... some LunarLander specific implementation
}
}
update
... and somewhere else, a factory method does something like:
if (MarsLander.probe(radio)) {
... create an instance of MarsLander and work with it
} else if (LunarLander.probe(radio)) {
... create an instance of LunarLander and work with it
}
In my application, creating an instance invokes a lot of machinery, so I need to call probe() on a class method before I create an instance of the class.
I looked over Is there a way to make sure classes implementing an Interface implement static methods? and most of the responses were "why would you want to do that?".
I hope this example makes it clear(er). Perhaps there's' a more Java-esque way to remind the developer that a class-visible probe() method is required?

There is no way to require a static method.
But you can create a factory class with non-static methods:
public abstract class LanderFactory<L extends Lander> {
public abstract L createLander();
public abstract boolean probe(Radio radio);
}
You can even have a registry of LanderFactory implementations, so they effectively act as singletons:
public abstract class LanderFactory<L extends Lander> {
private static final Map<Class<? extends Lander>,
LanderFactory<? extends Lander>> registry
= Map.of(MarsLander.class, new MarsLanderFactory(),
LunarLander.class, new LunarLanderFactory());
public static LanderFactory<? extends Lander> getInstance(
Class<? extends Lander> type) {
LanderFactory<? extends Lander> factory = registry.get(type);
if (factory == null) {
throw new IllegalArgumentException("No factory known for " + type);
}
return factory;
}
public abstract L createLander();
public abstract boolean probe(Radio radio);
}
public class MarsLanderFactory extends LanderFactory<MarsLander> {
#Override
public MarsLander createLander() {
return new MarsLander();
}
#Override
public boolean probe(Radio radio) {
// ...
}
}
public class LunarLanderFactory extends LanderFactory<LunarLander> {
#Override
public LunarLander createLander() {
return new LunarLander();
}
#Override
public boolean probe(Radio radio) {
// ...
}
}

"methods that operate on all members of the class"
This is not right. static methods operate on no members of a class.
Use of this is prohibited in static contexts.
I feel your question itself has the answer you are looking for. static abstract doesnt exists because on one hand you want it to behave differently (based on if it is a LunarProbe or a MarsProbe) and on the other hand you want it to be independent of the instance of LunarProbe and MarsProbe. static abstract contradicts itself.
Besides
static boolean probe() {
// ... some MarsLander specific implementation
// what goes here? you dont have access to any instance of this class.
}
static abstract violates a tenet of OOP called polymorphism.

I tried your code(and other things surrounding this), and I'd like to tell you there's a way but I don't think there is.
I would instead recommend using a utility class that supports this static functionality you're looking for.For example, a LanderUtility class that has static methods in it might solve this in a reasonable way.
Mostly though, I don't think of using static methods in that way in Java. The real power in what is going on with this abstract class is that you can count on(somewhat) a certain type of behavior from a child. Notably, for this to matter, the child needs to be instantiated in the first place and you can use a normal, non-static method and have the child implement that instead.

In Java, static methods is allowed to be invoked directly on class references. It does not matter whether it is a abstract class or a normal class.
Due to this reason, static and abstract modifiers can't be used together. Otherwise, what would be the output of this code if the lander method is not implemented ? That's why it is illegal.
Lander.probe();
But, I think we can throw an exception from our super class static method with some hint in the exception message. We can enforce implementation of the static method.
public abstract class Lander {
static boolean probe() {
throw new RuntimeException("Oh oh !!! Why am I here ??");
}
}
public class MarsLander extends Lander {
static boolean probe() {
// ... some MarsLander specific implementation
}
}
public class LunarLander extends Lander {
static boolean probe() {
// ... some LunarLander specific implementation'
}
}
That way, if there is a Lander implementation which has not implemented a static probe method, the probe method will get inherited but this will throw a run time exception.

Methods can't be both abstract and static, because abstract methods have to be overridden, and static methods in Java cannot be overridden. Which class implementation of a static method is used is determined at compile time. You can declare a static method with the same signature in a subclass, but it is not considered overriding because there won’t be any run-time polymorphism. The static method of the superclass is 'masked' in the subclass. If a derived class defines a static method with the same signature as a static method in the base class, the method in the derived class hides the method in the base class.

What's the benefit (if there's any) of using interface only with static method? [duplicate]

I was learning through interfaces when I noticed that you can now define static and default methods in an interface.
public interface interfacesample2 {
public static void method() {
System.out.println("hello world");
}
public default void menthod3() {
System.out.println("default print");
}
}
Kindly explain the difference of the two and also if there's an example of when we would use this would be nice. A little confused on Interfaces.

