as it is explained here, in java static methods are not overridden but hidden by child implementations
that means that you can't use #Override annotations with them
the following code
#Override
public static void test(String value1, String value2) {
gives this compiler error.
The method test(String, String) of type Child must override or implement a supertype method Child.java
is there some equivalent annotation that I could use to make sure my class is "hiding" an existing static method from parent class?
This is an easy one: No.
Check out the list of standard compiler annotations
I am not aware of any annotations that you can add. But simply creating the method with the exact same name and inputs (the same method signature) should hide it. The only way to know for sure is to know the details of the superclass method for sure, and then perhaps test your method... Thats all I can think of. The only other annotations I can think of would be to suppress warnings which would be the opposite of what you want I imagine. Whenever I hide a method like this I generally get a warning from my IDE telling me that I am hiding another method.
Related
This is my first time dealing with inheritance and I'm trying to create a subclass of the ArrayList class, but I'm getting stopped pretty early on.
import java.util.ArrayList;
/**
* extending to ArrayList
*/
public class SortedArrayList<E> extends ArrayList<E>
{
/**
* Inheriting the supers constructors
*/
public SortedArrayList()
{
super();
}
public void add(){
SortedArrayList.add(); // testing out inherited method.
}
}
however when I try to compile this I get an error, "non-static method add() can not be referenced from a static context"
What am I doing wrong?
Just as a matter of style ... I don't think that you really want to do this.
While ArrayList is documented for inheritance, this is not typical. Subclassing across package boundaries from base classes that are not expressly designed for inheritance can make your code fragile, error prone, and insecure. The reason for this is that inheritance breaks encapsulation. Your subclass becomes dependent upon the implementation details of the base class.
If you are writing new code, I recommend the following stylistic alternatives to subclassing ArrayList:
Subclass AbstractList instead (which IS documented and designed for inheritance)
Make ArrayList an implementation detail of SortedArrayList by making it a private field (i.e. favor composition over inheritance); adding instrumentation to an existing class is typically best done via composition and forwarding method calls to the contained instance.
These recommendations are very much in line with the advice given in Effective Java, 2nd Ed.
There is no no-arg add method in ArrayList to call. If there were, then you would write super.add();. But overriding a method just to call super.add() and nothing else is the same as not overriding the method at all and letting your subclass inherit the method.
I suspect you want something like this:
#Override
public void add(E e) {
// Write code here to find the index where it belongs, in an "index" variable.
// Then call super.add with the proper index so it remains sorted.
super.add(index, e);
}
The correct way to invoke a method on your parent class is
super.add();
Couple of problems:
The error message gives you a major clue; static members are associated with the class, rather than an object instance, which is typically done by referencing class name specifically, much as you have done (a simple example would be MyClass.myStaticMethod()). However, the add() method in your case is non-static; it is an instance method.
Its unclear what you're trying to do, specifically, in this method, presently. If you remove the SortedArrayList portion of the method call, you'll end up in an infinite recursive loop, because it'll continuously call itself.
I assume you're attempting to call the superclass's add method, however you're missing the method signature. If you are attempting to override the ArrayList add method, you should note that there are two version; if you want to override these methods, they should contain the same signature, and look more like this:
First Version
public boolean add(E e){
// your code
}
Or the second version
public void add(int index, E element){
// your code
}
When you write SortedArrayList.add(); you are calling the static method "add" of "SortedArrayList" class... but it doesn't exist.
If you have to call the add() method of superclass (non-static), then you should use super.add().
In java or Android there are #Override annotations. What does it mean? I found that it is used when method is from subclass or inherited interface's method, I want to know further and other is #SuppressWarnings its also Anonation, if yes how many annonation used by java and for which purpose.
This question is also answered here, and quite succinctly:
Android #Override usage
It's an annotation that you can use to tell the compiler and your IDE that you intend the method that has that annotation to be an override of a super class method. They have warning/errors in case you make mistakes, for example if you intend to override a method but misspell it, if the annotation is there the IDE or the compiler will tell you that it is not in fact overriding the super class method and thus you can determine why and correct the misspelling.
This is all the more important for Android applications and activities for example, where all of the calls will be based on the activity lifecycle - and if you do not properly override the lifecycle methods they will never get called by the framework. Everything will compile fine, but your app will not work the way you intend it to. If you add the annotation, you'll get an error.
In other words, if you add #Override this helps you make sure you are really overriding an existing method! Pretty darn useful.
