Can have an abstract class implementing all of its methods-- with no abstract methods in it.
Eg.:
public abstract class someClass {
int a;
public someClass (int a) { this.a = a; }
public void m1 () { /* do something */ }
private void m2 () { /* do something else */ }
}
What's the advantage, if any, of having such an abstract class compared to having the same class as a concrete one instead?
One i can think of is that, when i declare it as abstract, it won't be instantiated.
however, i can have the same effect by making it concrete and its constructor(s) private.
TIA.
//==================
EDIT: One other use I can think of:
it may be extending another abstract class or implementing an interface without implementing that class's abstract methods-- although it is implementing all methods of its own. for whatever it' worth.
It has a conceptual meaning: this class has a behaviour which makes no sense on its own.
Granted, it's difficult to imagine such a scenario without well-defined extension points (i.e. abstract methods), but occasionally it will be a reasonably accurate model of your problem.
You can have something like this:
public abstract class ObjectWithId {
private final String id;
public ObjectWithId( String id ) {
this.id = id;
}
public final String getId() {
return id;
}
}
And then you can extend it to declare different types of objects that have ids. Here you have a fully specified and implemented behaviour but no restriction on any other behaviours subclasses may exhibit.
Note though that a much neater way to model the same thing is to use composition instead of inheritance.
public final class ObjectWithId<T> {
private final String id;
private final T ob;
public ObjectWithId( String id, T ob ) {
this.id = id;
this.ob = ob;
}
public String getId() {
return id;
}
public T getObject() {
return ob;
}
}
But before generics were introduced (up to Java version 1.4), this wouldn't have been as elegant and obviously better than the abstract class solution because you'd have had to trade in type safety.
you can declare to implement an interface and don't provide implementation and then each child implicitly gets interface extended
you prevent to create instance of this class
you in future provide common implementation to all children
As you pointed out, you can prevent the class from being instantiated by making it's constructor private. Othere than that, there is no benefit whatsoever. This is probably supported just to provide language completeness.
We generally use Abstraction concept with inheritance
Consider using abstract classes if any of these statements apply to
your situation:
You want to share code among several closely related classes.
To answer your question,
Why declare a class with concrete methods Abstract?
One possible reason is to support inheritance without actually creating objects
Assume you have two classes one Abstract and other Concrete
Abstract class : AbsClass
abstract class AbsClass {
int a = 5;
//Constructor
public AbsClass() {
System.out.println(a);
}
void methodA() {
System.out.println(a + 10);
}
}
and
Concrete class : ConcreteClass
class ConcreteClass {
int a = 10;
//Made the constructor Private to prevent from creating objects of this class
private ConcreteClass() {
System.out.println(a);
}
void methodA() {
System.out.println(a + 10);
}
}
The above two classes should function similarly (?) Until you try to Subclass them
class AbsImplementer extends AbsClass {
//Works fine
}
class ConcImplementer extends ConcreteClass {
//Compilation Error Implicit super constructor ConcreteClass() is not visible
}
The practical difference is that you can't create an instance of it. You would have to subclass it and create an instance of the subclass.
As to WHY you would want to do this, in practice ... I'm hard pressed to think of a good reason. You could say that the class is only meaningful if someone creates a subclass that implements some function. But then why not make that function abstract in the super-class?
I wouldn't rule out the possibility that someone might come up with some example where this makes sense, but I can't think of one. Just because it's possible to write a piece of code and that code compiles successfully doesn't mean that that it makes sense. I mean, I can write "total_price = item_price * zip_code + customer_height_in_cubits - 7.879", but that doesn't mean such a line of code would be meaningful.
Well assume that you don't care whether the methods of the abstract class are implemented or abstract, but by design it has to be abstract so that when someone extends it, they have to add more methods or override the existing ones or use as is. If they don't want to override the methods then the default behavior is already provided in that abstract class.
In this abstract class, the only criteria you enforce is - one simply cannot instantiate that class and they have to have their only version of class before using it.
So in general, abstract class with few or all methods being implemented, is much better than having an interface which has no methods implemented at all. This is based on the assumption that you are using it as a single inheritance.
Consider something similar to the NVI pattern (not sure what you'd call it in Java):
public abstract class A {
public final void doSomething() {
System.out.println("required");
doOptional();
}
protected void doOptional() {
System.out.println("optional");
}
}
public class B extends A {
#Override
protected void doOptional() {
System.out.println("overridden");
}
}
For your public API, you only expose a public final method which cannot be overridden. It performs some required work inside there and an optional method. When extending this class, you can only override doOptional().
