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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.
This question already has answers here:
Why is method overloading and overriding needed in java? [duplicate]
(2 answers)
Closed 8 years ago.
Recently I was asked this question "why should one override a method? "
I replied, if I have a class with 10 methods and I want to use all of its functionality except one method, then I will override that method to have my own functionality.
Then the interviewer replied in that case why cant we write a new method with a different name and use that method instead.
Yes this is also right. Now I am confused. What is the real objective in overriding a method?
Can anyone please tell me? Thank you all in advance.
If you will give another name to the method in derived class, you cant invoke it with same interface. You can always invoke it through base class pointer.
i.e.
Base p = new Derived();
p.overrideMethod();
If Derived class is derived from Base then it will automatically call the derived version and not of Base. In case of different name, it is not possible. It is called code against interfaces and not implementations.
.
why cant we write a new method with a different name and use that method instead
It is because we want to use polymorphism. You could tell the interviewer this example: There is a module that calls specific methods on objects you give it; now imagine you can't change that module (e.g. no source). You can't tell it to use a different method but you can give it an object of a subclass which has overridden that method. To the module it will appear that nothing changed.
In practice it is also often the case that you could change that module but dont want to.
I replied, if I have a class with 10 methods and I want to use all of
its functionality except one method, then I will override that method
to have my own functionality.
=> Very often a way to break the Liskov Substitution principle ... => very bad OO design
You have many examples on the web of this "break" but a you can find a good explanation here.
The benefit of overriding is: ability to define a behavior that's specific to the subclass type which means a subclass can implement a parent class method based on its requirement.
One uses interfaces to allow for multiple implementations and one uses overriding to simplify the implementation of an interface (e.g. when implementing a WindowListener, one typically extends and overrides a method of WindowAdapter so that one does not need to provide definitions for the cases where the default behavior is sufficient). Adding a new method rather than overriding would not work in this case, because the caller understands the interface and invokes its methods; the whole point of overriding here is to change the behavior for the calls to the interface. If you simply added a new function, then the caller would have to know about it, which defeats the entire isolation between the consumer of a piece of functionality and the provider of that functionality which is what interfaces are intended to provide.
Overriding is a feature that is available while using Inheritance.
It is used when a class that extends from another class wants to use most of the feature of the parent class and wants to implement specific functionality in certain cases.
In such cases we can create methods with the same name and signature as in the parent class. This way the new method masks the parent method and would get invoked by default.
The main objective of overriding is code reuseablity which can be advatageous in big projects,it also provide flexiblity means you can pass different sets of input from any class and get the output
I've been asked a question. It is the following:
The API documentation of an abstract class tells you whether a method
is abstract. When and why would you need to know this?
Any help would be appreciated.
You need to know what methods are abstract because you will need to provide implementations for those methods when inheriting the class.
As an extension to Fredrik's answer, it also specifies which behaviour is intended to be changed.
You can usually override a method (if the method is not final and the class is not final) but in practice that can be very tricky if the class is not specifically designed for changes. It may be that existing methods assume some kind of behaviour of the method you override, which is not specified (it happens) and that you do not provide.
By explicitly declaring a method to be abstract you express the intention that the method will be implemented by someone else. It also usually means that the documentation of an abstract method is a bit more complete with regards to expected behaviour.
If you call the abstract method you need to take into account that the actual implementation is elsewhere and may have some variation in behavior.
you have know if the method is abstract, because in that case you have to implement it in your concrete (inherited) class.
I advice you to take a look on the following books about Design Patterns, because they mention these stuff and have practices too:
http://oreilly.com/catalog/9780596007126
As part of a larger project, I am attempting to achieve something that I'm not sure is possible, so am eager to see if anyone has any suggestions!
The overall system:
As a whole, my system should be able to be provided with a JUnit test class, that matches some provided interface. Classes will be then given that do not implement this interface, but need to be checked to see if they would be able to (a.k.a. if they implement all necessary methods). If so, some transformation should take place such that the JUnit test class can be run against it.
So far I have implemented:
- A package that loads other classes given a path and name, using URLClassLoader
- A package that runs a JUnit test case and returns the results, using JUnitCore
The problem:
1. At first, how could I run the JUnit test against a class that does implement the interface when the test is designed to match the interface? How do I (at runtime) dictate that the instance being tested by the interface is the loaded class?
