I am a Java programmer who is new to the corporate world. Recently I've developed an application using Groovy and Java. All through the code I wrote used quite a good number of statics. I was asked by the senior technical lot to cut down on the number of statics used. I've googled about the same, and I find that many programmers are fairly against using static variables.
I find static variables more convenient to use. And I presume that they are efficient too (please correct me if I am wrong), because if I had to make 10,000 calls to a function within a class, I would be glad to make the method static and use a straightforward Class.methodCall() on it instead of cluttering the memory with 10,000 instances of the class, right?
Moreover statics reduce the inter-dependencies on the other parts of the code. They can act as perfect state holders. Adding to this I find that statics are widely implemented in some languages like Smalltalk and Scala. So why is this opposition to statics prevalent among programmers (especially in the world of Java)?
PS: please do correct me if my assumptions about statics are wrong.
Static variables represent global state. That's hard to reason about and hard to test: if I create a new instance of an object, I can reason about its new state within tests. If I use code which is using static variables, it could be in any state - and anything could be modifying it.
I could go on for quite a while, but the bigger concept to think about is that the tighter the scope of something, the easier it is to reason about. We're good at thinking about small things, but it's hard to reason about the state of a million line system if there's no modularity. This applies to all sorts of things, by the way - not just static variables.
Its not very object oriented:
One reason statics might be considered "evil" by some people is they are contrary the object-oriented paradigm. In particular, it violates the principle that data is encapsulated in objects (that can be extended, information hiding, etc). Statics, in the way you are describing using them, are essentially to use them as a global variable to avoid dealing with issues like scope. However, global variables is one of the defining characteristics of procedural or imperative programming paradigm, not a characteristic of "good" object oriented code. This is not to say the procedural paradigm is bad, but I get the impression your supervisor expects you to be writing "good object oriented code" and you're really wanting to write "good procedural code".
There are many gotchyas in Java when you start using statics that are not always immediately obvious. For example, if you have two copies of your program running in the same VM, will they shre the static variable's value and mess with the state of each other? Or what happens when you extend the class, can you override the static member? Is your VM running out of memory because you have insane numbers of statics and that memory cannot be reclaimed for other needed instance objects?
Object Lifetime:
Additionally, statics have a lifetime that matches the entire runtime of the program. This means, even once you're done using your class, the memory from all those static variables cannot be garbage collected. If, for example, instead, you made your variables non-static, and in your main() function you made a single instance of your class, and then asked your class to execute a particular function 10,000 times, once those 10,000 calls were done, and you delete your references to the single instance, all your static variables could be garbage collected and reused.
Prevents certain re-use:
Also, static methods cannot be used to implement an interface, so static methods can prevent certain object oriented features from being usable.
Other Options:
If efficiency is your primary concern, there might be other better ways to solve the speed problem than considering only the advantage of invocation being usually faster than creation. Consider whether the transient or volatile modifiers are needed anywhere. To preserve the ability to be inlined, a method could be marked as final instead of static. Method parameters and other variables can be marked final to permit certain compiler optimiazations based on assumptions about what can change those variables. An instance object could be reused multiple times rather than creating a new instance each time. There may be compliler optimization switches that should be turned on for the app in general. Perhaps, the design should be set up so that the 10,000 runs can be multi-threaded and take advantage of multi-processor cores. If portablity isn't a concern, maybe a native method would get you better speed than your statics do.
If for some reason you do not want multiple copies of an object, the singleton design pattern, has advantages over static objects, such as thread-safety (presuming your singleton is coded well), permitting lazy-initialization, guaranteeing the object has been properly initialized when it is used, sub-classing, advantages in testing and refactoring your code, not to mention, if at some point you change your mind about only wanting one instance of an object it is MUCH easier to remove the code to prevent duplicate instances than it is to refactor all your static variable code to use instance variables. I've had to do that before, its not fun, and you end up having to edit a lot more classes, which increases your risk of introducing new bugs...so much better to set things up "right" the first time, even if it seems like it has its disadvantages. For me, the re-work required should you decide down the road you need multiple copies of something is probably one of most compelling reasons to use statics as infrequently as possible. And thus I would also disagree with your statement that statics reduce inter-dependencies, I think you will end up with code that is more coupled if you have lots of statics that can be directly accessed, rather than an object that "knows how to do something" on itself.
