Is there a way to capture the type of an anonymous class?
In the following example, how can i invoke the method g2 of the anonymous class? can't think of a specific case that it would be absolutely useful. and i'm aware that anonymous classes are for "on-the-fly" use. however, wondering.
If i can't invoke it, what's the use of being able to define it (if any-- other than being a helper to other methods of the anonymous class itself) in the anonymous class?
// http://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html
public class SomeClass {
abstract class HelloWorld { abstract public void greet(); }
public void sayHello() {
class EnglishGreeting extends HelloWorld { // local class
String name = "world";
public void greet() { System.out.println("Heya " ); }
public void gg() { System.out.println("do this as well.. ");} }
HelloWorld englishGreeting = new EnglishGreeting();
HelloWorld frenchGreeting = new HelloWorld() { // anonymous class
public void g2() { System.out.println("do this too.. ");}
public void greet() { System.out.println("Salute "); }
};
englishGreeting.greet();
((EnglishGreeting)englishGreeting).gg();
frenchGreeting.greet();
// ((frenchGreeting.getClass())frenchGreeting).g2(); // gives a checked error
}
public static void main(String... args) {
SomeClass myApp = new SomeClass();
myApp.sayHello();
}
}
Note: saw Can't call anonymous class method & Anonymous Inner Classes Inside Methods along with some other relevant discussions.
TIA.
//==============================================
EDIT:
the below worked-- one step closer to it for whatever its worth. not looking up its reference type when the method is invoked right on the new object.
HelloWorld frenchGreeting = new HelloWorld() {
public HelloWorld g2() { System.out.println("do this too.. "); return this; }
public void greet() { System.out.println("Salute "); }
}.g2();
You can only call it directly, e.g.
new HelloWorld() {
// ...
}.g2();
However, notice that you can't assign the variable and call it directly, and you can't call it elsewhere in the method. Still, this is the closest thing I could think of to answering your question.
no, you cannot call g2. actually, anonymous class in java serves as a short hand to implement an instance of an interface only used in one place. with specified interface, the method you want to be called should be specifically defined by that interface.
the reason to allow you define non-interface method is because designers considered the case when you want to implement helper method.
i think anonymous interface is widely used.
Is there a way to capture the type of an anonymous class?
No. You can only access it for reflection.
If i can't invoke it, what's the use of being able to define it (if any-- other than being a helper to other methods of the anonymous class itself) in the anonymous class?
Since they can only be referenced from within the anonymous type itself, they are of no use outside of it. So, as you say, they may be used to organize the logic within the class, but that's about it. The only exception is the edge case that #bcsb1001 describes, in which you invoke the method directly on the anonymous object creation expression (and not the variable to which it is assigned).
the below worked...
That's because the type of the expression new HelloWorld() { ... } is the type of the anonymous class. Since the creation expression has the actual anonymous class type, you can use it to access any members it declares. However, since the class is anonymous, it has no name, so you cannot declare a variable of the concrete anonymous type. The closest you can get is declaring a variable of HelloWorld. Since the variable is declared as HelloWorld, you can only use it to access members declared on HelloWorld or one of its supertypes.
If Java added support for inferred types in declarations, you could write something like var g = new HelloWorld() { ... }; (C# style) or auto g = new HelloWorld() { ... }; (C++ style), and the type of g would be inferred from the assignment. That would allow you to capture the anonymous type without needing a type name. However, Java has no such capabilities at this time.
If I have the following class:
public class TestObject {
public String Hooray() {
return "Hooray!";
}
}
I can obviously instantiate the object, and I know some sort of subclassing must be going on since I can override the Hooray method, but if there's subclassing, why can't I create a new method inside the anonymous class?
TestObject a = new TestObject() {
public String Boo() {
return "Booooo";
}
};
System.out.println(a.Boo());
returns a syntax error
You can create the method, there's nothing wrong with your Boo method (apart from the fact that it has a capital letter at the front). The problem is that outside of the anonymous class, the Boo method is not available (it is not exposed as part of the API of the class).
This is the same with any class that implements an interface... if the class has methods (even public methods) that are not part of the interface then you need to cast the instance to the specific class in order to access these methods.
Unfortunately, because this is an anonymous class, you can't cast it (you don't know what to cast it to).
These internal methods can still be useful, but you have to call them from inside the anonymous class, not from outside.
Because the class has no name, you cannot refer to its type definition at compile time. The compiler can only know it as a TestObject, which has no boo() method
You have this:
public class TestObject {
public String Hooray() {
return "Hooray!";
}
}
TestObject a = new TestObject() {
public String Boo() {
return "Booooo";
}
}
System.out.println(a.Boo());
You can't do this. You can create new methods in anonymous inner classes, and, in fact, you are. But you wouldn't be able to call a.Boo() from outside, since a is a TestObject and TestObject has no method named Boo. It's the same reason you can't do this:
public class Base {
public void something ();
}
public class Derived extends Base {
public void another ();
}
Base b = new Derived();
b.another(); // b is a Base, it must be cast to a Derived to call another().
In the above you have to cast b to a Derived to call the new method added to the derived class:
((Derived)b).another();
The reason that you couldn't do this with anonymous inner classes (which are just syntactic shortcuts for deriving new subclasses) is precisely because they are anonymous - there is no type available for you to cast them to.
The reason you can't access another() through type Base, by the way, is pretty simple when you think about it. While Derived is a Base, the compiler has no way of knowing that Base b is holding a Derived as opposed to some other subclass of Base that doesn't have an another() method.
Hope that helps.
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.
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.