Overriding means that you are changing the behavior of a method inherited from a parent class, without changing the signature. The #Override annotation is used to mark this. It is strongly linked with the concept of polymorphism. Example:
public class A {
public void foo() {
System.out.println("A");
}
}
public class B extends A {
#Override
public void foo() { // I want to change the way foo behaves
System.out.println("B"); // I want to print B instead of A
}
}
public static void main(String[] args) {
A a = new A();
a.foo(); // prints A
A b = new B(); // I can use type B because it extends A
b.foo(); // I have overriden foo so it prints B now
}
Just to ensure that you are actually overriding it at compile time, and to improve readability
Example:
class Animal{
public void eat(Food food){
}
}
class Person extends Animal {
#Override
public void eat(String food){
}
}
This will give you compile time error since you are not actually overriding it (see the type of food)
#override its an annotation i.e meta data introduce in jdk 1.6 . If you don't write it before override method , it won't make any difference but it just use to increase the readability of compiler.
To mark that you really implement or change a method. Like meantined it's checked at compile time. That is you for instance you get an error if you want to implement #Override public void equals(final Car pObject); instead of #Override public void equals(final Object pObject);.
Just go the source for the definition of both annotations, besides other additional details: the #Override and the #SuppressWarnings from the Java specs.
It sounds like your question is more about annotations in general, so I'll answer that. Annotations provide extra meta data about the item that is being annotated. This allows other code to use that information to decide how to run. More detailed description. There are a large number build into the language, but you can write your own.
The two examples you gave tell the compiler extra information about the code it is compiling. When it sees #Override, it checks to ensure that the method is actually overriding a method. When it sees #SuppressWarnings, it know that it should ignore any compiler warnings, of the given type, that exist inside the block of code.
They can be used outside of compilers as well. There are a number of libraries that have you annotate a class object and it uses that meta data to build a database or parse an xml file.
This question already has answers here:
When do you use Java's #Override annotation and why?
(27 answers)
Closed 9 years ago.
This is a newbie question.
I read that JVM's execution starts from searching for the methodname from lowest class in the hierarchy and if the method is not available in that class it traverses to the parent class looking for the method.
If this is the case then why do we need to use "#override" for adding custom logic to the inherited class ?
The below example illustrates my question
class superclassA
{
method()
{
}
}
class subclassB extends superclassA
{
#Override
//While executing if JVM starts looking for the method name from the lowest hierarchy
//then why do we have to use "override" as the methodname will be matched from the lowest level itself?
method()
{
--custom subclass specific code...
}
}
If this is the case then why do we need to use "#override" for adding custom logic to the inherited class?
We don't. The #Override annotation has no technical meaning - it exists to document the fact that this method overrides one in the superclass, which has some advantages:
If you look at the code, it tells you there is an superclass method that might be important to understand what this method does
You will get a compiler error if the superclass method's signature changes in a way that the subclass method does in fact not override it anymore.
You can get a compiler warning if you override a method without using the annotation, in case you do it inadvertantly.
#Override simply helps Java compiler detect errors in source code, compilers are generate an error if a method annotated with #Override does not override it in fact.
It is not mandatory to annotate a method that overrides a supertype methods with #Override.
You don't need #Override. But it's a useful annotation that causes the compiler to check whether or not you are really overriding the method that you say you are. When you #Override a method that is not actually overriding a method, the compiler will inform you of this discrepancy. Additionally, it just makes your code more clear: since all methods in Java are implicitly virtual, and a method in a derived class with the same signature as that of a non-final method in a super class implicitly overrides it1, adding #Override makes the code easier for humans to understand.
1: To be clear, you can not have a method in a derived class with the same signature as a final method in a super class.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
What's “#Override” there for in java?
I've never put "#Override" before a method until now. I see some code examples with it, but I don't understand its utility. I'd love some explanation.
Many thanks,
JDelage
Indicates that a method declaration is intended to override a method declaration in a superclass. If a method is annotated with this annotation type but does not override a superclass method, compilers are required to generate an error message.
http://download.oracle.com/javase/6/docs/api/java/lang/Override.html
The case I like to explain its use is when overriding equals.
This will error because equals expects an Object parameter:
public class Foo{
#Override
public boolean equals(Foo f){
return true;
}
}
First, you can't annotate a class with #Override. This annotation indicates that a method declaration is intended to override a method declaration in a superclass.