Evil is a subjective term.
You don't control statics in terms of creation and destruction. They live at the behest of the program loading and unloading.
Since statics live in one space, all threads wishing to use them must go through access control that you have to manage. This means that programs are more coupled and this change is harder to envisage and manage (like J Skeet says). This leads to problems of isolating change impact and thus affects how testing is managed.
These are the two main issues I have with them.
No. Global states are not evil per se. But we have to see your code to see if you used it properly. It is quite possible that a newbie abuses global states; just like he would abuses every language feature.
Global states are absolute necessity. We cannot avoid global states. We cannot avoid reasoning about global states. - If we care to understand our application semantics.
People who try to get rid of global states for the sake of it, inevitably end up with a much more complex system - and the global states are still there, cleverly/idiotically disguised under many layers of indirections; and we still have to reason about global states, after unwrapping all the indirections.
Like the Spring people who lavishly declare global states in xml and think somehow it's superior.
#Jon Skeet if I create a new instance of an object now you have two things to reason about - the state within the object, and the state of the environment hosting the object.
If you are using the ‘static’ keyword without the ‘final’ keyword, this should be a signal to carefully consider your design. Even the presence of a ‘final’ is not a free pass, since a mutable static final object can be just as dangerous.
I would estimate somewhere around 85% of the time I see a ‘static’ without a ‘final’, it is WRONG. Often, I will find strange workarounds to mask or hide these problems.
Please don’t create static mutables. Especially Collections. In general, Collections should be initialized when their containing object is initialized and should be designed so that they are reset or forgotten about when their containing object is forgotten.
Using statics can create very subtle bugs which will cause sustaining engineers days of pain. I know, because I’ve both created and hunted these bugs.
If you would like more details, please read on…
Why Not Use Statics?
There are many issues with statics, including writing and executing tests, as well as subtle bugs that are not immediately obvious.
Code that relies on static objects can’t be easily unit tested, and statics can’t be easily mocked (usually).
If you use statics, it is not possible to swap the implementation of the class out in order to test higher level components. For example, imagine a static CustomerDAO that returns Customer objects it loads from the database. Now I have a class CustomerFilter, that needs to access some Customer objects. If CustomerDAO is static, I can’t write a test for CustomerFilter without first initializing my database and populating useful information.
And database population and initialization takes a long time. And in my experience, your DB initialization framework will change over time, meaning data will morph, and tests may break. IE, imagine Customer 1 used to be a VIP, but the DB initialization framework changed, and now Customer 1 is no longer VIP, but your test was hard-coded to load Customer 1…
A better approach is to instantiate a CustomerDAO, and pass it into the CustomerFilter when it is constructed. (An even better approach would be to use Spring or another Inversion of Control framework.
Once you do this, you can quickly mock or stub out an alternate DAO in your CustomerFilterTest, allowing you to have more control over the test,
Without the static DAO, the test will be faster (no db initialization) and more reliable (because it won’t fail when the db initialization code changes). For example, in this case ensuring Customer 1 is and always will be a VIP, as far as the test is concerned.
Executing Tests
Statics cause a real problem when running suites of unit tests together (for example, with your Continuous Integration server). Imagine a static map of network Socket objects that remains open from one test to another. The first test might open a Socket on port 8080, but you forgot to clear out the Map when the test gets torn down. Now when a second test launches, it is likely to crash when it tries to create a new Socket for port 8080, since the port is still occupied. Imagine also that Socket references in your static Collection are not removed, and (with the exception of WeakHashMap) are never eligible to be garbage collected, causing a memory leak.
This is an over-generalized example, but in large systems, this problem happens ALL THE TIME. People don’t think of unit tests starting and stopping their software repeatedly in the same JVM, but it is a good test of your software design, and if you have aspirations towards high availability, it is something you need to be aware of.
These problems often arise with framework objects, for example, your DB access, caching, messaging, and logging layers. If you are using Java EE or some best of breed frameworks, they probably manage a lot of this for you, but if like me you are dealing with a legacy system, you might have a lot of custom frameworks to access these layers.
If the system configuration that applies to these framework components changes between unit tests, and the unit test framework doesn’t tear down and rebuild the components, these changes can’t take effect, and when a test relies on those changes, they will fail.
