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Closed 10 years ago.
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Why would one mark local variables and method parameters as “final” in Java?
I used PMD in my code and it always tells me to make method local variables final. Can someone tell me how this effects the general performance apart from making the code more readable.
There's no effect on performance and it's debatable whether it's more readable. Java should have made variables final by default.
The biggest value of final is that it prevents programming errors. Regarding performance, I'd think the compiler can figure out the last write to a variable in most cases and do the necessary optimizations.
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Does use of final keyword in Java improve the performance?
(14 answers)
Closed 5 years ago.
Does declaring variables as final inside a method brings any benefit from a performance/memory point of view in the latest java versions?
I am not talking here about any other benefit.
This question Does use of final keyword in Java improve the performance? was addresed almost 7 years ago, some progress has been done since then.
No, final keyword on a local variable has no performance impact, and it cannot have even in theory, since .class files do not retain this information.
See this answer for details.
final keyword has no direct performance impact if we will talk about variables, but can improve performance in some use cases.
Example:
When iterating in a loop, if the variable is final then JIT will not check for any cause which can change the value of the list (reference).
final allows a variable to be inlined into the calling code, so that could cause some memory and performance improvement. This is used when you have a number of public static final Strings in a class (reference).
final keyword improves performance. Not just JVM can cache final variable but also application can cache frequently use final variables (check here for 3 and 4).
final keyword allows JVM to optimized method, variable or class.
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Why must local variables, including primitives, always be initialized in Java?
(8 answers)
Closed 6 years ago.
In JAVA world, field variables have default values if you don't initialize them, while local variables don't.
I have considered a lot and searched a lot but I still don't understand. Why does JAVA world design like that? By the way, I think it has something to do with heap and stack.
Local variables are much easier to check that a variable is always initialised in a relatively limited scope when you can determine the code paths. It doesn't always get it right but does a good job unless the code is confusing.
final fields also have to be initialised, though only once. When one constructor calls another, it can get confused.
For non final fields, it is very hard to ensure a field is initialised before it is used for all possible code paths. e.g. how can it ensure a setter is always called before a getter if those calls are made from another class which might be changed in the future.
Instead the JVM leaves default values and makes it your problem to worry about.
Well, it is very good design in my opinion.
The compiler is trying to check if programmer did not make any mistakes. Local variable should be initialized manually to avoid unexpected problems because it is usually used to do some additional calculations or actions and with default value it can be very difficult to track such a bug in the future. It is programmer responsibility to initialize local variable properly and use it in short block of code.
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When should one use final for method parameters and local variables?
(15 answers)
Closed 10 years ago.
I use PMD tool to find errors in java code if any. One common suggestion PMD gives is that "Local variable {local_variable} could be declared final". Is it necessary to declare all local variables as final if it's state is not changed further?
Is it necessary to declare all local variables as final if it's state is not changed further?
No it is not necessary, except in one situation: if you need to access that variable within an anonymous class.
Is it good practice to make all local variable final when they don't change?
This is obviously subjective - I personally think that it clutters the code unnecessarily and that if you follow good coding practice, your methods should be short enough and self-explanatory enough that making your variables final should not be required.
Well it's also a problem of the language design to set variables explicitly to final and to have the final keyword appear all around. In scala the default is to have everything final and immutable, pushing a more functional design. If you wonder why use final, have a look at scala - that should give you some ideas.
I would consider it bad style not to use final by the way. Final variables - well or constants in that sense - cannot be changed. That is kind of a programming contract. You simply have less side-effects in code with final variables. If you leave that out you could also leave out all the private fields. Why bother?
In eclipse you can add all the required final fields and lots of other things considered "cleaner" automatically, when you open the dialog "Source/Clean Up..." (so I guess it's called "Clean Up" because not using final would be considered unclean).
It's up to you - use it - or leave it. Use it - and look especially at the code where the final could not be applied. Maybe that code could be improved to be final?
So: Yes! It is good practice!
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Closed 11 years ago.
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Is there any performance reason to declare method parameters final in Java?
Why would one mark local variables and method parameters as “final” in Java?
I am using PMD to see the code violations.
Inside a webService Method, I have this below code
public ServiceRequest getData()
{
Status status = new Status();
// code
}
What PMD is suggesting me is that, this local variable status could be declared as final.
My question is, making it final would result in any performance improvements or if not what benefits the code could get?
Taken from the following article: http://www.javapractices.com/topic/TopicAction.do?Id=23
clearly communicates your intent
allows the compiler and virtual machine to perform minor optimizations
clearly flags items which are simpler in behaviour - final says, "If you are looking for complexity, you won't find it here."
This is also discussed in this question: Can excessive use of final hurt more than do good?
final indicates the local variable won't be changed. My feeling is that methods should be so short you should be able to easily understand them and so making the variable final may be a bit redundant.
I prefer to make fields final because making the whole class so short, is a serious limitation. Also fields can have thread safety issues which local variables do not.
I dont know about performance-benefits by making status final, but PMD is suggesting you this, because probably you are never writing on status after its first initialization.
So what you gain by making it final is just that your code is less error-prone - if you declare it final, you cant overwrite it by mistake...
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Closed 12 years ago.
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Why is String final in Java?
I'm just wondering why java.lang.String is made final? Is it to prevent from being inherited? Why?
Yes indeed. This allows code in security managers and classloaders to work with the String type without having to worry that it's actually dealing with a malicious subclass that's specifically designed to trick it into allowing evil code through.
You should not be extending the string class. Just write your own methods in some other class that manipulate strings.
The reason is that the string class is a stable one which should not be tampered with as you may re-define some methods which would have unknown side effects on some other transactions.
Aside from security aspects that were already mentioned, I suspect performance was another important reason. For older JVMs especially final classes (where all methods are final by definition) made it much easier to inline code on-the-fly. And since String is one of most heavily used objects, which affects overall performance of many applications, this was seen as an area where improvements would have big overall effect.