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 in a Introduction to Java class and I was doing a bit of research on variables. It seems that knowledgeable programers state that it is bad practice to define the variables in public visibility. I see them stating it is bad practice but I can not find a rhyme or reason to their claims. This is how I defined my variables in a application for my course.
public class DykhoffWk3Calculator
{
/*
* This class is used to define the variables in a static form so all
* classes can access them.
*/
public static double commissionRate = .03, startSalary = 45000,
accelerationFactor = 1.25;
public static double annualSales, commissionTotal, totalCompensation,
total, count, count2;
private static Object input; Object keyboard;
public static class UserInput
{ //Then continue with my other classes
I thought this was a logical method of defining them so all classes, not just main, could access them. Can someone explain to me why this is bad practice, and where variables should be defined? Any assistance would be greatly appreciated.
In short: because all of your public "surface area" for a class effectively defines its API.
If you expose things through methods, then you can change the details of how they work later. But if you expose a field, and some other class outside of your control (and quite possibly outside of your knowledge) starts referencing that field, then you're stuck with exposing that field for the rest of time. Or until you decide to break backwards-compatibility.
I thought this was a logical method of defining them so all classes, not just main, could access them.
As a general rule, you don't want "all classes" to access them. The vast majority of work with software, is spent maintaining code, not writing it for the first time. And so experienced developers realise that best practices for code, are generally the ones that make it most maintainable, not necessarily the ones that make it most convenient to write in the first place.
And if you have a variable that could be accessed from anywhere, at any time, and you want to make some tweaks to how it is modified - how can you be sure that this is safe? How long will it take you to track down all the ways that this is referenced, and determine what the effects of your change will be? (And specific to public fields, you can kiss goodbye to any sort of reusability regarding running at the same time from multiple threads, or running reentrantly.)
Broadly speaking, reducing the "surface area" of classes is a really good thing to do. Restricting the ways that other classes can interact with this one, makes it much easier to control and understand the relationships, as well as making it easier to make internal changes "invisible" to those other classes. Think about what this class does, what it will provide to other classes, as defining an interface (whether an actual interface or not). And only expose to other classes, the bare minimum that is required to fulfil those requirements.
And that never involves letting them have arbitrary access to variables.
So the general point is that you in fact DON'T want anyone to be able to access those values. Not only can I see those variables, but I can also change them to anything I like. This can lead to problems in larger, more complex programs.
Furthermore, if you wanted to later change how the class uses/stores these values, you couldn't without having to go out and change all the other classes that access those public variables directly. Instead, you should offer methods that provide just the amount of access that you want to give.
The standard analogy is that of driving a car. You know how to turn the wheel, hit the brake, etc, but not how the car actually does these things. So if the engine needed to be dramatically changed, or you got in a new car, then you'd still know how to drive. You don't need to worry about what's happening behind the scenes.
Firstly you state it wrong.
its bad to make your variable public i.e:
public String name = null; this is bad. You should always do it as
private String name = null;
To understand why, you need to dig a bit into the ideology of OOPs
OPPS ideology states that each object of your class will have 2 things:
Properties: something which we also call variables or state.
Behavior: something which we call methods or functions.
Properties identify the object over a period of time. Behaviors allow you to manage the properties of the object so that the same object over time can appear to be in different states.e.g: a Product object over a period of can be an 'Available line item' or 'Added to cart' or 'Sold' or 'Out of stock' depending on its state. Since state is critically important to the object so the object should not allow direct nonsense mutation operations on its state. Objects should keep their variables private to them and expose behaviors that the outside world can use to interact with the object and change the state based on the operation executed in the behavior. e.g: calling the 'addToCart()' behavior on the Product object that was in 'Available line item' state would probably mean: changing not just its state to 'Added to cart' but probably making other users aware that the number of this Products now available is 1 less.
So long story short: don't expose properties directly to outside work for mutation unless needed. This means dont make them public and also dont give setter methods if not needed.
By Convention Fields, methods and constructors declared public (least restrictive) within a public class are visible to any class in the Java program, whether these classes are in the same package or in another package.Which means that a change in the value of a field will definitely affect other classes accessing that field..thus breaking the whole sense of encapsulation.
Public variables in general in a class are a bad idea. Since this means other classes/programs, can modify the state of instances.