Differences between static and default methods in Java 8:
1) Default methods can be overriden in implementing class, while static cannot.
2) Static method belongs only to Interface class, so you can only invoke static method on Interface class, not on class implementing this Interface, see:
public interface MyInterface {
default void defaultMethod(){
System.out.println("Default");
}
static void staticMethod(){
System.out.println("Static");
}
}
public class MyClass implements MyInterface {
public static void main(String[] args) {
MyClass.staticMethod(); //not valid - static method may be invoked on containing interface class only
MyInterface.staticMethod(); //valid
}
}
3) Both class and interface can have static methods with same names, and neither overrides other!
public class MyClass implements MyInterface {
public static void main(String[] args) {
//both are valid and have different behaviour
MyClass.staticMethod();
MyInterface.staticMethod();
}
static void staticMethod(){
System.out.println("another static..");
}
}

A static method is a method that applies to the class 'namespace', so to speak. So a static method foo of interface Interface is accessed by Interface.foo(). Note that the function call does not apply to any particular instance of the interface.
A default implementation bar on the other hand, is called by
Interface x = new ConcreteClass();
x.bar();
A static interface method cannot know about the this variable, but a default implementation can.

1. explain the difference of the two
Static interface methods are like static class methods(here they belong to Interface only). Where as the default interface methods provide default implementation of interface methods (which implementing classes may override)
But remember in case a class is implementing more than one interface with same default method signature then the implementing class needs to override the default method
You can find a simple example below (can DIY for different cases)
public class Test {
public static void main(String[] args) {
// Accessing the static member
I1.hello();
// Anonymous class Not overriding the default method
I1 t = new I1() {
#Override
public void test() {
System.out.println("Anonymous test");
}
};
t.test();
t.hello("uvw");
// Referring to class instance with overridden default method
I1 t1 = new Test2();
t1.test();
t1.hello("xyz");
}
}
interface I1 {
void test();
//static method
static void hello() {
System.out.println("hello from Interface I1");
}
// default need not to be implemented by implementing class
default void hello(String name) {
System.out.println("Hello " + name);
}
}
class Test2 implements I1 {
#Override
public void test() {
System.out.println("testing 1234...");
}
#Override
public void hello(String name) {
System.out.println("bonjour" + name);
}
}
2. when we would use this would be nice.
That depends on your problem statement. I would say Default methods are useful, if you need same implementation for a method in your specification in all the classes in that contract, Or it may be used like Adapter classes.
here is a good read: https://softwareengineering.stackexchange.com/questions/233053/why-were-default-and-static-methods-added-to-interfaces-in-java-8-when-we-alread
also below oracle doc explains default & static methods for evolving existing interfaces:
Users who have classes that implement interfaces enhanced with new
default or static methods do not have to modify or recompile them to
accommodate the additional methods.
http://docs.oracle.com/javase/tutorial/java/IandI/nogrow.html

Here is my view:
static method in interface:
You can call it directly (InterfacetA.staticMethod())
Sub-class will not be able to override.
Sub-class may have method with same name as staticMethod
default method in interface:
You can not call it directly.
Sub-class will be able to override it
Advantage:
static Method: You don't need to create separate class for utility method.
default Method: Provide the common functionality in default method.

This link has some useful insights, have listed few of them here.
default & static methods have bridged down the differences between interfaces and abstract classes.
Interface default methods:
It helps in avoiding utility classes, such as all the Collections class method can be provided in the interfaces itself.
It helps in extending interfaces without having the fear of breaking implementation classes.
Interface static methods:
They are part of interface, we can’t use it for implementation class objects.
It helps in providing security by not allowing implementation classes to override them.
Like to quote another useful reference.

As per Java14 JLS doc:
Default Method:
It is an instance method declared in an interface with the default
modifier
It can be accessed only by the instance of the implementing class
only
Its body is always represented by a block, which provides a default
implementation or behaviour for any implementing class without
overriding the method
It can never be static or private
Static Method:
It can be invoked by interface without reference to a particular
object, just like class static methods
Static method can be private
The implementing class can not access static method
Lets understand it with the help of below example code:
public interface MyInterface {
private void privateMethod() {
System.out.println("Hi, this is privateMethod");
}
private static void staticPrivateMethod() {
System.out.println("Hi, this is staticPrivateMethod");
}
static void staticMethod() {
//privateMethod(); // Non-static method cannot be referenced from a static contex
System.out.println("Hi, this is staticMethod");
staticPrivateMethod();
}
default void defaultMethod() {
System.out.println("Hi, this is defaultMethod");
}
}
public class MyInterfaceImpl implements MyInterface{
public static void main(String[] args) {
MyInterface.staticMethod();
// myInterface.staticMethod(); // Not allowed
MyInterface myInterface = new MyInterfaceImpl();
myInterface.defaultMethod();
// MyInterface.defaultMethod(); // Not allowed
}
}