You don't have to annotate overriding methods but if you use this annotation and your annotated method does not override a superclass method, then the compiler will generate an error message.
The best example - overriding equals().
If you write a class like this:
public class Foo
{
public String bar;
public boolean equals(Foo other)
{
return this.bar.equals(other.bar);
}
}
then you've overloaded the equals method, rather than overriding Object.equals as was intended.
If you annotate the equals method with #Override, the compiler will give you an error stating (correctly) that you haven't overridden a superclass method.
In Java 6, you can use this for implementing interface methods too - this is handy when you're only adding a method to your class to satisfy some interface, and hence the compiler can check that it's required and alert you to the interface changing.
As with all annotations it's effectively a programmatic comment, but having the compiler check that your assumptions are (still) correct is very handy in these cases.
It's there to express that you expect the method to be overriding a superclass method. It does come in handy when you make a mistake spelling the method name or give it the wrong parameters so that it does not override what you thought it was overriding.
It's a conventional comment. Some compilers make sure that the function followed by #Override is actually an override... just a failsafe
I have a class that will be subclassed. All subclasses must contain a static method with the same signature, but differnt for each one.
I would like to have an abstract instance method in the superclass that subclasses will override, but it seems mot possible in Java, I wonder why.
A silly example:
Image{
abstract String getExtension();...
RGBImage extends Image{
static String getExtension(){return "RGB"};..
PNGImage extends Image{
static String getExtension(){return "PNG"};...
Have a look at this explanation. You could use the Builder pattern for your purpose.
As others have said, Java does not support overriding or abstract for static methods. However, I don't really understand what you would achieve with an "abstract" static method anyway.
The normal use of an abstract method is to force every subclass to implement a method with the same signature. But why would it even it matter for a static (hence non-polymorphic) method? If you forget to provide the method for one of the subclasses, EITHER it doesn't matter because you don't call it, OR it does matter but you'll get a compilation error at the point you try to call the missing method.
The only use-case I can think of where it might matter is if you are calling the method reflectively in a pseudo-polymorphic way. But if you are doing that you'd be better off using real polymorphism and instance methods.
I don't do much Java, but (using C++ logic) you could make the base class implementation a dummy implementation that cannot be executed.
static void DoStuff(arg_type arg)
{
std::cerr << "Method DoStuff() must be overriden\n";
::abort();
}
That isn't as good as an abstract method, but it will achieve some of the same results.
Unfortunately, this is not possible in Java. You have to make them instance methods to be able to define as being abstract.
It's not possible to override static methods as they pertain to a class, not an instance.
Furthermore, an instance method cannot be made static in the sub-classes as you suggest in your example.
Can you provide more information as to why all the subclasses need to have the same static method?
You want to override instance method with static method ? That makes no sense. Static methods are not polymorphic. How would you like to call it? It is impossible in Java to enforce class to have a static method with given signature.
as others have said, static is not polymorphic... it means it is the same for the class, not specific to an instance. i am not sure what you would accomplish by using a static method... but you could have have your extended method implementation return a static value:
public String getExtension() {
return STATIC_CONSTANT;
}
I guess it depends on what you are trying to accomplish with the static method. From your post: "Let me say, everything works OK implemnting a instance getExtension() method in each subclass. But the point is that the method is in fact static in the subclass, has no dependence on the instance. I know I can't do that, I only saying it seems no so strange idea to me."
The problem is you are asking for polymorphic behavior (a change in the behavior of a method for each subclass) from something that defines behavior for the class, not the instance. By definition, static cannot be polymorphic. I hope this helps you with your understanding here of why this is not possible.
The reason I want to have the abstract method declared in the superclass is because the class has to do some extra work with the result of the abstract method.
Lets put it simple even the example has no sense:
Image{
abstract String getExtension();
int process(){
return getExtension().length
}
The algorithm in process() is common for all subclasses XXXImage of Image, so its right place is the superclass.
Let me say, everything works OK implemnting a instance getExtension() method in each subclass. But the point is that the method is in fact static in the subclass, has no dependence on the instance.
I know I can't do that, I only saying it seems no so strange idea to me.
Yes thaks, this is what I'm doing, retun a static constat. In may case it returns the class object of a inner enum that lists the name of sections of a given file format: the file format of the subclass.
But you put it very clear in your example:
public String getExtension() {
return STATIC_CONSTANT;
}
I was expecting to be able to put "static" before String...