It is considered to be a good practice to
use #Override annotation on methods which are being overriden in
subclass.
But why is same not applied to the classes that come with Java Library. For e.g. String Class. It overrides methods of Object class but does not use #Override annotation on these methods.
Is this so to maintain backward compatibility with previous releases of Java such as 1.4 etc.
Thanks
Within an API, it does not offer much to the user (of that API). However when you implement a method, and you 'intend' do override that of a super class, it is easy to miss out on the method signature, which is supposed to match.
In this case the #Override comes to the rescue as at compile time, it will fail or give a warning when the override does not happen. Also many IDE's recognize the #Override and give you enough support to flag and correct those situations before you even compile.
So the #Override in essence declares your intention that this method overrides something. The user of the API would care less what your intent is, as long as it works.
Actually, probably the true reason is this: The Retention of the #Override annotation is set to SOURCE. Which means the #Override flag is discarded when compiled into a class file.
#Target(value=METHOD)
#Retention(value=SOURCE)
public #interface Override
That is not much more of a cosmetic annotation, it's useful when generating documentation, to give hints through your Java IDE and to explicitly state when a method is overriden.
From the runtime/standard library implementors point of view, it was not worth the effort to modify all existing classes just to add something cosmetic.
Furthermore, regarding backward compatibility of annotations in general, considering that annotations are an optional and extended attribute present in .class file (when their retention policy is either CLASS or RUNTIME and available for Class and Method as Runtime(In)VisibleAnnotations and for Parameter as Runtime(In)VisibleParameterAnnotations) previous releases of the JVM would simply ignore that attribute during the .class file parsing performed the first time that Class is needed.
But actually, that 1.4 JVM class parser will not even reach the point where those Annotation .class attribute are located inside the structure because the parsing will end abruptly when the JVM will notice that the .class version is greater than the supported one.
#override annotation is used to provide some extra information, mainly while generating documentations and also for informing the developer that the code intends to override a method from the superclass.
This is mentioned in oracle documentation.
#Override #Override annotation informs the compiler that the element
is meant to override an element declared in a superclass. Overriding
methods will be discussed in Interfaces and Inheritance.
// mark method as a superclass method // that has been
overridden #Override int overriddenMethod() { }
While it is 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.
Refer to this discussion in SO itself.
This question already has answers here:
When do you use Java's #Override annotation and why?
(27 answers)
Closed 9 years ago.
Let's say I have this method in my runnable.
I always delete that #override. But I want to know if there is any negative effects to this?
I tried to search but didnt find nothing.
#Override
public void run() {
while (running) {
}
}
The #Override annotation does not make any difference to your production code. In fact, the annotation is not even encoded in the java byte code of your class, this is stated on the definition of the annotation:
#Target(value=METHOD)
#Retention(value=SOURCE)
public #interface Override
It's only retained in the source code.
So what's the point? Well it's something that's checked at compile-time. This allows you to state that you are intentionally overriding a method in a super-class.
You'll notice that if you add the #Override annotation to a method that is not overriding a parent method, then you'll get a compilation error.
The idea behind this is that if you are extending a class from a third-party library, and overriding one of it's methods, then you need to be warned if that method no longer exists. Consider upgrading the version of the external library you're using, and instead of void run() the method was now void run(String threadName).
In that case, your overridden method would never get invoked, as it no longer overrides the parent method (not the best of examples, as you're implementing an interface - so compilation would fail anyway, but imagine if it was a class).
This annotation can be set when you override methods from classes, as well as interfaces (although this is only supported in Java 1.6 and above).
Functionally, there is no difference.
However, with the annotation, you get the added checks that there is a run() method to override, which can catch bugs where you aren't using the interface you think you are using.
If you add the clause, you are telling the compiler implicitly that you are overriding a method from a super type, which leads to make it check for a method with the same name from the superclass. This means that if you misspelled something or the method doesn't have the same return type, the compiler will tell you.
You can, but this will disable the compile-time check and makes it less readable.
Since it doesn`t cost you anything to leave it as it is, you should not spend any effort with deleting the annotation.
Let's say you you have a class called Vehicle that outlines a protected (could also be abstract) method called speed. And, the sub class Car overrides that method to define its own implementation.
Now, assume that the need to have a speed method in the Vehicle class is no longer required (let's say all vehicles will be stationary for the whatever reason).
If I remove the method speed from Vehicle, I would like to throw a compile error so that so that the developer who has removed the method knows that sub class(es) are potentially relying on it to perform certain actions.