According to Oracle's Javadocs: http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.
A static method is a method that is associated with the class in which it is defined rather than with any object. Every instance of the class shares its static methods.
Normally, static method in interface is used as Helper methods while default method are used as a default implementation for classes that implements that interface.
Example:
interface IDemo {
//this method can be called directly from anywhere this interface is visible
static int convertStrToInt(String numStr) {
return Integer.parseInt(numStr);
}
//getNum will be implemented in a class
int getNum();
default String numAsStr() {
//this.getNum will call the class's implementation
return Integer.toString(this.getNum());
}
}

Interface default methods:
It helps in avoiding utility classes, such as all the Collections class method can be provided in the interfaces itself.
It helps in extending interfaces without having the fear of breaking implementation classes.
Interface static methods:
They are part of interface, we can’t use it for implementation class objects.
It helps in providing security by not allowing implementation classes to override them.
Now how static method providing security. Let's see an example.
interface MyInterface {
/*
* This is a default method so we need not to implement this method in the implementation classes
*/
default void newMethod() {
System.out.println("Newly added default method in Interface");
}
/*
* This is a static method. Static method in interface is similar to default method except that we cannot override them in the implementation classes. Similar to default methods, we need to implement these methods in implementation classes so we can safely add them to the existing interfaces.
*/
static void anotherNewMethod() {
System.out.println("Newly added static method in Interface");
}
/*
* Already existing public and abstract method We must need to implement this method in implementation classes.
*/
void existingMethod(String str);
}
public class Example implements MyInterface {
// implementing abstract method
public void existingMethod(String str) {
System.out.println("String is: " + str);
}
public void newMethod() {
System.out.println("Newly added default method in Class");
}
static void anotherNewMethod() {
System.out.println("Newly added static method in Class");
}
public static void main(String[] args) {
Example obj = new Example();
// calling the default method of class
obj.newMethod();
// calling the static method of class
obj.anotherNewMethod();
// calling the static method of interface
MyInterface.anotherNewMethod();
// calling the abstract method of interface
obj.existingMethod("Java 8 is easy to learn");
}
}
Here obj.newMethod(); printing class implementation logic, means we can change the logic of that method inside implementation class.
But obj.anotherNewMethod(); printing class implementation logic ,but not changed interface implementation. So if any encryption-decryption logic written inside that method you can't change.

we cannot execute Interfacesample2.menthod3(); because it is not static method. In order to execute method3() we need an instance of Interfacesample2 interface.
Please find the following practical example:
public class Java8Tester {
public static void main(String args[]){
// Interfacesample2.menthod3(); Cannot make a static reference to the non-static method menthod3 from the type Interfacesample2
new Interfacesample2(){ }.menthod3();// so in order to call default method we need an instance of interface
Interfacesample2.method(); // it
}
}
interface Interfacesample2 {
public static void method() {
System.out.println("hello world");
}
public default void menthod3() {
System.out.println("default print");
}
}

Starting Java 8 interface can also have static method. Like static method of a class, static method of an interface can be called using Interface name.
Example
public interface Calculator {
int add(int a, int b);
int subtract(int a, int b);
default int multiply(int a, int b) {
throw new RuntimeException("Operation not supported. Upgrade to UltimateCalculator");
}
static void display(String value) {
System.out.println(value);
}
}
Difference between static and default method of interface is default method supports inheritance but static method does not. Default method can be overridden in inheriting interface.
Here is good read about interface default method and static method. Interface Default Method in Java 8

All good answers here. I would like to add another practical usage of the static function in the interface. The tip is coming from the book - Effective Java, 3rd Edition by Joshua Bloch in Chapter2: Creating and Destroying Object.
Static functions can be used for static factory methods.
Static factory method are methods which return an object. They work like constructor. In specific cases, static factory method provides more readable code than using constructor.
Quoting from the book - Effective Java, 3rd Edition by Joshua Bloch
Prior to Java 8, interfaces couldn’t have static methods. By
convention, static factory methods for an interface named Type were
put in a noninstantiable companion class (Item 4) named Types.
Author gives an example of Collections where such static factory method is implemented. Checking on the code, Josh Bloch can be seen as first author of Collections class. Although Collections is a class and not interface. But the concept still applies.
For example, the Java Collections Framework has forty-five utility
implementations of its interfaces, providing unmodifiable collections,
synchronized collections, and the like. Nearly all of these
implementations are exported via static factory methods in one
noninstantiable class (java.util.Collections). The classes of the
returned objects are all nonpublic.
Further he explains that API is not only smaller, it helps with the code readability and API ease..
It is not just the bulk of the API that is reduced but the conceptual
weight: the number and difficulty of the concepts that programmers
must master in order to use the API. The programmer knows that the
returned object has precisely the API specified by its interface, so
there is no need to read additional class documentation for the
implementation class.
Here is one of the static method from java.util.Collections class:
public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
return new UnmodifiableCollection<>(c);
}

Static Interface Method:
It is a static method which belongs to the interface only. We can
write implementation of this method in interface itself.
Static method can invoke only on interface class not on class.
Interface and implementing class , both can have static method with the same name without overriding each other.
It can be used as a utility method
Default Method:
It is a method with default keyword and class can override this method.
It can be invoked on interface as well as class.
We can override the default method in implementing class.
It can be used to provide common functionality in all implementing classes.
There is a link with detailed explanation. For detailed example: Default method vs static method in an interface in Java?