Even non-framework components are subject to this problem. Imagine a static map called OpenOrders. You write one test that creates a few open orders, and checks to make sure they are all in the right state, then the test ends. Another developer writes a second test which puts the orders it needs into the OpenOrders map, then asserts the number of orders is accurate. Run individually, these tests would both pass, but when run together in a suite, they will fail.
Worse, failure might be based on the order in which the tests were run.
In this case, by avoiding statics, you avoid the risk of persisting data across test instances, ensuring better test reliability.
Subtle Bugs
If you work in high availability environment, or anywhere that threads might be started and stopped, the same concern mentioned above with unit test suites can apply when your code is running on production as well.
When dealing with threads, rather than using a static object to store data, it is better to use an object initialized during the thread’s startup phase. This way, each time the thread is started, a new instance of the object (with a potentially new configuration) is created, and you avoid data from one instance of the thread bleeding through to the next instance.
When a thread dies, a static object doesn’t get reset or garbage collected. Imagine you have a thread called “EmailCustomers”, and when it starts it populates a static String collection with a list of email addresses, then begins emailing each of the addresses. Lets say the thread is interrupted or canceled somehow, so your high availability framework restarts the thread. Then when the thread starts up, it reloads the list of customers. But because the collection is static, it might retain the list of email addresses from the previous collection. Now some customers might get duplicate emails.
An Aside: Static Final
The use of “static final” is effectively the Java equivalent of a C #define, although there are technical implementation differences. A C/C++ #define is swapped out of the code by the pre-processor, before compilation. A Java “static final” will end up memory resident in the JVM's class memory, making it (usually) permanent in ram. In that way, it is more similar to a “static const” variable in C++ than it is to a #define.
Summary
I hope this helps explain a few basic reasons why statics are problematic up. If you are using a modern Java framework like Java EE or Spring, etc, you may not encounter many of these situations, but if you are working with a large body of legacy code, they can become much more frequent.
There are 2 main problems with static variables:
Thread Safety - static resources are by definition not thread-safe
Code Implicity - You do not know when a static variables is instantiated and whether or not it will be instantiated before another static variable
Summarising few basic Advantages & Disadvantages of using Static methods in Java:
Advantages:
Globally accessible i.e. not tied with any particular object instance.
One instance per JVM.
Can be accessed by using class name (No object require).
Contains a single value applicable to all instances.
Load up on JVM startup and dies when JVM shuts down.
They doesn't modify state of Object.
Disadvantages:
Static members are always part of memory whether they are in use or not.
You can not control creation and destruction of static variable. Usefully they have been created at program loading and destroyed when program unload (or when JVM shuts down).
You can make statics thread safe using synchronize but you need some extra efforts.
If one thread change value of a static variable that can possibly break functionality of other threads.
You must know “static“ before using it.
You cannot override static methods.
Serialization doesn't work well with them.
They don't participate in runtime polymorphism.
There is a memory issue (to some extent but not much I guess) if a large number of static variables/methods are used. Because they will not be Garbage Collected until program ends.
Static methods are hard to test too.
Static variables are generally considered bad because they represent global state and are therefore much more difficult to reason about. In particular, they break the assumptions of object-oriented programming. In object-oriented programming, each object has its own state, represented by instance (non-static) variables. Static variables represent state across instances which can be much more difficult to unit test. This is mainly because it is more difficult to isolate changes to static variables to a single test.
That being said, it is important to make a distinction between regular static variables (generally considered bad), and final static variables (AKA constants; not so bad).
Since no one* has mentioned it: concurrency. Static variables can surprise you if you have multiple threads reading and writing to the static variable. This is common in web applications (e.g., ASP.NET) and it can cause some rather maddening bugs. For example, if you have a static variable that is updated by a page, and the page is requested by two people at "nearly the same time", one user may get the result expected by the other user, or worse.
statics reduce the inter-dependencies on the other parts of the code. They can act as perfect state holders
I hope you're prepared to use locks and deal with contention.
*Actually, Preet Sangha mentioned it.
if I had to make 10,000 calls to a function within a class, I would be
glad to make the method static and use a straightforward
class.methodCall() on it instead of cluttering the memory with 10,000
instances of the class, Right?
You have to balance the need for encapsulating data into an object with a state, versus the need of simply computing the result of a function on some data.
Moreover statics reduce the inter-dependencies on the other parts of the code.