Calling B.doSomething() will always print "required" before it proceeds.
Since doOptional() is not abstract, there's no purely code reason that class A needs to be abstract. But it might be desired for your particular project. For example, a base service that is always extended into specific sub-projects.
This can be useful for cases when the classes derived from the abstract base class must have some behaviour that is different from each other but that behaviour can not be abstracted as residing within a method that has the same signature for all the classes. Being unable to share a signature can occur if the different behaviour requires methods that are passed different primitive types. Because they use primitive types you can not use generics to express the similarity.
An abstract base class without any abstract methods is acting a bit like a marker interface, in that it is declaring that implementing classes must provide some behaviour without having that behaviour encapsulated within a new method with a signature that is the same for all implementations. You would use an abstract base class rather than a marker interface when the implementing classes have some behaviour in common, especially if the base class can implement it for the derived classes.
For example:
abstract class Sender {
protected final void beginMessage() {
...
}
protected final void endMessage() {
...
}
protected final void appendToMessage(int x) {
...
}
}
final class LongSender extends Sender {
public void send(int a, int b, int c) {
beginMessage();
appendToMessage(a);
appendToMessage(b);
appendToMessage(c);
endMessage();
}
}
final class ShortSender extends Sender {
public void send(int a) {
beginMessage();
appendToMessage(a);
endMessage();
}
}
It can be useful if you consider it an utility class.
This question already has answers here:
What is the use of interface constants?
(13 answers)
Closed 9 years ago.
Recently I saw in somebody's code that he implements his final class variable fields inside an interface ex:
public interface commentSchema{
static final String DB_TABLE_NAME = "comment";
...
}
& he had implemented a class which does need these variables something like this:
public class DbComment implements commentSchema {
// use the DB_TABLE_NAME value here ...
...
}
as you know If someone make an instance of DbComment class because of inheritance aspect, he's gonna be able to access the DB_TABLE_NAME which is not proper because we want to use those values only inside DbComment methods.
now I have several questions:
1) is this implementation proper & ok ?
2) if It's not, how we have to declare these variables outside of DbComment class & make this class be the only class who does see these values. (we don't want to use abstract class coz a class can only extends one other class in java)
3) why we do need to use static for values & methods which exist inside an interface ? (when we implements certain interface for a specific class why do we need to make it static to be seen everywhere?)
4) is there any specification that exactly determine kinds of different declarations for java methods, classes, enums, etc ?
1) is this implementation proper & ok ?
Yes, will work just fine.
2) if It's not how I'm gonna declare these variable outside of DbComment class & make this class be the only class who does see DB_TABLE_NAME value. (I don't want to use abstract class coz a class can only extends one other class in java)
No need as the implementation used works as expected.
3) why we do need to use static for values & methods which exist inside an interface ? (when we implements certain interface for a specific class why do we need to make it static to be seen everywhere?)
You can't make methods in interfaces final or static. The only qualifiers allowed for methods are public and abstract which have, by the way, no effect at all.
For fields, static exists, but also has no effect. All fields declared in a interface will be accessible statically by the implementors and are considered constants.
4) is there any specification that exactly determine kinds of different declarations for java methods, classes, enums, etc ?
The official spec has a chapter on Names and Declaration.
By default, any field declared inside an interface is marked as public static final even if the programmer doesn't do it. This means, any field in the interface will be constant and impossible to modify.
If you don't want that functionality (whatever reasons you could have) then it would be better to have an abstract class instead and mark the field as protected static final.
public abstract class CommentSchema{
protected static final String DB_TABLE_NAME = "comment";
}
Still, if you want to base your design to interfaces, then you can have the interface without this field, then an abstract class that implements this interface and adds the field. By doing this, every class that extends the abstract class will implement the interface and have access to the field:
public interface CommentSchema {
foo();
}
public abstract class CommentSchemaAbstractImpl implements CommentSchema {
protected static final String DB_TABLE_NAME = "comment";
}
public class CommentSchemaRealImpl extends CommentSchemaAbstractImpl {
#Override
public void foo() {
//do something...
baz(DB_TABLE_NAME);
}
private void baz(String s) {
//fancy code here...
}
}
At last, you can forget about all this and create an enum to handle your constants.
public enum TableName {
COMMENT("comment");
private String tableName;
private TableName(String tableName) {
this.tableName = tableName;
}
}
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.