Why cannot we override static method in the derived class [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Can we override static method in Java?
We cannot override the static methods of the base class.
Actually I tried something like this:
// Base class
public class StaticExampleImpl {
protected String name="overriding";
public static void display(){
System.out.println("static method display : base class");
}
}
Then the derived class is as follows:
//derived class
public class StaticDemo extends StaticExampleImpl {
// cannot override the static methods...
//#Override
public static void display(){
System.out.println("child!!! static method display");
}
public static void main(String[] args) {
StaticDemo d=new StaticDemo();
d.display(); // derived class display is called rather than Base class.
}
}
So, when I uncomment the #Override method, it gives error as "Static methods cannot be overriden". But with commenting it works fine. So, when we create the Objects and call the static methods with the instances, those work fine. so what is the difference??

because static methods are not get inherited.
When you uncomment #Override it means you are trying to override the
static method which is not possible thats why you are getting an
error.
But when you comment //#Override it means you are declaring a new
method in child class.

Static methods does not belong to an instance of a class, it belongs to the actual class.
When you call d.display();, you are really calling the static method of the StaticDemo d reference's static method.
if you did :
StaticExampleImpl d2 = new StaticDemo();d2.display(), you will find that it calls the base class's display.
However, don't do this. It leads to confusing code, and is a bad way to implement inheritance poorly.

Overriding depends the an instance of a class. polymorphismis that you can subclass a class and the objects implementing those subclasses will have different behaviors for those method defined in the superclass (and overridden in the subclasses) .static methods does not belong to an instance of a class so the concept is not applicable.

Static methods cannot be inherited. If you want to call the 'base' class static method, you have to explicitely call StaticExampleImpl.display().

Static methods are bound to class they can't be inherited thats why you can't have base class static method in derived class.

If you are trying to override a static method, there is probably something wrong with your design.
OOP and Polymorphism allows you to do the following:
public class MyClass1 {
public String toString() { return "MyClass1 Instance"; }
}
public class MyClass2 extends MyClass1 {
#Override
public String toString() { return "MyClass1 Instance"; }
}
public void printSomething(MyClass1 myclass1){
System.out.println(myclass1);
}
Inside printSomething, the toString method which is going to be called is the one on the runtime type of myClass1: when you pass inside printSomething an instance of MyClass2, its compile-type will be MyClass1 but its runtime type will be MyClass2
It is clear that to use polymorphism you need objects instances, where the actual runtime type could different from the compile type. Static methods however do not belong to any object instance, but to the class. Why don't you explain us what you are trying to achieve?

The following code:
StaticExampleImpl one = new StaticExampleImpl();
StaticDemo two = new StaticDemo();
StaticExampleImpl three = two;
one.display();
two.display();
three.display();
Will yield the following output:
static method display : base class
child!!! static method display
static method display : base class
As you can see, the method does not get inherited. This is why they are called 'static methods': they are called statically, not dynamically, as instance methods would be. The compile-time type of the class is what matters when calling static methods, not the runtime type.
This is also why you shouldn't call static methods through object instances. Always call them like this:
StaticExampleImpl.display();
StaticDemo.display();
This completely takes away the confusion that might (will) come up when people expect inheritance to work for these methods.

any static block in java, may be static variables, methods are loaded when the class is loaded. You probably know about class loader in java. So thing is static block (methods, variables or anything is static) is loaded once. So you can’t actually override any static block.
Commenting #Override means that you are writing another static method in sub class, but not just overriding base class method.

How to define nested static classes with static methods, inherited from a nested interface in Java?