So does encapsulation. In large applications, statics tend to produce spaghetti code and don't easily allow refactoring or testing.
The other answers also provide good reasons against excessive use of statics.
In my opinion it's hardly ever about performance, it's about design. I don't consider the use of static methods wrong as apposed of the use of static variables (but I guess you are actually talking about method calls).
It's simply about how to isolate logic and give it a good place. Sometimes that justifies using static methods of which java.lang.Math is a good example. I think when you name most of your classes XxxUtil or Xxxhelper you'd better reconsider your design.
I have just summarized some of the points made in the answers. If you find anything wrong please feel free to correct it.
Scaling: We have exactly one instance of a static variable per JVM. Suppose we are developing a library management system and we decided to put the name of book a static variable as there is only one per book. But if system grows and we are using multiple JVMs then we dont have a way to figure out which book we are dealing with?
Thread-Safety: Both instance variable and static variable need to be controlled when used in multi threaded environment. But in case of an instance variable it does not need protection unless it is explicitly shared between threads but in case of a static variable it is always shared by all the threads in the process.
Testing: Though testable design does not equal to good design but we will rarely observe a good design that is not testable. As static variables represent global state and it gets very difficult to test them.
Reasoning about state: If I create a new instance of a class then we can reason about the state of this instance but if it is having static variables then it could be in any state. Why? Because it is possible that the static variable has been modified by some different instance as static variable is shared across instances.
Serialization: Serialization also does not work well with them.
Creation and destruction: Creation and destruction of static variables can not be controlled. Usually they are created and destroyed at program loading and unloading time. It means they are bad for memory management and also add up the initialization time at start up.
But what if we really need them?
But sometimes we may have a genuine need of them. If we really feel the need of many static variables that are shared across the application then one option is to make use of Singleton Design pattern which will have all these variables. Or we can create some object which will have these static variable and can be passed around.
Also if the static variable is marked final it becomes a constant and value assigned to it once cannot be changed. It means it will save us from all the problems we face due to its mutability.
Seems to me that you're asking about static variables but you also point out static methods in your examples.
Static variables are not evil - they have its adoption as global variables like constants in most cases combined with final modifier, but as it said don't overuse them.
Static methods aka utility method. It isn't generally a bad practice to use them but major concern is that they might obstruct testing.
As a example of great java project that use a lot of statics and do it right way please look at Play! framework. There is also discussion about it in SO.
Static variables/methods combined with static import are also widely used in libraries that facilitate declarative programming in java like: make it easy or Hamcrest. It wouldn't be possible without a lot of static variables and methods.
So static variables (and methods) are good but use them wisely!
Static variables most importantly creates problem with security of data (any time changed,anyone can change,direct access without object, etc.)
For further info read this
Thanks.
It might be suggested that in most cases where you use a static variable, you really want to be using the singleton pattern.
The problem with global states is that sometimes what makes sense as global in a simpler context, needs to be a bit more flexible in a practical context, and this is where the singleton pattern becomes useful.
Yet another reason: fragility.
If you have a class, most people expect to be able to create it and use it at will.
You can document it's not the case, or protect against it (singleton/factory pattern) - but that's extra work, and therefore an additional cost.
Even then, in a big company, chances are someone will try at some point to use your class without fully paying attention to all the nice comments or the factory.
If you're using static variables a lot, that will break. Bugs are expensive.
Between a .0001% performance improvement and robustness to change by potentially clueless developers, in a lot of cases robustness is the good choice.
I find static variables more convenient to use. And I presume that they are efficient too (Please correct me if I am wrong) because if I had to make 10,000 calls to a function within a class, I would be glad to make the method static and use a straightforward class.methodCall() on it instead of cluttering the memory with 10,000 instances of the class, Right?
I see what you think, but a simple Singleton pattern will do the same without having to instantiate 10 000 objects.
static methods can be used, but only for functions that are related to the object domain and do not need or use internal properties of the object.
ex:
public class WaterContainer {
private int size;
private int brand;
...etc
public static int convertToGallon(int liters)...
public static int convertToLiters(int gallon)...
}
The issue of 'Statics being evil' is more of an issue about global state. The appropriate time for a variable to be static, is if it does not ever have more than one state; IE tools that should be accessible by the entire framework and always return the same results for the same method calls are never 'evil' as statics. As to your comment:
I find static variables more convenient to use. And I presume that they are efficient too
Statics are the ideal and efficient choice for variables/classes that do not ever change.