Technically speaking a compile error is not needed, but some sort of notification that acts as a hurdle when such re factoring is happening. Is there a programming pattern that can be employed to handle such a situation?
UPDATE: I am using Java 1.4 (Sorry!)
The #Override annotation is expressly for this purpose.
If you're not using Java 1.5+, then no, although you could use AOP to instrument those methods to throw an exception, or just use reflection and classpath scanning to go over all subclasses and check for the presence of said method.
If it's abstract, then there is no implementation that can be removed from the parent class, and your risk comes down to new subclasses not implementing it. If it's protected and defined in the parent, there are two cases that should already throw compiler errors if the parent implementation is removed.
1) A subclass calls that method without defining its own implementation. Method does not exist.
2) A subclass defines the method, but includes a call to super. Again, the method does not exist.
You can write super.speed() in nested classes and leave this method empty in parent. If you delete now this method in parent, you'll have an Exception. But there is a disadvantage - you must call it from all overrided methods. Try it, perhaps this will help you
Use #Override annotation for methods in subclases.
Once you remove the method from a base-class, tools like Eclipse and javac will issue a warining for those no-longer-overriding methods.
Edit: While you cannot use #Override before Java 1.5.0, there is a tool called xdoclet. Back in the days of J2EE and EJB 2.1 this was used to "simulate" annotations and do magical things with code based on javadoc-like markers. Look at it, maybe you can use it.
Edit 2: In Java 1.4.x, you can also use JavaDoc tag {#inheritDoc} for this kind of verification. Instead of annotating your method with #Override annotate it with #inheritDoc, like this:
public class MyAwesomeClass extends BaseClass
{
/** {#inheritDoc} */
protected void myAweSomeMethod()
{
//...
}
}
Now, if you change the myAweSomeMethod signature in BaseClass, or remove it, you will get warnings from JavaDoc tool, similar to this:
/home/npe/java-tests/MyAwesomeClass.java:4: warning - #inheritDoc used but myAwesomeMethod does not override or implement any method.
public class NaiveAlien extends Alien
{
#Override
public void harvest(){}
}
I was trying to understand my friend's code, and I do not get the syntax, #Override in the code. What does that do and why do we need in coding?
Thanks.
It's a hint for the compiler to let it know that you're overriding the method of a parent class (or interface in Java 6).
If the compiler detects that there IS no function to override, it will warn you (or error).
This is extremely useful to quickly identify typos or API changes. Say you're trying to override your parent class' method harvest() but spell it harvset(), your program will silently call the base class, and without #Override, you wouldn't have any warning about that.
Likewise, if you're using a library, and in version 2 of the library, harvest() has been modified to take an integer parameter, you would no longer override it. Again, #Override would quickly tell you.
This feature is called an annotation. #Override is the syntax of using an annotation to let the compiler know, "hey compiler, I'm changing what harvest does in the parent class", then the compiler can immediately say, "dude, you are naming it incorrectly". The compiler won't compile until you name it correctly.
So, without this #Override annotation, the compiler won't error and it will be considered a new method declaration. It would be difficult to recognize the error at this point.
#Override means you are overriding the base class method. In java6, it also mean you are implementing a method from an interface. It protects you from typos when you think are overriding a method but you mistyped something.
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What are the best practices for using Java's #Override annotation and why?
It seems like it would be overkill to mark every single overridden method with the #Override annotation. Are there certain programming situations that call for using the #Override and others that should never use the #Override?
Use it every time you override a method for two benefits. Do it so that you can take advantage of the compiler checking to make sure you actually are overriding a method when you think you are. This way, if you make a common mistake of misspelling a method name or not correctly matching the parameters, you will be warned that you method does not actually override as you think it does. Secondly, it makes your code easier to understand because it is more obvious when methods are overwritten.
Additionally, in Java 1.6 you can use it to mark when a method implements an interface for the same benefits. I think it would be better to have a separate annotation (like #Implements), but it's better than nothing.
I think it is most useful as a compile-time reminder that the intention of the method is to override a parent method. As an example:
protected boolean displaySensitiveInformation() {
return false;
}
You will often see something like the above method that overrides a method in the base class. This is an important implementation detail of this class -- we don't want sensitive information to be displayed.
Suppose this method is changed in the parent class to
protected boolean displaySensitiveInformation(Context context) {
return true;
}
This change will not cause any compile time errors or warnings - but it completely changes the intended behavior of the subclass.
To answer your question: you should use the #Override annotation if the lack of a method with the same signature in a superclass is indicative of a bug.