I have a Java problem with nested classes.
My first class structure looked like this:
public class TopClass {
public void mainMethod() {
// uses the different "method" methods from
// NestedClass-implementing nested classes
}
private interface NestedClass {
public void method();
}
private class NestedClass1 {
public void method() {
}
}
private class NestedClass2 {
public void method(){
}
}
}
But now I want these method() methods to be static because they should be principally.
I cannot make them static without having them in a static class, but that's no problem, I made the classes static, they should be anyway.
It looks like this right now:
public class TopClass {
public void mainMethod() {
// uses the different "method" methods from
// NestedClass-implementing nested classes
}
private static interface NestedClass {
public void method();
}
private static class NestedClass1 {
public static void method() {
}
}
private static class NestedClass2 {
public static void method(){
}
}
}
But then the trouble begins. A static method does not inherit correctly from a non-static interface method, as I get this message This static method cannot hide the instance method from TopClass.NestedClass in Eclipse.
When I make the interface method static, it gives me this error: Illegal modifier for the interface method method; only public & abstract are permitted
So I thought of an abstract class, and tried this:
public class TopClass {
public void mainMethod() {
// uses the different "method" methods from
// NestedClass-implementing nested classes
}
private static abstract class NestedClass {
public static abstract void method();
}
private static class NestedClass1 {
public static void method() {
}
}
private static class NestedClass2 {
public static void method(){
}
}
}
But again, seemingly abstract methods cannot be declared static: The abstract method method in type NestedClass can only set a visibility modifier, one of public or protected.
Leaving the static away (in the abstract class method), errors this on the method methods in the NestedClass1 & 2: This static method cannot hide the instance method from TopClass.NestedClass.
Isn't there any way to declare some kind of superstructure for covering static methods?
EDIT:
The problem I actually try to solve it the lack of possibility of Java for storing references to methods. So instead I have those classes everyone with just one method, but to store them in a List f.e. they must be able to be "caught" by a superstructure.
I got the hint to try anonymous classes or enums, gonna try that now.

Interfaces and statics don't go together. At all. There is no Java support for creating / imposing patterns on static methods.

A static method declaration must always be followed by a definition. It cannot be implemented by subclasses.
I think you're just not approaching your problem right. Try a different approach!
Make NestedClass an interface NestedInterface and store your different implementations as anonymous classes implementing this interface:
public static final NestedInterface firstNested = new NestedInterface() {
#Override
public void method() {
// ...
}
};
Make NestedClass an enumeration NestedEnum and store your different implementations as enumeration values implementing an abstract method from the enumeration. This only works if you have a fixed number of implementations you which to choose from and you do not want to accept NestedClass implementations from outside sources.
public enum NestedEnum {
FIRST {
#Override
public void method() {
// ...
}
};
public abstract void method();
}
EDIT: In reply to your comment:
The classes itself are static as well..
static in the context of a nested class means that this class can be instantiated without an instance of the containing class.
A regular nested class such as in your first example can be instantiated through TopClass.this.new NestedClass1(). Normally you'd simply write new NestedClass1() from within the constructor or an instance method of TopClass, but in this verbose form you can clearly see the dependence on TopClass.this. This can also be seen from any method of NestedClass1, as you have access to the containing class with TopClass.this.
A static nested class such as in your second example can be instantiated through new TopClass.NestedClass1(). Once again, you could just write new NestedClass1() but the verbose form clearly shows that the construction only depends on TopClass and is not associated with an instance of TopClass. You could even create an instance from an outside class using the same snippet new TopClass.NestedClass1() without ever creating a TopClass instance.
I suggest you take a look at this question on inner classes and static nested classes.

The fact the your interface/abstract class is nested is irrelevant to the problem.
You just can't. There is no way in Java to enforce some class to implement static methods. Just cry and surrender and use instance methods.

static abstract is a contradiction. Static methods are not like other languages' class methods. When you make a static method it goes on a single class, it doesn't get inherited by or have its implementation deferred to subclasses.
You don't explain why you want these methods to be static. If you want these methods to be defined by subclasses then they shouldn't be.

Why can't static methods be abstract in Java?

The question is in Java why can't I define an abstract static method? for example
abstract class foo {
abstract void bar( ); // <-- this is ok
abstract static void bar2(); //<-- this isn't why?
}

Because "abstract" means: "Implements no functionality", and "static" means: "There is functionality even if you don't have an object instance". And that's a logical contradiction.

Poor language design. It would be much more effective to call directly a static abstract method than creating an instance just for using that abstract method. Especially true when using an abstract class as a workaround for enum inability to extend, which is another poor design example. Hope they solve those limitations in a next release.

You can't override a static method, so making it abstract would be meaningless. Moreover, a static method in an abstract class would belong to that class, and not the overriding class, so couldn't be used anyway.

The abstract annotation to a method indicates that the method MUST be overriden in a subclass.
In Java, a static member (method or field) cannot be overridden by subclasses (this is not necessarily true in other object oriented languages, see SmallTalk.) A static member may be hidden, but that is fundamentally different than overridden.
Since static members cannot be overriden in a subclass, the abstract annotation cannot be applied to them.
As an aside - other languages do support static inheritance, just like instance inheritance. From a syntax perspective, those languages usually require the class name to be included in the statement. For example, in Java, assuming you are writing code in ClassA, these are equivalent statements (if methodA() is a static method, and there is no instance method with the same signature):
ClassA.methodA();
and
methodA();
In SmallTalk, the class name is not optional, so the syntax is (note that SmallTalk does not use the . to separate the "subject" and the "verb", but instead uses it as the statemend terminator):
ClassA methodA.
Because the class name is always required, the correct "version" of the method can always be determined by traversing the class hierarchy. For what it's worth, I do occasionally miss static inheritance, and was bitten by the lack of static inheritance in Java when I first started with it. Additionally, SmallTalk is duck-typed (and thus doesn't support program-by-contract.) Thus, it has no abstract modifier for class members.