The problem with global state is the inherent inconsistency that it can create. Documentation about unit tests often address this issue, since any time there is a global state that can be accessed by more than multiple unrelated objects, your unit tests will be incomplete, and not 'unit' grained. As mentioned in this article about global state and singletons, if object A and B are unrelated (as in one is not expressly given reference to another), then A should not be able to affect the state of B.
There are some exceptions to the ban global state in good code, such as the clock. Time is global, and--in some sense--it changes the state of objects without having a coded relationship.
My $.02 is that several of these answers are confusing the issue, rather than saying "statics are bad" I think its better to talk about scoping and instances.
What I would say is that a static is a "class" variables - it represenst a value that is shared across all instances of that class. Typically it should be scoped that way as well (protected or private to class and its instances).
If you plan to put class-level behavior around it and expose it to other code, then a singleton may be a better solution to support changes in the future (as #Jessica suggested). This is because you can use interfaces at the instance/singleton level in ways that you can not use at the class level - in particular inheritance.
Some thoughts on why I think some of the aspects in other answers are not core to the question...
Statics are not "global". In Java scoping is controlled separately from static/instance.
Concurrency is no less dangerous for statics than instance methods. It's still state that needs to be protected. Sure you may have 1000 instances with an instance variable each and only one static variable, but if the code accessing either isn't written in a thread-safe way you are still screwed - it just may take a little longer for you to realize it.
Managing life cycle is an interesting argument, but I think it's a less important one. I don't see why its any harder to manage a pair of class methods like init()/clear() than the creation and destroying of a singleton instance. In fact, some might say a singleton is a little more complicated due to GC.
PS, In terms of Smalltalk, many of its dialects do have class variables, but in Smalltalk classes are actually instances of Metaclass so they are really are variables on the Metaclass instance. Still, I would apply the same rule of thumb. If they are being used for shared state across instances then ok. If they are supporting public functionality you should look at a Singleton. Sigh, I sure do miss Smalltalk....
There are two main questions in your post.
First, about static variables.
Static variables are completelly unnecesary and it's use can be avoided easily. In OOP languajes in general, and in Java particularlly, function parameters are pased by reference, this is to say, if you pass an object to a funciont, you are passing a pointer to the object, so you dont need to define static variables since you can pass a pointer to the object to any scope that needs this information. Even if this implies that yo will fill your memory with pointers, this will not necesary represent a poor performance because actual memory pagging systems are optimized to handle with this, and they will maintain in memory the pages referenced by the pointers you passed to the new scope; usage of static variables may cause the system to load the memory page where they are stored when they need to be accessed (this will happen if the page has not been accesed in a long time). A good practice is to put all that static stuf together in some little "configuration clases", this will ensure the system puts it all in the same memory page.
Second, about static methods.
Static methods are not so bad, but they can quickly reduce performance. For example, think about a method that compares two objects of a class and returns a value indicating which of the objects is bigger (tipical comparison method) this method can be static or not, but when invoking it the non static form will be more eficient since it will have to solve only two references (one for each object) face to the three references that will have to solve the static version of the same method (one for the class plus two, one for each object). But as I say, this is not so bad, if we take a look at the Math class, we can find a lot of math functions defined as static methods. This is really more eficient than putting all these methods in the class defining the numbers, because most of them are rarelly used and including all of them in the number class will cause the class to be very complex and consume a lot of resources unnecesarilly.
In concluson: Avoid the use of static variables and find the correct performance equilibrium when dealing with static or non static methods.
PS: Sorry for my english.
There's nothing wrong with static variables per se. It's just the Java syntax that's broken. Each Java class actually defines two structures- a singleton object which encapsulates static variables, and an instance. Defining both in the same source block is pure evil, and results in a code that's hard to read. Scala did that right.
everything (can:) have its purpose, if you have bunch of threads that needs to share/cache data and also all accessible memory (so you dont split into contexts within one JVM) the static is best choice-> of course you can force just one instance, but why?
i find some of the comments in this thread evil, not the statics ;)
Static variables are not good nor evil. They represent attributes that describe the whole class and not a particular instance. If you need to have a counter for all the instances of a certain class, a static variable would be the right place to hold the value.
