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.
Related
Although the class B implements the interface A, it does not provide the implementation of the method show(). Again, D extends C but in class D the implementation of displayNothing() is the same as the implementation of displayNothing() method in its superclass C.
But in both the cases #Override annotation work properly, why?
//code snippet 1
interface A
{
void show();
}
abstract class B implements A
{
#Override
abstract public void show();
}
//code snippet 2
class C
{
void displayNothing()
{
}
}
class D extends C
{
#Override
void displayNothing()
{
}
}
#Override is only bothered with whether your function name along with the data type is just the same as the method in your super class (with no Alphabetical error). In your case it is the same.Try changing void as int in your class D. It shows a compilation error. As for your abstract class method, it needn't define anything coz the name says it all, it is abstract.
The purpose of the #Override annotation is to prevent bugs. It's presence informs the compiler that there must be a inherited method with that same exact signature that this class is overriding. It does matter if there is an implementation of a method or not.
It is perfectly legal to not include an #Override annotation. The potential problem you might encounter, though, is if you accidentally had a typo or inadvertently changed the method signature, you would be overloading the method, not overriding it. The compiler would not be able to catch this bug. The program might crash or it might even run, but run incorrectly.
Because this might be a difficult bug to spot, especially with methods containing several parameters, the #Override annotation was created. The #Override annotation informs the compiler that this method is an override, not an overload and if the compiler fails to find a method to override, return a compilation error.
All these methods would compile and could be run but none would override the displayNothing() method.
void displayNothng();
void displaynothing();
void displayNothing(String value);
The #Override annotation "works properly" in both presented cases, because java reuses this annotation for interfaces and classes, even if you feel that the first and the second snippet of code do something different it's just not as important to introduce separated keywords/annotations to differentiate between overriding from an interface and from a class. Even an abstract class doesn't change things here.
The #Override indicates that a given method has a compatible representation in the super class/interface, it's used to ensure we don't have any syntax issue in the definition.
B is abstract. Abstract classes don’t need to provide implementations. If an abstract class doesn’t implement a required method it doesn’t matter. If a class isn’t abstract, then implementations of all abstract methods need to be there.
D provides an override of the displayNothing method in C. The compiler doesn’t check whether the overridden version is the same as the overriding version, and it doesn't care if the overriding implementation is empty. All the Override annotation does is confirm that there is a method with the same signature above the annotated one in the class hierarchy.
The purpose of the annotation is to confirm that you’re actually overriding an existing method and haven’t introduced a bug by misspelling the method name or otherwise not matching the signature of the method you mean to override. See the Java Language Specification, 9.6.4.4, where it says:
Programmers occasionally overload a method declaration when they mean to override it, leading to subtle problems. The annotation type Override supports early detection of such problems.
The classic example concerns the equals method. Programmers write the following in class Foo:
public boolean equals(Foo that) { ... }
when they mean to write:
public boolean equals(Object that) { ... }
This is perfectly legal, but class Foo inherits the equals implementation from Object, which can cause some very subtle bugs.
(Be aware when I describe what #Override does I'm skipping over override-equivalence because you didn't ask about it and there are already perfectly good answers that cover that.
Im new to android development. I would like to know why do we have to precede every overriden method with #Override annotation in android ?. In regular java this is not a requirement.
Please Help
Thanks
The #Override notation is used so the compiler can warn you if the method-signature isn't the same as the super class method-signature. It can warn you about tedious bugs, and it is not required in Android, but it is good practice both in "normal" Java programming and Android Programming.
If you in "normal" Java had misspelled the toString-method e.g. public String toString(int n) {...} and you had the #Override the compiler will warn you because you are not overriding a method in the superclass.
It's a best practice and it's safe. Assume that you have a class:
public class MyClass {
...
...
public void doSomething() {
...
...
}
}
Now assume you extend it:
public class MyExtendedClass extends MyClass {
...
...
public void doSomthing() {
...
}
}
This code will compile, but you'll have problems because you actually haven't overridden the method! Note the misspelling. Now if you annotate it with #Override:
public class MyExtendedClass extends MyClass {
...
...
#Override
public void doSomthing() {
...
}
}
The java compiler will complain because you are trying to override a method that does not exist in the base class. Essentially using #Override lets you catch problems like these at compile-time.
This is especially useful when you refactor. If you change a method signature or a name, but don't have the #Override annotation, some methods may slip-by leading to hard-to-find bugs (of course, with modern IDE's a lot of this pain is mitigated, but still). If you judiciously use the #Override annotation, you will now get compiler errors and so you will be able to fix the method signatures in your derived classes.