Is it possible to then extend this, such that I could i) verify that it does match the interface (I assume using Reflection to check for corresponding methods?) and then ii) modify that class such that it can be tested using the JUnit test class?
Thanks for any advice that might help towards part of this problem. I appreciate my description may be lacking, so please comment if you have any extra information that would help you give any answer!
You can do everything you want with the reflection API. It sounds like you should start with the tutorial, and then come back here for specific questions. Given a Class object you can check if it implements a given interface, create an instance of it, and then treat it like any other class.
Edit: I don't think I got that from your question, but in that case you are looking for the Proxy part of the reflection API.
how could I run the JUnit test against
a class that does implement the
interface when the test is designed to
match the interface
Since you have the class you can use the isAssignableFrom method offered by the class such that
Class loadedJunitClass = clazz;
MyInterface impl = null;
if(MyInterface.class.isAssignableFrom(loadedJunitClass )){
impl = (MyInterface) loadedJunitClass.newInstance();
}
For the second question, you can check each method and see 1. If there exists a method with the same method name as defined in the interface, 2. If the method return type is the same from the interface and 3. If the method parameter types and length are the same. Of course 2 and 3 can be tricky to get right.
At that point I would just create an instance of that interface (anonymous or a private class), create a newInstance of that matching class. And invoke the methods through reflection within the interface's methods.
Now that is how you can get it done with reflection. I am not advicating reflection as you can imagine :)
For the first part of your question; if you have the loaded Class instance for the class you want to test you can construct one with newInstance() if it has a default constructor, or via the getConstructor methods if you need to pass parameters. You should be able to get this Class instance from the class loader.
For the second part. You should be able to check the public methods via getMethods() (again on the Class instance) then look through the returned array for the methods you want. There are methods on the Method class that will return information about parameters, exceptions and return type to verify they are what you require.
However, I am pretty certain it is not possible to modify the class at runtime to add the interface. It might be possible by modifying the byte code, but I don't know about that.
An alternative would be to write your test to call all method via reflection, then it doesn't matter what the type of the object is just that it has the right methods (which you've already checked).
If you want to make arbitrary class to implement given interface at runtime if its public API matches the interface, you have several options in Java. Creating java.lang.Proxy to bridge the target class, exposing YourInterface is the easiest way.
YourInterface i = (YourInterface) Proxy.newProxyInstance(
this.getClass().getClassLoader(),
new Class[]{YourInterface.class},
new InvocationHandler() {
#Override
public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
//run method on your target class here using reflection
}
});
You can also use mixins in AspectJ or subclass your target class using CGLIB and add interface at runtime. But the proxy approach is not that hard-core to implement.
When I run static analysis on the following code:
public ExtractDBScripts(String resBundleName)
{
super();
m_mainBundle = ResourceBundle.getBundle(resBundleName);
}
I get the following error:
"JAVA 0058 Constructor 'ExtractDBScripts' calls super()".
What is wrong with calling super() from a constructor?
Probably just that it's completely unnecessary--that is java's default behavior (it will call super for you). You want to use the explicit call to super() if you need to pass a parameter to a non-default constructor.
A static analysis tool will often point out code that does absolutely nothing or is unnecessary to help you reduce clutter. It will also point out a=a; there is nothing wrong with saying a=a; but it's not actually doing anything.
Absolutely nothing wrong with it - although it is implicitly there as the first line of a constructor if you don't declare it yourself (i.e. there's no need to declare it)
I presume the tool you are using is objecting to that line of code because it is not required - if you remove it the compiler will automatically insert it.
See the "Subclass Constructors" section here.
There is nothing wrong with calling super() within a constructor so long as it is the first line in your constructor, so long as you are actually extending a class that has a non private constructor.
If the class you are extending has only one constructor, and it's private then you can't call super().
I would suggest calling super() in your code on purpose, so that it is obvious to other coders what you want to do. It is part of the programming practices where I work and seems to make sense. There may be a way for you to set up the static analysis tool to ignore calls to super() also depending on the software. IntelliJ has annotations for ignoring specific things when it does static analysis.