Why is this valid?
Foo.java
public class Foo {
public Bar getBar() {
return new Bar();
}
private class Bar {}
}
If Bar is private, how will users of this class use this method? Polymorphism can be used of course, but shouldn't this be invalid and the declaration should indicate this as returning an Object?
I've just been doing a bit of research on this and have not been able to find a definitive answer. It seems most likely that it is just an oversight on the part of the Java language designers and since it doesn't actually do any harm it has been left. It's no different really from putting a public method into a private class. Nothing stops you doing this, even though there is no way to actually access that public method.
Certainly NetBeans gives you the warning "Exporting non-public type through public API" when you try to do this. I expect most other environments will give a similar warning.
The returned object is entirely useless to anyone who tries to use it (unless they use reflection), pretty much all they can do is store it into an Object (or any other super class that they do have access to) and then pass that Object around.
You could potentially want to do this if the passed Object is being used as a "handle" that gets passed around but never operated on. In that case though it would still make much more sense to have the class public but make all the methods within it private to prevent them being acted on outside your class (or define a public interface to return and have the private class implement that).
So the answer seems to be:
It probably shouldn't be valid, but as it doesn't do any harm it has never been blocked.
There is a good answer here on a similar subject:
Exporting non-public type through public API
Why is this valid?
Because client code in the call place might be expecting an Object(or not expecting anything at all), there is no problem with calling this method from anywhere:
Object o = new Foo().getBar();
It is valid because Bar is visible to the Foo class. Thus it compiles.
Of course another class can not see Bar and thus can not use the return value.
But another class can still just invoke the method without using the return value.
public class FooBar {
public void invokeBar() {
Foo foo = new Foo();
foo.getBar();
}
}
A public method returning a private class can be useful it you need to be able to call the method from any scope (e.g. to mutate an internal state of Foo), and for internal usage if you need any kind of result in addition of simply calling the method.
public class Foo {
private String myString;
public String getMyString() {
return myString;
}
}
This is valid as well. Why should inner classes behave differently?
Making Bar private only makes it invisible to the outside world just as making fields private.
One important caveat is that even if you are able to call getBar() on a Foo object you can't call methods of that reference (because of the visibility).
So the main thing is that you can do that but you should not do so.
The only situation I can imagine is when Foo is also an inner class and the outer class of Foo wants to use Bar.
Inner class
Inner classes represent a special type of relationship that is it can access all the members (data members and methods) of outer class including private. Nested classes can lead to more readable and maintainable code because it logically group classes in one place only.
It is one of the form of nested types. This kind of class declaration is known as inner class. If you declare the inner class as static, then it would be known as top-level nested class. Other forms of nested types available in Java are local class; class declared and defined within a block,ie, a method or a constructor or an initializer block. The fourth form of nested type is anonymous class; a class without any name whose object is used where the class is defined.
As far as your case is considered, i.e., inner class all the classes within a class can be declared with public, private and protected access specifiers. All the classes with in the enclosing class as well as enclosing class itself share a trust relationship. That means, all the private members of inner class as well as private members of enclosing class is shared among each other. However you cannot access the object of inner class without an object of enclosing class.
When you will try to create an object of inner class compiler would report a compile-time error. However following example access the private members of each other class, i.e., enclosing class access private members of inner class and inner class access private members of enclosing class :
class Bar {
private static int x;
public void getFoo() {
System.out.println(new Foo().y);
}
private class Foo {
private int y;
public void getBar() {
System.out.println(Bar.x);
}
}
}
public class Test{
public static void main(String[] a) {
Bar b = new Bar();
//Bar.Foo f = new Bar.Foo(); This is completely illegal syntax.
}
}
Best example you could have for an inner class is the relationship of an Accounts class which is enclosing class and Transaction class which is inner class. One Accounts class can have more than one Transaction objects but Transaction object cannot exist without Accounts object.
Albeit, returning an object of private inner class is useless as it becomes invisible outside its class. As the above example of Accounts and Transaction class explains. Transaction cannot exists without Accounts object.
I have a perfectly valid use case for this, and I'm glad this is allowed.
Let's stay you have a class that creates UI pieces. It accepts somekind of domain object and creates a piece of UI:
public Node createPersonUI(Person person) {
BasePanel panel = new BasePanel();
// ... setup panel with values ...
return panel;
}
BasePanel is a subclass of Node and is some internal class that the caller has no business with, as this class determines how things will look.