I also asked the same question , here is why
Since Abstract class says, it will not give implementation and allow subclass to give it
so Subclass has to override the methods of Superclass ,
RULE NO 1 - A static method cannot be overridden
Because static members and methods are compile time elements , that is why Overloading(Compile time Polymorphism) of static methods are allowed rather then Overriding (Runtime Polymorphism)
So , they cant be Abstract .
There is no thing like abstract static <--- Not allowed in Java Universe

This is a terrible language design and really no reason as to why it can't be possible.
In fact, here is a pattern or way on how it can be mimicked in **Java ** to allow you at least be able to modify your own implementations:
public static abstract class Request {
// Static method
public static void doSomething() {
get().doSomethingImpl();
}
// Abstract method
abstract void doSomethingImpl();
/////////////////////////////////////////////
private static Request SINGLETON;
private static Request get() {
if ( SINGLETON == null ) {
// If set(request) is never called prior,
// it will use a default implementation.
return SINGLETON = new RequestImplementationDefault();
}
return SINGLETON;
}
public static Request set(Request instance){
return SINGLETON = instance;
}
/////////////////////////////////////////////
}
Two implementations:
/////////////////////////////////////////////////////
public static final class RequestImplementationDefault extends Request {
#Override void doSomethingImpl() {
System.out.println("I am doing something AAA");
}
}
/////////////////////////////////////////////////////
public static final class RequestImplementaionTest extends Request {
#Override void doSomethingImpl() {
System.out.println("I am doing something BBB");
}
}
/////////////////////////////////////////////////////
Could be used as follows:
Request.set(new RequestImplementationDefault());
// Or
Request.set(new RequestImplementationTest());
// Later in the application you might use
Request.doSomething();
This would allow you to invoke your methods statically, yet be able to alter the implementation say for a Test environment.
Theoretically, you could do this on a ThreadLocal as well, and be able to set instance per Thread context instead rather than fully global as seen here, one would then be able to do Request.withRequest(anotherRequestImpl, () -> { ... }) or similar.
Real world usually do not require the ThreadLocal approach and usually it is enough to be able to alter implementation for Test environment globally.
Note, that the only purpose for this is to enable a way to retain the ability to invoke methods DIRECTLY, EASILY and CLEANLY which static methods provides while at the same time be able to switch implementation should a desire arise at the cost of slightly more complex implementation.
It is just a pattern to get around having normally non modifiable static code.

An abstract method is defined only so that it can be overridden in a subclass. However, static methods can not be overridden. Therefore, it is a compile-time error to have an abstract, static method.
Now the next question is why static methods can not be overridden??
It's because static methods belongs to a particular class and not to its instance. If you try to override a static method you will not get any compilation or runtime error but compiler would just hide the static method of superclass.

A static method, by definition, doesn't need to know this. Thus, it cannot be a virtual method (that is overloaded according to dynamic subclass information available through this); instead, a static method overload is solely based on info available at compile time (this means: once you refer a static method of superclass, you call namely the superclass method, but never a subclass method).
According to this, abstract static methods would be quite useless because you will never have its reference substituted by some defined body.

I see that there are a god-zillion answers already but I don't see any practical solutions. Of course this is a real problem and there is no good reason for excluding this syntax in Java. Since the original question lacks a context where this may be need, I provide both a context and a solution:
Suppose you have a static method in a bunch of classes that are identical. These methods call a static method that is class specific:
class C1 {
static void doWork() {
...
for (int k: list)
doMoreWork(k);
...
}
private static void doMoreWork(int k) {
// code specific to class C1
}
}
class C2 {
static void doWork() {
...
for (int k: list)
doMoreWork(k);
...
}
private static void doMoreWork(int k) {
// code specific to class C2
}
}
doWork() methods in C1 and C2 are identical. There may be a lot of these calsses: C3 C4 etc. If static abstract was allowed, you'd eliminate the duplicate code by doing something like:
abstract class C {
static void doWork() {
...
for (int k: list)
doMoreWork(k);
...
}
static abstract void doMoreWork(int k);
}
class C1 extends C {
private static void doMoreWork(int k) {
// code for class C1
}
}
class C2 extends C {
private static void doMoreWork(int k) {
// code for class C2
}
}
but this would not compile because static abstract combination is not allowed.
However, this can be circumvented with static class construct, which is allowed:
abstract class C {
void doWork() {
...
for (int k: list)
doMoreWork(k);
...
}
abstract void doMoreWork(int k);
}
class C1 {
private static final C c = new C(){
#Override void doMoreWork(int k) {
System.out.println("code for C1");
}
};
public static void doWork() {
c.doWork();
}
}
class C2 {
private static final C c = new C() {
#Override void doMoreWork(int k) {
System.out.println("code for C2");
}
};
public static void doWork() {
c.doWork();
}
}
With this solution the only code that is duplicated is
public static void doWork() {
c.doWork();
}

