This question already has answers here:
Difference between static class and singleton pattern?
(41 answers)
Closed 9 years ago.
Should a singleton in Java use static variables or member variables? Are there any advantages to either?
You should use member variables. A singleton is an object (i.e. an instance of a class) and so should be modelled as such; even if you only ever intend to create one of them.
Statics should be used for class level variables.
There needs to be a static reference to the singleton instance, but the instance itself should use instance variables, just like a regular class.
The reason is that the singleton instance is after all an object, so the usual good design principles still apply to its class.
Further, today it's a singleton, but tomorrow it may be a ThreadLocal, or not have any kind of instance creation restrictions. The change between these architectural choices is very low if the class is designed in the usual way. If you use static fields, such changes would require more maintenance work to make the fields non-static.
You can avoid using static variables and use Enum instead:
public enum MySingleton {
INSTANCE;
}
You can access this singleton as MySingleton.INSTANCE.
Enum is thread safe and implementation of Singleton through Enum ensures that your singleton will have only one instance even in a multithreaded environment.
There is no mandate to use static variables or member variables. As singleton is going to have only one instance so logically, using static or member variable does not make any difference. Apart form that, static and instance variable difference hold, static variables will be initialized during class loading but instance variables will be initialized during instance creation.
But as a general programming rule, you should decided whether you need a static variable or not. Don't simply create public static variables and end up with unnecessary problems. So in my personal opinion, instance variables should be preferred to keep things simple and in control.
It depends on concrete variable. Definitely the most common usage is that singleton is normal object that holds member variables. You can, for example, easily replace one object with another (with all other properties).
Every class can have static variables but it does not deppend wheter it is singleton or not.
Singleton pattern in its nature deal with instance of object. Statics are the metter of classes --> no relation with singleton pattern.
Its not mandatory to use static variables.You can use enum also
see this
A lot of information is already given: use static variables or a static variable pointing to your instance or use an enum.
A big difference is when your single ton is a member of a class that is a subclass of another class.
Hence your singleton instance inherits from the super class.
This can have a huge advantage.
Enums are not allowed to extend each other, but you can use an enum that implements interfaces.
Related
I was reading a code snippet:
Class MyDAO{
public static final MyDAO DAO = new MyDAO();
public void loadData(){
//Hibernate Code to do something
}
...
}
So I'm amazed, and questions I'm facing right now is:
as static keyword denotes that a "member variable, or method, can be accessed without requiring an instantiation of the class to which it belongs". In simple terms, it means that you can call a method, even if you've never created the object to which it belongs.
What is the use of declaring a data member as static and instantiating a class then? is it a design pattern or what? and Most Importantly what is use of that? Any how when you say Class.staticMember how is the class loaded into the memory of JVM?
It can be used to implement Singleton pattern.
Data member is to be declared when you want it to hold same value across all instances(shared between different object of the class)
Declaring static is not a design pattern, but it is a way to design your application.
Further to this if your member is only consumed by methods in class and it is not expected to be accessed directly you can make it private static.
This is a way of providing the DAO object to the rest of the application as a global object. It is crude and ugly and makes testing hard, because it's difficult to provide a mock implementation of the DAO.
Static members are initialized at the time that the class is loaded. Classes are loaded lazily when they are first referenced. Other classes in the application can access this DAO without having to initialize it.
Code like this is why Dependency Injection frameworks (Spring, Guice, Hivemind, etc.) were created. This code makes the application depend on a specific implementation instead of on an abstraction, using dependency injection reduces coupling and increases testability by having the application depend on an abstraction and having the DI container be in charge of selecting the implementation and enforcing singleton scope.
What is the use of declaring a data member as static and instantiating a class then?
Having a data member being static does not give you any reason for not instantiating that class, because your class may still contain other non-static members.
So when do you not have to instantiate the class?
When all your members are static such as a utility class. An example is the Math class in Java.
Most Importantly what is use of that?