Now, I found myself needing to re-use part of this class when I needed to support a new object, PersonalProfile that contains much more information, but also contains the basic Person data:
public Node createPersonalProfileUI(PersonalProfile profile) {
BasePanel panel = new BasePanel();
// ... setup panel with values ...
return panel;
}
However, that code was partially duplicated, so I did:
public Node createPersonalProfileUI(PersonalProfile profile) {
BasePanel panel = (BasePanel)createPerson(profile.getPerson());
// ... only setup missing values ...
return panel;
}
The cast however is a bit ridiculous -- changing it to return BasePanel not only works, but doesn't expose any functionality of my private class. Instead it only exposes the methods from any public classes it inherits from... brilliant!
Full code:
public BasePanel createPersonUI(Person person) {
BasePanel panel = new BasePanel();
// ... setup panel with values ...
return panel;
}
public BasePanel createPersonalProfileUI(PersonalProfile profile) {
BasePanel panel = createPerson(profile.getPerson());
// ... only setup missing values ...
return panel;
}
private class BasePanel extends Node {
}
if we have 2 class Zoo & Moo as follows:
public class zoo {
String superString="super";
private String coolMethod(){
return "Zoo method";
}
}
public class Moo extends zoo{
public void useMyCoolMethod(){
zoo z=new zoo();
System.out.println(superString);//1
System.out.println(z.superString);//2
}
public static void main(String[]args){
new Moo().useMyCoolMethod();
}
}
at 1 & 2 we print the value of the String in the super class through inheritance and access, the question is , what is the benefit of Access although i can do the same thing by inheritance ? knowing that the Access approach isn't allowed if the two classes is in diff packages
By accessing through the object ref you are modifying the referenced object's state, by inheritance you are modifying the object you are currently in ( this reference ). So there is actually no benefit aside from encapsulation at class level and package level through out the access modifiers and that sort of thing, you just use it depending on the behavior you want for you're code, or in this case, how restrictive to modify the state of objects depending on the context.
But aside from that, i'm not sure if there is anything else.
Regarding your example there is no benefit at all in "access" the method, if you really wants to print "super" twice.
But normally OO programs contains more than two classes and the authors tries to modularize the code with defined interfaces (aka public methods or an API). Create modules with inheritance only is very hard and create clumsy code. So objects will need other instances and call there method by "access".
In line 1, you are using inheritance, i.e. you have a class animal and you have method move() which moves the animal using his four leg. But in case of Kangaroo, you want to use most of the feature of animal class but want to change how it moves based on the fact that it jumps and uses it hind leg for movement.
In Line 2, you are using composition, i.e. when you want to create car, you will need different component and they will interact with each other for that car to function. Here you can not inherit from GearBox or Engine but you have to use them as part of Car (or what you are calling access).
In the end its the relationship between Zoo and Moo will decide what method you want to use
Using this access and inheritance is same when you do Not want to modify the content of the inherited memeber..
eg:
public class A {
String s = "Hello";
}
public class B extends A{
String s = "Hi;
System.out.println(s); // Will print the s in class B
System.out.println(new A().s); // Will print the s in class A
}
Now as String s has no modifier, it is considered having Default modifier, that means it can be accessed by classes only with in its own package.
If you use protected access modifier, then you Need to extend the class and then use the inherited members, but you can Not use the protected member by creating an instance of the class that holds it and then use dot operator to access it, this will Not work..
eg :
package com.go;
public class A {
protected String s= "Hi";
}
package com.fo;
public class B extends A {
System.out.println(s); // This will work
A a = new A();
System.out.println(a.s); // This will NOT work
}
The access (default or package-private) would be useful if Moo was in the same package an didn't extend Zoo.
First of all, I think is a good practice to maintain class attributes with private visibility, and access them through getters and setters. In second place, you are not accessing the attribute by inheritance, you are creating an instance of Zoo, and accessing the superString attribute because of it package visibility, you could also access them from another class of the package that don't even extends Zoo class (and that's generally not a good thing) In third place, you don't need to create an instance of the super class to access his public or default attribute, you could simply do:
System.out.println(this.superString)
wich is absolutly the same as (if not local variable or parameter declared with the same name):
System.out.println(superString)
In conclusion, having default or public attributes, let client classes access them (read and write) without the class could do nothing about it, and this could bring side effects for the methods of the class that use those attributes.
Example 2 you have a separate instance of your zoo object, which is a bit weird, but since the method will always return the same thing there isn't much difference. If you changed your method to be based on constructor input or something you could see a difference in the 2.
public class zoo {
public String superString;
public zoo (String _superstring) {
superString = _superstring;
}
}
public class Moo extends zoo{
public void useMyCoolMethod(){
zoo z=new zoo("string1");
System.out.println(superString);//1
System.out.println(z.superString);//2
}
public Moo (String _superstring) {
superString = _superstring;
}
public static void main(String[]args){
new Moo("string2").useMyCoolMethod();
}
}
Will return
string2
string1
Let's say I have a base class named Entity. In that class, I have a static method to retrieve the class name:
class Entity {
public static String getClass() {
return Entity.class.getClass();
}
}
Now I have another class extend that.
class User extends Entity {
}
I want to get the class name of User:
System.out.println(User.getClass());
My goal is to see "com.packagename.User" output to the console, but instead I'm going to end up with "com.packagename.Entity" since the Entity class is being referenced directly from the static method.