Assume there are two classes, Parent and Child. Parent is abstract. The declarations are as follows:
abstract class Parent {
abstract void run();
}
class Child extends Parent {
void run() {}
}
This means that any instance of Parent must specify how run() is executed.
However, assume now that Parent is not abstract.
class Parent {
static void run() {}
}
This means that Parent.run() will execute the static method.
The definition of an abstract method is "A method that is declared but not implemented", which means it doesn't return anything itself.
The definition of a static method is "A method that returns the same value for the same parameters regardless of the instance on which it is called".
An abstract method's return value will change as the instance changes. A static method will not. A static abstract method is pretty much a method where the return value is constant, but does not return anything. This is a logical contradiction.
Also, there is really not much of a reason for a static abstract method.

An abstract class cannot have a static method because abstraction is done to achieve DYNAMIC BINDING while static methods are statically binded to their functionality.A static method means
behavior not dependent on an instance variable, so no instance/object
is required.Just the class.Static methods belongs to class and not object.
They are stored in a memory area known as PERMGEN from where it is shared with every object.
Methods in abstract class are dynamically binded to their functionality.

Declaring a method as static means we can call that method by its class name and if that class is abstract as well, it makes no sense to call it as it does not contain any body, and hence we cannot declare a method both as static and abstract.

As abstract methods belong to the class and cannot be overridden by the implementing class.Even if there is a static method with same signature , it hides the method ,does not override it.
So it is immaterial to declare the abstract method as static as it will never get the body.Thus, compile time error.

A static method can be called without an instance of the class. In your example you can call foo.bar2(), but not foo.bar(), because for bar you need an instance.
Following code would work:
foo var = new ImplementsFoo();
var.bar();
If you call a static method, it will be executed always the same code. In the above example, even if you redefine bar2 in ImplementsFoo, a call to var.bar2() would execute foo.bar2().
If bar2 now has no implementation (that's what abstract means), you can call a method without implementation. That's very harmful.

I believe I have found the answer to this question, in the form of why an interface's methods (which work like abstract methods in a parent class) can't be static. Here is the full answer (not mine)
Basically static methods can be bound at compile time, since to call them you need to specify a class. This is different than instance methods, for which the class of the reference from which you're calling the method may be unknown at compile time (thus which code block is called can only be determined at runtime).
If you're calling a static method, you already know the class where it's implemented, or any direct subclasses of it. If you define
abstract class Foo {
abstract static void bar();
}
class Foo2 {
#Override
static void bar() {}
}
Then any Foo.bar(); call is obviously illegal, and you will always use Foo2.bar();.
With this in mind, the only purpose of a static abstract method would be to enforce subclasses to implement such a method. You might initially think this is VERY wrong, but if you have a generic type parameter <E extends MySuperClass> it would be nice to guarantee via interface that E can .doSomething(). Keep in mind that due to type erasure generics only exist at compile time.
So, would it be useful? Yes, and maybe that is why Java 8 is allowing static methods in interfaces (though only with a default implementation). Why not abstract static methods with a default implementation in classes? Simply because an abstract method with a default implementation is actually a concrete method.
Why not abstract/interface static methods with no default implementation? Apparently, merely because of the way Java identifies which code block it has to execute (first part of my answer).

Because abstract class is an OOPS concept and static members are not the part of OOPS....
Now the thing is we can declare static complete methods in interface and we can execute interface by declaring main method inside an interface
interface Demo
{
public static void main(String [] args) {
System.out.println("I am from interface");
}
}

Because abstract mehods always need implementation by subclass.But if you make any method to static then overriding is not possible for this method
Example
abstract class foo {
abstract static void bar2();
}
class Bar extends foo {
//in this if you override foo class static method then it will give error
}

Static Method
A static method can be invoked without the need for creating an instance of a class.A static method belongs to the class rather than the object of a class.
A static method can access static data member and also it can change the value of it.
Abstract Keyword is used to implement abstraction.
A static method can't be overriden or implemented in child class. So, there is no use of making static method as abstract.