Problems appear when you try to use static variables for holding instance related values.
a) Reason about programs.
If you have a small- to midsize-program, where the static variable Global.foo is accessed, the call to it normally comes from nowhere - there is no path, and therefore no timeline, how the variable comes to the place, where it is used. Now how do I know who set it to its actual value? How do I know, what happens, if I modify it right now? I have grep over the whole source, to collect all accesses, to know, what is going on.
If you know how you use it, because you just wrote the code, the problem is invisible, but if you try to understand foreign code, you will understand.
b) Do you really only need one?
Static variables often prevent multiple programs of the same kind running in the same JVM with different values. You often don't foresee usages, where more than one instance of your program is useful, but if it evolves, or if it is useful for others, they might experience situations, where they would like to start more than one instance of your program.
Only more or less useless code which will not be used by many people over a longer time in an intensive way might go well with static variables.
All the answers above show why statics are bad. The reason they are evil is because it gives the false impression that you are writing object oriented code, when in fact you are not.
That is just plain evil.
There are plenty of good answers here, adding to it,
Memory:
Static variables are live as long as the class loader lives[in general till VM dies], but this is only in-case of bulk objects/references stored as static.
Modularization:
consider concepts like IOC, dependencyInjection, proxy etc.. All are completely against tightly coupling/static implementations.
Other Con's: Thread Safety, Testability
I've played with statics a lot and may I give you a slightly different answer--or maybe a slightly different way to look at it?
When I've used statics in a class (Members and methods both) I eventually started to notice that my class is actually two classes sharing responsibility--there is the "Static" part which acts a lot like a singleton and there is the non-static part (a normal class). As far as I know you can always separate those two classes completely by just selecting all the statics for one class and non-statics for the other.
This used to happen a lot when I had a static collection inside a class holding instances of the class and some static methods to manage the collection. Once you think about it, it's obvious that your class is not doing "Just one thing", it's being a collection and the doing something completely different.
Now, let's refactor the problem a little: If you split your class up into one class where everything is static and another which is just a "Normal Class" and forget about the "Normal Class" then your question becomes purely Static class vs Singleton which is addressed in length here (and probably a dozen other questions).
Static fields are de facto GC roots (see the How Garbage Collection Works section earlier in this chapter), which means they are never garbage-collected! For convenience alone, static fields and collections are often used to hold caches or share state across threads. Mutable static fields need to be cleaned up explicitly. If the developer does not consider every possibility (a near certainty), the cleanup will not take place, resulting in a memory leak. This sort of careless programming means that static fields and collections have become the most common cause of memory leaks!
In short, never use mutable static fields—use only constants. If you think you need mutable static fields, think about it again, and then again! There's always a more appropriate technique.
I think excessive uses of global variables with static keyword will also leads to memory leakage at some point of instance in the applica
From my point of view static variable should be only read only data or variables created by convention.
For example we have a ui of some project, and we have a list of countries, languages, user roles, etc. And we have class to organize this data. we absolutely sure that app will not work without this lists. so the first that we do on app init is checking this list for updates and getting this list from api (if needed). So we agree that this data is "always" present in app. It is practically read only data so we don't need to take care of it's state - thinking about this case we really don't want to have a lot of instances of those data - this case looks a perfect candidate to be static.
I am working on my first mutlithreaded program and got stuck about a couple of aspects of synchronization. I have gone over the multi-threading tutorial on oracle/sun homepage, as well as a number of questions here on SO, so I believe I have an idea of what synchronization is. However, as I mentioned there are a couple of aspects I am not quite sure how to figure out. I formulated them below in form of clear-cut question:
Question 1: I have a singleton class that holds methods for checking valid identifiers. It turns out this class needs to hold to collections to keep track of associations between 2 different identifier types. (If the word identifier sounds complicated; these are just strings). I chose to implement two MultiValueMap instances to implement this many-to-many relationship. I am not sure if these collections have to be thread-safe as the collection will be updated only at the creation of the instance of the singleton class but nevertheless I noticed that in the documentation it says:
Note that MultiValueMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. This class may throw exceptions when accessed by concurrent threads without synchronization.
Could anyone elaborate on this "appropriate synchronization"? What exactly does it mean? I can't really use MultiValueMap.decorate() on a synchronized HashMap, or have I misunderstood something?