If this wasn't a static method, this could easily be solved by using the this keyword within the Entity class (i.e.: return this.class.getClass()). However, I need this method to remain static. Any suggestions on how to approach this?
Don't make the method static. The issue is that when you invoke getClass() you are calling the method in the super class - static methods are not inherited. In addition, you are basically name-shadowing Object.getClass(), which is confusing.
If you need to log the classname within the superclass, use
return this.getClass().getName();
This will return "Entity" when you have an Entity instance, "User" when you have a User instance, etc.
Not possible. Static methods are not runtime polymorphic in any way. It's absolutely impossible to distinguish these cases:
System.out.println(Entity.getClass());
System.out.println(User.getClass());
They compile to the same byte code (assuming that the method is defined in Entity).
Besides, how would you call this method in a way where it would make sense for it to be polymorphic?
This works for me
this.getClass().asSubclass(this.getClass())
But I'm not sure how it works though.
Your question is ambiguous but as far as I can tell you want to know the current class from a static method. The fact that classes inherit from each other is irrelevant but for the sake of the discussion I implemented it this way as well.
class Parent {
public static void printClass() {
System.out.println(Thread.currentThread().getStackTrace()[2].getClassName());
}
}
public class Test extends Parent {
public static void main(String[] args) {
printClass();
}
}
Create a member String variable in the superclass.
Add the this.getClass().getName() to a constructor that stores the value in the member String variable.
Create a getter to return the name.
Each time the extended class is instantiated, its name will be stored in the String and accessible with the getter.
The superclass should not even know of the existence of the subclass, much less perform operations based on the fully qualified name of the subclass. If you do need operations based on what the exact class is, and can't perform the necessary function by inheritance, you should do something along these lines:
public class MyClassUtil
{
public static String doWorkBasedOnClass(Class<?> clazz)
{
if(clazz == MyNormalClass.class)
{
// Stuff with MyNormalClass
// Will not work for subclasses of MyNormalClass
}
if(isSubclassOf(clazz, MyNormalSuperclass.class))
{
// Stuff with MyNormalSuperclass or any subclasses
}
// Similar code for interface implementations
}
private static boolean isSubclassOf(Class<?> subclass, Class<?> superclass)
{
if(subclass == superclass || superclass == Object.class) return true;
while(subclass != superclass && subclass != Object.class)
{
subclass = subclass.getSuperclass();
}
return false;
}
}
(Untested code)
This class doesn't know about its own subclasses, either, but rather uses the Class class to perform operations. Most likely, it'll still be tightly linked with implementations (generally a bad thing, or if not bad it's not especially good), but I think a structure like this is better than a superclass figuring out what all of its subclasses are.
Why do you want to implement your own getClass() method? You can just use
System.out.println(User.class);
Edit (to elaborate a bit): You want the method to be static. In that case you must call the method on the class whose class name you want, be it the sub-class or the super-class. Then instead of calling MyClass.getClass(), you can just call MyClass.class or MyClass.class.getName().
Also, you are creating a static method with the same signature as the Object.getClass() instance method, which won't compile.
A static method is associated with a class, not with a specific object.
Consider how this would work if there were multiple subclasses -- e.g., Administrator is also an Entity. How would your static Entity method, associated only with the Entity class, know which subclass you wanted?
You could:
Use the existing getClass() method.
Pass an argument to your static getClass() method, and call an instance method on that object.
Make your method non-static, and rename it.
If I understand your question correctly, I think the only way you can achieve what you want is to re-implement the static method in each subclass, for example:
class Entity {
public static String getMyClass() {
return Entity.class.getName();
}
}
class Derived extends Entity {
public static String getMyClass() {
return Derived.class.getName();
}
}
This will print package.Entity and package.Derived as you require. Messy but hey, if those are your constraints...
If i am taking it right you want to use your sub class in base class in static method
I think you can do this by passing a class parameter to the method
class Entity {
public static void useClass(Class c) {
System.out.println(c);
// to do code here
}
}
class User extends Entity {
}
class main{
public static void main(String[] args){
Entity.useClass(Entity.class);
}
}
My context: superclass Entity with subclasses for XML objects.
My solution:
Create a class variable in the superclass
Class<?> claz;
Then in the subclass I would set the variable of the superclass in the constructor
public class SubClass {
public SubClass() {
claz = this.getClass();
}
}
it is very simple done by
User.getClass().getSuperclass()
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.