If you class contains non-static members, how do you expect those data to be accessed if you do not instantiate the class? Remember that your non-static members will not exist in memory if the class they belongs to is not instantiated.
is it a design pattern or what?
It has no direct relation with design patterns. Generally, you declare a member as static when it is a behaviour or property of the class itself and not individual objects.
Any how when you say Class.staticMember class is loaded into the memory of JVM.
I won't know exactly how Java memory works internally, but static members exist even before object instantiation which means variables declared as static are already created and loaded during runtime.
That also implies static members actually belongs to the class (not individual instance).
This question already has answers here:
What does the 'static' keyword do in a class?
(22 answers)
Closed 8 years ago.
I'm quite new to Java and I'm trying to understand static variables and methods. Could someone give me a brief explanation of why I'd use the static method over the instance method and also what would be the ideal situation to use static variables? I know that static variables and methods are part of the class and not an instance of said class but I'm having trouble understanding when I'd use it.
Thanks!
These are the main differences
1.Static variables take memory only 1 time during the lifetime of program.
2.You can call static variables or methods. without creating the object of that class by classname only.
3.Static variables can be called in static and non-static context.
Mainly static variables are used to save memory because every object takes memory .
Static members relate to a type not a concrete instance. So instance methods and variables concern the identity of the instance, e.g. an instance method can alter the state of the object.
To some extend this is a religious question. Some people prefer to create static methods in a class when they do not access any instance variable. Other people do not like this approach. Another usual usage is creating factory methods, able to create an instance of an object while itself being called statically.
Often there are utility classes that contain static methods to serve clients with some functionality that is not bound to any type context. But all too often these utility classes get abused by using them as a trash bin for all sorts of methods a programmer did not find a fitting class for. This may be a result of poor design.
A popular use of static variables is the definition of an logger instance (e.g. SLF4J) for a class because loggers are thread safe and can be used by all instances and static methods alike. One advantage is (see a comparison) that only one instance has to be created for all instances of the containing class.
For static variable or method, you can think of that there is only one copy of the variable or method during the whole life time of the program.
So if you there is variable whose value should be shared across all the processes or thread, you can declare a static variable. For example, when you want to use a singleton pattern, you will need a static variable to indicate if there is instance of the class already.
For static methods, I would use them when state of the class is not important. I think the best example is java.lang.Math, they are all static methods, and returns the expected values to you no matter what you called before.
I know how to create a singleton object by declaring its constructor private. But my doubt is: can we create a singleton object by declaring all its methods and variables static. If so, what challenges we will face?
If you declare all methods and variables of a class static, you can still create arbitrary many instances of that class. These objects will have only the inherited methods and variables. But all newly declared methods and variable are global. That is very similar to a singleton object, but not the same.
E.g. let's say you have a singleton class implementing the Collection interface, than you can have only one instance. But you are free to give it to any method requiring a collection instance. That is impossible when you make everything in a class static.
Making members of classes static means you use the class not as class but as a namespace.
by declaring all variables and methods static, you are practiacally making it not an object. It will be similar to a c program with global variables more then a singleton pattern - very not OOP style!
You will also make it impossible for this class to implement any interface [remember there is no overriding with static methods...] again, not very OOP style.
When you do that, you can create as many instances of the class as you like, though the underlying things will remain unchanged.
Even if you don't create any instance, the attributes will remain: because static elements are properties of the class, not the object.
I guess that you're in the case where singleton is not needed because almost all your methods are static, that say that you don't have to retains any state in any object... like a singleton is designed for.
A singleton is a stateful object offering static methods, like would be a ConnectionPool's singleton for instance, that retains information about the underlying backend service.
So, I'd do is to decide whether you need some shared state to be kept in order to execute your methods. It will drive your implementation.
So I have a question regarding best practices. Basically I'm doing the following to make accessing members of different classes easier:
class myClass1 {
public static int var1;
public static String var2;
//...
public static void method1() {
//...