There are many good answers here, so let me offer another way to look at it...
There is no overkill when you are coding. It doesn't cost you anything to type #override, but the savings can be immense if you misspelled a method name or got the signature slightly wrong.
Think about it this way: In the time you navigated here and typed this post, you pretty much used more time than you will spend typing #override for the rest of your life; but one error it prevents can save you hours.
Java does all it can to make sure you didn't make any mistakes at edit/compile time, this is a virtually free way to solve an entire class of mistakes that aren't preventable in any other way outside of comprehensive testing.
Could you come up with a better mechanism in Java to ensure that when the user intended to override a method, he actually did?
Another neat effect is that if you don't provide the annotation it will warn you at compile time that you accidentally overrode a parent method--something that could be significant if you didn't intend to do it.
I always use the tag. It is a simple compile-time flag to catch little mistakes that I might make.
It will catch things like tostring() instead of toString()
The little things help in large projects.
Using the #Override annotation acts as a compile-time safeguard against a common programming mistake. It will throw a compilation error if you have the annotation on a method you're not actually overriding the superclass method.
The most common case where this is useful is when you are changing a method in the base class to have a different parameter list. A method in a subclass that used to override the superclass method will no longer do so due the changed method signature. This can sometimes cause strange and unexpected behavior, especially when dealing with complex inheritance structures. The #Override annotation safeguards against this.
To take advantage from compiler checking you should always use Override annotation. But don’t forget that Java Compiler 1.5 will not allow this annotation when overriding interface methods. You just can use it to override class methods (abstract, or not).
Some IDEs, as Eclipse, even configured with Java 1.6 runtime or higher, they maintain compliance with Java 1.5 and don’t allow the use #override as described above. To avoid that behaviour you must go to: Project Properties ->Java Compiler -> Check “Enable Project Specific Settings” -> Choose “Compiler Compliance Level” = 6.0, or higher.
I like to use this annotation every time I am overriding a method independently, if the base is an interface, or class.
This helps you avoiding some typical errors, as when you are thinking that you are overriding an event handler and then you see nothing happening. Imagine you want to add an event listener to some UI component:
someUIComponent.addMouseListener(new MouseAdapter(){
public void mouseEntered() {
...do something...
}
});
The above code compiles and run, but if you move the mouse inside someUIComponent the “do something” code will note run, because actually you are not overriding the base method mouseEntered(MouseEvent ev). You just create a new parameter-less method mouseEntered(). Instead of that code, if you have used the #Override annotation you have seen a compile error and you have not been wasting time thinking why your event handler was not running.
#Override on interface implementation is inconsistent since there is no such thing as "overriding an interface" in java.
#Override on interface implementation is useless since in practise it catches no bugs that the compilation wouldn't catch anyway.
There is only one, far fetched scenario where override on implementers actually does something: If you implement an interface, and the interface REMOVES methods, you will be notified on compile time that you should remove the unused implementations. Notice that if the new version of the interface has NEW or CHANGED methods you'll obviously get a compile error anyways as you're not implementing the new stuff.
#Override on interface implementers should never have been permitted in 1.6, and with eclipse sadly choosing to auto-insert the annotations as default behavior, we get a lot of cluttered source files. When reading 1.6 code, you cannot see from the #Override annotation if a method actually overrides a method in the superclass or just implements an interface.
Using #Override when actually overriding a method in a superclass is fine.
Its best to use it for every method intended as an override, and Java 6+, every method intended as an implementation of an interface.
First, it catches misspellings like "hashcode()" instead of "hashCode()" at compile-time. It can be baffling to debug why the result of your method doesn't seem to match your code when the real cause is that your code is never invoked.
Also, if a superclass changes a method signature, overrides of the older signature can be "orphaned", left behind as confusing dead code. The #Override annotation will help you identify these orphans so that they can be modified to match the new signature.
If you find yourself overriding (non-abstract) methods very often, you probably want to take a look at your design. It is very useful when the compiler would not otherwise catch the error. For instance trying to override initValue() in ThreadLocal, which I have done.
Using #Override when implementing interface methods (1.6+ feature) seems a bit overkill for me. If you have loads of methods some of which override and some don't, that probably bad design again (and your editor will probably show which is which if you don't know).
#Override on interfaces actually are helpful, because you will get warnings if you change the interface.
Another thing it does is it makes it more obvious when reading the code that it is changing the behavior of the parent class. Than can help in debugging.