Since it is the responsibility of a class to protect its state and ensure the state is "consistent", one can enforce this by defining public setters (since this allows to run code to check/repair state).
By setting the variables public, the state is not protected. If later not all representable states are valid states, one has a problem.
Example:
Say you want to implement an ArrayList<T>, then it will look like (not fully implemented):
public class ArrayList<T> {
public int size = 0;
public Object[] data = new Object[5];
}
Now one can modify the size of the arrayList. Without adding an element. Now if you would ask the ArrayList<T> instance to remove/add/copy/...whatever, the data on which it works can be wrong.
Perhaps you can claim that a programmer is nice: he will not modify the object unless he needs to and according to the "rules". But such things eventually always go wrong, and what if you decide to modify your definition of the ArrayList (for instance using two int's for the size). In that case you would need to rewrite all code that sets such fields.
To conclude: private/protected is invented to protect a class instance from other instances that would turn the instance corrupt/invalid/inconsistent/...
In OOP why need to put something Private , for example. I know that any private member can not be accessed but with the same class objects. But why I need to do that while I am the only coder of my project. The same question extends to Protected, protected from who!
private and protected are not there to prevent other coders from accessing the internals of a class, but (also) to prevent yourself from creating a program without clearly defined interfaces.
If every class in your project can modify every other class, you're not only prone to introduce bugs because of the huge state space, but also preventing yourself from:
Changing the implementation (while keeping the interface the same) of any class.
Ever introducing anyone not familiar with all the internals of all the classes to the project. Unless you have a perfect memory and can recite every line of code you've ever written, that includes future you.
Mocking up objects for unit testing
Interacting with other versions of your program/library. Suppose you do change internals of one class, and you manage to track down every reference to that internal property in your project. Even then, you may need to interface with the old version of your program again. This becomes especially hard if used properties instead of getter/setter methods.
Access modifiers achieve two different things:
They limit the amount of code that can cause side effects, making it easier to establish invariants.
They protect clients of the class from changes to the internal representation.
For small projects, these advantages might not be immediately visible, especially for beginners.
Protected from your future self, who could otherwise accidentally forget what parts of an object are a detail that should be decoupled from the rest of the system, and which parts are a solid interface that can be relied on by the rest of the system.
the language tries to force you, to write "good" code. "good" means that the code is structured, clean and not susceptible to error. so you have to declare types, private members and so on. if you don't want that, you could use a language thats lesser in this aspects, like python. but this means, that your program could (could!) be more insecure or if it gets very big, easy to misunderstand. it's the same as with comments. you haven't to write them. especially when you are the only programmer. but it's a good style and you will be very thankfull for that if you read your program again, in a half year.
You mark the members of a class private that must not be accessed from outside the class. E.g., you use it to hide implementation details, so you can change the implementation without affecting other code using your class. Hiding implementation details is one of the key aspects of OOP (encapsulation). If you create a Car class, and then you write a lot of code that uses the Car class, and you suddenly realize that your implementation performs very poorly and you need to refactor it, if all of the Car implementation details are private to the Car class, you know that none of the code using Car accesses those things and you can change them at will. If you didn't mark them private, you might have used some of them in that other code, which would then break.
Similarly, protected (in Java, anyway) is for the same purpose but allows classes derived from your class to access those members. This is fairly weak protection, because of course it means that you can't change the implementation details of the base class without affecting derived classes.
Think about it this way: The protection level defines what you can change later without care for any other piece of code besides this class (private), without care for any other piece of code besides this class and every class inheriting from this class (protected) and without care for any other piece of code besides every piece of code using this class (public).
private or protected come from encapsulation concept. and it comes from data hiding concept. I believe this intoduction is clear and useful at least for me :
Encapsulation is the process of combining data and functions into a
single unit called class. Using the method of encapsulation, the
programmer cannot directly access the data. Data is only accessible
through the functions existing inside the class. Data encapsulation
led to the important concept of data hiding. Data hiding is the
implementation details of a class that are hidden from the user. The
concept of restricted access led programmers to write specialized
functions or methods for performing the operations on hidden members
of the class. Attention must be paid to ensure that the class is
designed properly. (Sripriya Rajagopalan)
Note: Answers are well, and this answer is to complete them
If you define a member (variable or method) of a class as private, you won't be able to use it from outside, using another class, using the dot operator. Protected helps you to protect the member variable or method from being inherited.