Question 2: I have another class that extends a HashMap to hold my experimental values, that are parsed in when the software starts. This class is meant to provide appropriate methods for my analysis, such as permutation(), randomization(), filtering(criteria) etc. Since I want to protect my data as much as possible, the class is created and updated once, and all the above mentioned methods return new collections. Again, I am not sure if this class needs to be thread-safe, as it's not supposed to be updated from multiple threads, but the methods will most certainly be called from a number of threads, and to be "safe" I have added synchronized modifier to all my methods. Can you foresee any problems with that? What kind of potential problems should I be aware of?
Thanks,
Answer 1: Your singleton class should not expose the collections it uses internally to other objects. Instead it should provide appropriate methods to expose the behaviours you want. For example, if your object has a Map in it, don't have a public or protected method to return that Map. Instead have a method that takes a key and returns the corresponding value in the Map (and optionally one that sets the value for the key). These methods can then be made thread safe if required.
NB even for collections that you do not intend to write to, I don't think you should assume that reads are necessarily thread safe unless they are documented to be so. The collection object might maintain some internal state that you don't see, but might get modified on reads.
Answer 2: Firstly, I don't think that inheritance is necessarily the correct thing to use here. I would have a class that provides your methods and has a HashMap as a private member. As long as your methods don't change the internal state of the object or the HashMap, they won't have to be synchronised.
It's hard to give general rules about synchronization, but your general understanding is right. A data-structure which is used in a read-only way, does not have to be synchronized. But, (1) you have to ensure that nobody (i.e. no other thread) can use this structure before it is properly initialized and (2) that the structure is indeed read-only. Remember, even iterators have a remove method.
To your second question: In order to ensure the immutability, i.e. that it is read-only, I would not inherit the HashMap but use it inside your class.
Synchronization commonly is needed when you either could have concurrent modifications of the underlying data or one thread modifies the data while another reads and needs to see that modification.
In your case, if I understand it correctly, the MultiValueMap is filled once upon creation and the just read. So unless reading the map would modify some internals it should be safe to read it from multiple threads without synchronization. The creation process should be synchronized or you should at least prevent read access during initialization (a simple flag might be sufficient).
The class you descibe in question 2 might not need to be synchronized if you always return new collections and no internals of the base collection are modified during creation of those "copies".
One additional note: be aware of the fact that the values in the collections might need to be synchronized as well, since if you safely get an object from the collection in multiple thread but then concurrently modify that object you'll still get problems.
So as a general rule of thumb: read-only access does not necessarily need synchronization (if the objects are not modified during those reads or if that doesn't matter), write access should generally be synchronized.
If your maps are populated once, at the time the class is loaded (i.e. in a static initializer block), and are never modified afterwards (i.e. no elements or associations are added / removed), you are fine. Static initialization is guaranteed to be performed in a thread safe manner by the JVM, and its results are visible to all threads. So in this case you most probably don't need any further synchronization.
If the maps are instance members (this is not clear to me from your description), but not modified after creation, I would say again you are most probably safe if you declare your members final (unless you publish the this object reference prematurely, i.e. pass it to the outside world from the cunstructor somehow before the constructor is finished).
When a getter returns a property, such as returning a List of other related objects, should that list and it's objects be immutable to prevent code outside of the class, changing the state of those objects, without the main parent object knowing?
For example if a Contact object, has a getDetails getter, which returns a List of ContactDetails objects, then any code calling that getter:
can remove ContactDetail objects from that list without the Contact object knowing of it.
can change each ContactDetail object without the Contact object knowing of it.
So what should we do here? Should we just trust the calling code and return easily mutable objects, or go the hard way and make a immutable class for each mutable class?
It's a matter of whether you should be "defensive" in your code. If you're the (sole) user of your class and you trust yourself then by all means no need for immutability. However, if this code needs to work no matter what, or you don't trust your user, then make everything that is externalized immutable.
That said, most properties I create are mutable. An occasional user botches this up, but then again it's his/her fault, since it is clearly documented that mutation should not occur via mutable objects received via getters.
It depends on the context. If the list is intended to be mutable, there is no point in cluttering up the API of the main class with methods to mutate it when List has a perfectly good API of its own.
However, if the main class can't cope with mutations, then you'll need to return an immutable list - and the entries in the list may also need to be immutable themselves.