Because it is a good programming practice. Overriding a method without using #Override makes for a potentially difficult-to-find bug in the future if the base method's signature changes (I've wasted hours on these sorts of bugs before).
This question already has answers here:
When do you use Java's #Override annotation and why?
(27 answers)
Closed 4 years ago.
(Newbie to Java, old time C# guy.)
I have noticed a lot of the use of #Override in Android example code. I thought that all Java methods were by default "Virtual"?
What then does #Override do?
Example:
private class HelloWebViewClient extends WebViewClient {
#Override
public boolean shouldOverrideUrlLoading(WebView view, String url) {
view.loadUrl(url);
return true;
}
}
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.
This code for the beginner who really want to understand about the #Override process, this will help you! (Remind inheritance concept of Java.)
For example, the Fish class might have two subclasses: FreshwaterFish and SaltwaterFish.
These subclasses would have all the features of the Fish class, but could further customize the objects through new attributes and behaviors or modified behaviors from the parent class Fish. For example, the FreshwaterFish class might include information about the type of freshwater environment lived in (e.g. river, lake, pond, or puddle).
Similarly, the SaltwaterFish class might customize the makeBabyFish() method such that the fish eats its mate after breeding (as defined in the super class) by using the override mechanism, like this:
public class SaltwaterFish extends Fish
{
#Override
public void makeBabyFish(Fish fishSpouse, int numBabies) {
// call parent method
super.makeBabyFish(fishSpouse, numBabies);
// eat mate
eat(fishSpouse);
}
}
The Override-Annotation is just a hint for the compiler that you want to overwrite a certain function. The compiler will then check parent-classes and interfaces if the function exists there. If not, you will get a compile-error.
Its basically just a safety mechanism.
For reference, see this article (override is explained somewhere in the middle)
Override is mostly used in case of defining a method.Overriding is similar to way its meaning.
I will try to explain in very lame way.Suppose if you have Oncreate() Method already defined and have the associated properties with it. and Again when you call Oncreate() method in your code for certain object,the code which you have written now... will override the formally defined property or inherited property of Oncreate() for your application.
In .NET, one can specify a "mustoverride" attribute to a method in a particular superclass to ensure that subclasses override that particular method.
I was wondering whether anybody has a custom java annotation that could achieve the same effect. Essentially what i want is to push for subclasses to override a method in a superclass that itself has some logic that must be run-through. I dont want to use abstract methods or interfaces, because i want some common functionality to be run in the super method, but more-or-less produce a compiler warning/error denoting that derivative classes should override a given method.
I don't quite see why you would not want to use abstract modifier -- this is intended for forcing implementation by sub-class, and only need to be used for some methods, not all. Or maybe you are thinking of C++ style "pure abstract" classes?
But one other thing that many Java developers are not aware of is that it is also possible to override non-abstract methods and declare them abstract; like:
public abstract String toString(); // force re-definition
so that even though java.lang.Object already defines an implementation, you can force sub-classes to define it again.
Ignoring abstract methods, there is no such facility in Java. Perhaps its possible to create a compile-time annotation to force that behaviour (and I'm not convinced it is) but that's it.
The real kicker is "override a method in a superclass that itself has some logic that must be run through". If you override a method, the superclass's method won't be called unless you explicitly call it.
In these sort of situations I've tended to do something like:
abstract public class Worker implements Runnable {
#Override
public final void run() {
beforeWork();
doWork();
afterWork();
}
protected void beforeWork() { }
protected void afterWork() { }
abstract protected void doWork();
}
to force a particular logic structure over an interface's method. You could use this, for example, to count invocations without having to worry about whether the user calls super.run(), etc.
... and if declaring a base class abstract is not an option you can always throw an UnsupportedOperationException
class BaseClass {
void mustOverride() {
throw new UnsupportedOperationException("Must implement");
}
}
But this is not a compile-time check of course...
I'm not sure which attribute you're thinking about in .NET.
In VB you can apply the MustOverride modifier to a method, but that's just the equivalent to making the method abstract in Java. You don't need an attribute/annotation, as the concept is built into the languages. It's more than just applying metadata - there's also the crucial difference that an abstract method doesn't include any implementation itself.
If you do think there's such an attribute, please could you say which one you mean?
Android has a new annotation out as announced in the Google I/O 2015:
#callSuper
More details here:
http://tools.android.com/tech-docs/support-annotations
If you need some default behaviour, but for some reason it should not be used by specializations, like a implementation of a logic in a non abstract Adapter class just for easy of prototyping but which should not be used in production for instance, you could encapsulate that logic and log a warning that it is being used, without actually having to run it.