Also, in Joshua Block's book Effective Java (2nd edition), item 36 gives more details on the benefits of the annotation.
It makes absolutely no sense to use #Override when implementing an interface method. There's no advantage to using it in that case--the compiler will already catch your mistake, so it's just unnecessary clutter.
Whenever a method overrides another method, or a method implements a signature in an interface.
The #Override annotation assures you that you did in fact override something. Without the annotation you risk a misspelling or a difference in parameter types and number.
I use it every time. It's more information that I can use to quickly figure out what is going on when I revisit the code in a year and I've forgotten what I was thinking the first time.
The best practive is to always use it (or have the IDE fill them for you)
#Override usefulness is to detect changes in parent classes which has not been reported down the hierarchy.
Without it, you can change a method signature and forget to alter its overrides, with #Override, the compiler will catch it for you.
That kind of safety net is always good to have.
I use it everywhere.
On the topic of the effort for marking methods, I let Eclipse do it for me so, it's no additional effort.
I'm religious about continuous refactoring.... so, I'll use every little thing to make it go more smoothly.
Used only on method declarations.
Indicates that the annotated method
declaration overrides a declaration
in supertype.
If used consistently, it protects you from a large class of nefarious bugs.
Use #Override annotation to avoid these bugs:
(Spot the bug in the following code:)
public class Bigram {
private final char first;
private final char second;
public Bigram(char first, char second) {
this.first = first;
this.second = second;
}
public boolean equals(Bigram b) {
return b.first == first && b.second == second;
}
public int hashCode() {
return 31 * first + second;
}
public static void main(String[] args) {
Set<Bigram> s = new HashSet<Bigram>();
for (int i = 0; i < 10; i++)
for (char ch = 'a'; ch <= 'z'; ch++)
s.add(new Bigram(ch, ch));
System.out.println(s.size());
}
}
source: Effective Java
Be careful when you use Override, because you can't do reverse engineer in starUML afterwards; make the uml first.
It seems that the wisdom here is changing. Today I installed IntelliJ IDEA 9 and noticed that its "missing #Override inspection" now catches not just implemented abstract methods, but implemented interface methods as well. In my employer's code base and in my own projects, I've long had the habit to only use #Override for the former -- implemented abstract methods. However, rethinking the habit, the merit of using the annotations in both cases becomes clear. Despite being more verbose, it does protect against the fragile base class problem (not as grave as C++-related examples) where the interface method name changes, orphaning the would-be implementing method in a derived class.
Of course, this scenario is mostly hyperbole; the derived class would no longer compile, now lacking an implementation of the renamed interface method, and today one would likely use a Rename Method refactoring operation to address the entire code base en masse.
Given that IDEA's inspection is not configurable to ignore implemented interface methods, today I'll change both my habit and my team's code review criteria.
The annotation #Override is used for helping to check whether the developer what to override the correct method in the parent class or interface. When the name of super's methods changing, the compiler can notify that case, which is only for keep consistency with the super and the subclass.
BTW, if we didn't announce the annotation #Override in the subclass, but we do override some methods of the super, then the function can work as that one with the #Override. But this method can not notify the developer when the super's method was changed. Because it did not know the developer's purpose -- override super's method or define a new method?
So when we want to override that method to make use of the Polymorphism, we have better to add #Override above the method.
I use it as much as can to identify when a method is being overriden. If you look at the Scala programming language, they also have an override keyword. I find it useful.
It does allow you (well, the compiler) to catch when you've used the wrong spelling on a method name you are overriding.
Override annotation is used to take advantage of the compiler, for checking whether you actually are overriding a method from parent class. It is used to notify if you make any mistake like mistake of misspelling a method name, mistake of not correctly matching the parameters
i think it's best to code the #override whenever allowed. it helps for coding. however, to be noted, for ecipse Helios, either sdk 5 or 6, the #override annotation for implemented interface methods is allowed. as for Galileo, either 5 or 6, #override annotation is not allowed.
Annotations do provide meta data about the code to the Compiler and the annotation #Override is used in case of inheritance when we are overriding any method of base class. It just tells the compiler that you are overriding method. It can avoide some kinds common mistakes we can do like not following the proper signature of the method or mispelling in name of the method etc. So its a good practice to use #Override annotation.
For me the #Override ensures me I have the signature of the method correct. If I put in the annotation and the method is not correctly spelled, then the compiler complains letting me know something is wrong.
Simple–when you want to override a method present in your superclass, use #Override annotation to make a correct override. The compiler will warn you if you don't override it correctly.