Don't forget, though, that you can return a custom List implementation that knows how to respond safely to mutation requests, whether by firing events or by performing any required actions directly. In fact, this is a classic example of a good time to use an inner class.
If you have control of the calling code then what matters most is that the choice you make is documented well in all the right places.
Joshua Bloch in his excellent "Effective Java" book says that you should ALWAYS make defensive copies when returning something like this. That may be a little extreme, especially if the ContactDetails objects are not Cloneable, but it's always the safe way. If in doubt always favour code safety over performance - unless profiling has shown that the cloneing is a real performance bottleneck.
There are actually several levels of protection you can add. You can simply return the member, which is essentially giving any other class access to the internals of your class. Very unsafe, but in fairness widely done. It will also cause you trouble later if you want to change the internals so that the ContactDetails are stored in a Set. You can return a newly-created list with references to the same objects in the internal list. This is safer - another class can't remove or add to the list, but it can modify the existing objects. Thirdly return a newly created list with copies of the ContactDetails objects. That's the safe way, but can be expensive.
I would do this a better way. Don't return a list at all - instead return an iterator over a list. That way you don't have to create a new list (List has a method to get an iterator) but the external class can't modify the list. It can still modify the items, unless you write your own iterator that clones the elements as needed. If you later switch to using another collection internally it can still return an iterator, so no external changes are needed.
In the particular case of a Collection, List, Set, or Map in Java, it is easy to return an immutable view to the class using return Collections.unmodifiableList(list);
Of course, if it is possible that the backing-data will still be modified then you need to make a full copy of the list.
Depends on the context, really. But generally, yes, one should write as defensive code as possible (returning array copies, returning readonly wrappers around collections etc.). In any case, it should be clearly documented.
I used to return a read-only version of the list, or at least, a copy. But each object contained in the list must be editable, unless they are immutable by design.
I think you'll find that it's very rare for every gettable to be immutable.
What you could do is to fire events when a property is changed within such objects. Not a perfect solution either.
Documentation is probably the most pragmatic solution ;)
Your first imperative should be to follow the Law of Demeter or ‘Tell don't ask’; tell the object instance what to do e.g.
contact.print( printer ) ; // or
contact.show( new Dialog() ) ; // or
contactList.findByName( searchName ).print( printer ) ;
Object-oriented code tells objects to do things. Procedural code gets information then acts on that information. Asking an object to reveal the details of its internals breaks encapsulation, it is procedural code, not sound OO programming and as Will has already said it is a flawed design.
If you follow the Law of Demeter approach any change in the state of an object occurs through its defined interface, therefore side-effects are known and controlled. Your problem goes away.
When I was starting out I was still heavily under the influence of HIDE YOUR DATA OO PRINCIPALS LOL. I would sit and ponder what would happen if somebody changed the state of one of the objects exposed by a property. Should I make them read only for external callers? Should I not expose them at all?
Collections brought out these anxieties to the extreme. I mean, somebody could remove all the objects in the collection while I'm not looking!
I eventually realized that if your objects' hold such tight dependencies on their externally visible properties and their types that, if somebody touches them in a bad place you go boom, your architecture is flawed.
There are valid reasons to make your external properties readonly and their types immutable. But that is the corner case, not the typical one, imho.
First of all, setters and getters are an indication of bad OO. Generally the idea of OO is you ask the object to do something for you. Setting and getting is the opposite. Sun should have figured out some other way to implement Java beans so that people wouldn't pick up this pattern and think it's "Correct".
Secondly, each object you have should be a world in itself--generally, if you are going to use setters and getters they should return fairly safe independent objects. Those objects may or may not be immutable because they are just first-class objects. The other possibility is that they return native types which are always immutable. So saying "Should setters and getters return something immutable" doesn't make too much sense.
As for making immutable objects themselves, you should virtually always make the members inside your object final unless you have a strong reason not to (Final should have been the default, "mutable" should be a keyword that overrides that default). This implies that wherever possible, objects will be immutable.
As for predefined quasi-object things you might pass around, I recommend you wrap stuff like collections and groups of values that go together into their own classes with their own methods. I virtually never pass around an unprotected collection simply because you aren't giving any guidance/help on how it's used where the use of a well-designed object should be obvious. Safety is also a factor since allowing someone access to a collection inside your class makes it virtually impossible to ensure that the class will always be valid.