The idea of having an abstract static method would be that you can't use that particular abstract class directly for that method, but only the first derivative would be allowed to implement that static method (or for generics: the actual class of the generic you use).
That way, you could create for example a sortableObject abstract class or even interface
with (auto-)abstract static methods, which defines the parameters of sort options:
public interface SortableObject {
public [abstract] static String [] getSortableTypes();
public String getSortableValueByType(String type);
}
Now you can define a sortable object that can be sorted by the main types which are the same for all these objects:
public class MyDataObject implements SortableObject {
final static String [] SORT_TYPES = {
"Name","Date of Birth"
}
static long newDataIndex = 0L ;
String fullName ;
String sortableDate ;
long dataIndex = -1L ;
public MyDataObject(String name, int year, int month, int day) {
if(name == null || name.length() == 0) throw new IllegalArgumentException("Null/empty name not allowed.");
if(!validateDate(year,month,day)) throw new IllegalArgumentException("Date parameters do not compose a legal date.");
this.fullName = name ;
this.sortableDate = MyUtils.createSortableDate(year,month,day);
this.dataIndex = MyDataObject.newDataIndex++ ;
}
public String toString() {
return ""+this.dataIndex+". "this.fullName+" ("+this.sortableDate+")";
}
// override SortableObject
public static String [] getSortableTypes() { return SORT_TYPES ; }
public String getSortableValueByType(String type) {
int index = MyUtils.getStringArrayIndex(SORT_TYPES, type);
switch(index) {
case 0: return this.name ;
case 1: return this.sortableDate ;
}
return toString(); // in the order they were created when compared
}
}
Now you can create a
public class SortableList<T extends SortableObject>
that can retrieve the types, build a pop-up menu to select a type to sort on and resort the list by getting the data from that type, as well as hainv an add function that, when a sort type has been selected, can auto-sort new items in.
Note that the instance of SortableList can directly access the static method of "T":
String [] MenuItems = T.getSortableTypes();
The problem with having to use an instance is that the SortableList may not have items yet, but already need to provide the preferred sorting.
Cheerio,
Olaf.

First, a key point about abstract classes -
An abstract class cannot be instantiated (see wiki). So, you can't create any instance of an abstract class.
Now, the way java deals with static methods is by sharing the method with all the instances of that class.
So, If you can't instantiate a class, that class can't have abstract static methods since an abstract method begs to be extended.
Boom.

As per Java doc:
A static method is a method that is associated with the class in which
it is defined rather than with any object. Every instance of the class
shares its static methods
In Java 8, along with default methods static methods are also allowed in an interface. This makes it easier for us to organize helper methods in our libraries. We can keep static methods specific to an interface in the same interface rather than in a separate class.
A nice example of this is:
list.sort(ordering);
instead of
Collections.sort(list, ordering);
Another example of using static methods is also given in doc itself:
public interface TimeClient {
// ...
static public ZoneId getZoneId (String zoneString) {
try {
return ZoneId.of(zoneString);
} catch (DateTimeException e) {
System.err.println("Invalid time zone: " + zoneString +
"; using default time zone instead.");
return ZoneId.systemDefault();
}
}
default public ZonedDateTime getZonedDateTime(String zoneString) {
return ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString));
}
}

Because 'abstract' means the method is meant to be overridden and one can't override 'static' methods.

Regular methods can be abstract when they are meant to be overridden by subclasses and provided with functionality.
Imagine the class Foo is extended by Bar1, Bar2, Bar3 etc. So, each will have their own version of the abstract class according to their needs.
Now, static methods by definition belong to the class, they have nothing to do with the objects of the class or the objects of its subclasses. They don't even need them to exist, they can be used without instantiating the classes. Hence, they need to be ready-to-go and cannot depend on the subclasses to add functionality to them.

Because abstract is a keyword which is applied over Abstract methods do not specify a body. And If we talk about static keyword it belongs to class area.

because if you are using any static member or static variable in class it will load at class loading time.

There is one occurrence where static and abstract can be used together and that is when both of these modifiers are placed in front of a nested class.

In a single line, this dangerous combination (abstract + static) violates the object-oriented principle which is Polymorphism.
In an inheritance situation, the JVM will decide at runtime by the implementation in respect of the type of instance (runtime polymorphism) and not in respect of the type of reference variable (compile-time polymorphism).
With #Overriding:
Static methods do not support #overriding (runtime polymorphism), but only method hiding (compile-time polymorphism).
With #Hiding:
But in a situation of abstract static methods, the parent (abstract) class does not have implementation for the method. Hence, the child type reference is the only one available and it is not polymorphism.
Child reference is the only one available:
For this reason (suppress OOPs features), Java language considers abstract + static an illegal (dangerous) combination for methods.

You can do this with interfaces in Java 8.
This is the official documentation about it:
https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html

Because if a class extends an abstract class then it has to override abstract methods and that is mandatory. And since static methods are class methods resolved at compile time whereas overridden methods are instance methods resolved at runtime and following dynamic polymorphism.