The base class constructor could check if the variable holding the logic points to the default one. (writing in very abstract terms as I think it should work on any language)
It would be something like this (uncompiled, untested and incomplete) Java (up to 7) example:
public interface SomeLogic {
void execute();
}
public class BaseClass {
//...private stuff and the logging framework of your preference...
private static final SomeLogic MUST_OVERRIDE = new SomeLogic() {
public void execute() {
//do some default naive stuff
}
};
protected SomeLogic getLogic() { return MUST_OVERRIDE; }
//the method that probably would be marked as MustOverride if the option existed in the language, maybe with another name as this exists in VB but with the same objective as the abstract keyword in Java
public void executeLogic() {
getLogic().execute();
}
public BaseClass() {
if (getLogic() == MUST_OVERRIDE) {
log.warn("Using default logic for the important SomeLogic.execute method, but it is not intended for production. Please override the getLogic to return a proper implementation ASAP");
}
}
}
public GoodSpecialization extends BaseClass {
public SomeLogic getLogic() {
//returns a proper implementation to do whatever was specified for the execute method
}
//do some other specialized stuff...
}
public BadSpecialization extends BaseClass {
//do lots of specialized stuff but doesn't override getLogic...
}
Some things could be different depending on the requirements, and clearly simpler, especially for languages with lambda expressions, but the basic idea would be the same.
Without the thing built in, there is always some way to emulate it, in this example you would get a runtime warning in a log file with a home-made-pattern-like-solution, that only your needs should point if it is enough or a more hardcore bytecode manipulation, ide plugin development or whatever wizardry is needed.
I've been thinking about this.
While I don't know of any way to require it with a compile error, you might try writing a custom PMD rule to raise a red-flag if your forgot to override.
There are already loads of PMD rules that do things like reminding you to implement HhashCode if you choose to override equals. Perhaps something could be done like that.
I've never done this before, so I'm not the one to write a tutorial, but a good place to start would be this link http://techtraits.com/programming/2011/11/05/custom-pmd-rules-using-xpath/ In this example, he basically creates a little warning if you decide to use a wildcard in an import package. Use it as a starting point to explore how PMD can analyze your source code, visit each member of a hierarchy, and identify where you forgot to implement a specific method.
Annotations are also a possibility, but you'd have to figure out your own way to implement the navigation through the class path. I believe PMD already handles this. Additionally, PMD has some really good integration with IDEs.
https://pmd.github.io/
public class Animal {
public void eat() { System.out.println("I eat like a generic Animal."); }
}
public class Wolf extends Animal {
#Override
public void eat() { System.out.println("I eat like a wolf!"); }
}
Does #Override actually have some functionality or it's just kinda comment?
From the Java Tutorials on annotations:
#Override — the #Override annotation
informs the compiler that the element
is meant to override an element
declared in a superclass (overriding
methods will be discussed in the the
lesson titled "Interfaces and
Inheritance").
// mark method as a superclass method
// that has been overridden
#Override
int overriddenMethod() { }
While it's not required to use this
annotation when overriding a method,
it helps to prevent errors. If a
method marked with #Override fails to
correctly override a method in one of
its superclasses, the compiler
generates an error.
Let's take a look at the example given in the Java Language specifications, 9.6.1.4 Override. Let's say you want to override a method, equals in that case, but you wrote:
public boolean equals(Foo that) { ... }
instead of:
public boolean equals(Object that) { ... }
While this code is legal, annotating the equals method declaration with #Override would trigger a compile time error because you're in fact not overriding it, you're overloading it. This can cause nasty bugs and the Override annotation type helps at detecting them early.
Override annotation is a compile time annotation which makes java compiler throw an error if the method having this annotation is actually not overriding a parent class method.
You can try to change the Wolf class to not extend the Animal class and if Override annotation is present it will show you a compile time error
something like it alerts at compile time by throwing compilation error if you are not really overriding the method.
similar Q here-When do you use Java's #Override annotation and why?
It works as a comment, but also as an assertion that you actually ARE overriding something. If you use #Override, but it does not actually override anything, the compiler will generate an error. See the documentation page for more details.
If you remove the "eat" method from the parent class or misspell it as "eats" in the subclass, your code will now fail to compile. Without the "#Override" annotation, you can do either and your code will still compile, it just won't do what you want (namely, override a method).
It does not do anything at run-time, but it helps you to catch errors:
If you thought you would override a method, but do not (because of a speling problem or parameter types), without the annotation, the code would compile to something useless (as in: your method would not be called, but the superclass method that you accidentally did not override gets called).