}
}
And then in other classes I can just access myClass1 members with myClass1.var1, myClass1.var2, myClass1.method1(). Another design pattern I see is to not use static at all and just do myClass1 instance = new myClass1(); and then do instance.method1(); or whatever.
I remember hearing something somewhere about static being bad... relating to global objects or whatever. But it's been a while since intro to computer science, heh.
Anyways, beginner Java programmer just looking to get some insight into best practices. Thanks.
The semantics of static vs. non-static member variables and methods are completely different. Non-static variables are members of instances of the class; each instance has its own copy. Static variables are members of the class itself; they're not tied to any particular instance.
Similarly, non-static methods operate on instances of the class, static methods aren't tied to a particular instance.
You should use static/non-static as the problem requires. This isn't a question of best practices.
In general having all the members fields/methods public static is considered bad practice for Object Oriented Programming paradigm. It takes all the notions of object encapsulation and data security. Any client of your class is free to tamper your data any time. This kind of practice is very similar to defining global variables and functions in procedural languages like C.
There are several reasons why this is a bad idea:
Public field. Using public fields makes it practically impossible to write thread-safe code. It also makes it hard to maintain or modify your code. On a theoretical level it violates encapsulation, which is one of the basic ideas of OO with all its consequences. For example if you have complex state, where not every combination of field values is valid, you're in trouble.
Static field. Although static fields have their legitimate uses, it should be kept to a minimum. They aren't inherited, which can easily lead to confusion and at the best of times it's a ticking time bomb.
All in all: don't use static fields unless it is necessary, and even then they should be private.
The notable exception is obviously static and final fields (aka. constants) which can be declared public without too many dangers.
Generally speaking one never makes static variables except for static final variables which are then like constants. The primary reason is that more than one thread can then change the state of the global variable at the same time leading to unpredictable state of every object instance of that class.
As per Object oriented fundamental concerns :
Variable should not be accessible outside the class. So they should be private not public except interface case. This is applicable to Static variable as well.
You want to access the variable use public method. In case of static variable you will static public method.
It depends entirely what you are doing. If your class just holds stateless utility functions then this may be OK. If you are trying to any kind of real OOP design, then this doesn't make sense.
If you use instances of classes to model objects in the 'real world', then they will need instance variables, and should have instance methods to act upon that data. Each instance encapsulates that data and provides suitable behaviour.
This question already has answers here:
Difference between static class and singleton pattern?
(41 answers)
Closed 5 years ago.
How is a singleton different from a class filled with only static fields?
Almost every time I write a static class, I end up wishing I had implemented it as a non-static class. Consider:
A non-static class can be extended. Polymorphism can save a lot of repetition.
A non-static class can implement an interface, which can come in handy when you want to separate implementation from API.
Because of these two points, non-static classes make it possible to write more reliable unit tests for items that depend on them, among other things.
A singleton pattern is only a half-step away from static classes, however. You sort of get these benefits, but if you are accessing them directly within other classes via `ClassName.Instance', you're creating an obstacle to accessing these benefits. Like ph0enix pointed out, you're much better off using a dependency injection pattern. That way, a DI framework can be told that a particular class is (or is not) a singleton. You get all the benefits of mocking, unit testing, polymorphism, and a lot more flexibility.
Let's me sum up :)
The essential difference is: The existence form of a singleton is an object, static is not. This conduced the following things:
Singleton can be extended. Static not.
Singleton creation may not be threadsafe if it isn't implemented properly. Static not.
Singleton can be passed around as an object. Static not.
Singleton can be garbage collected. Static not.
Singleton is better than static class!
More here but I haven't realized yet :)
Last but not least, whenever you are going to implement a singleton, please consider to redesign your idea for not using this God object (believe me, you will tend to put all the "interesting" stuffs to this class) and use a normal class named "Context" or something like that instead.
A singleton can be initialized lazily, for one.
I think, significant thing is 'object' in object oriented programing. Except from few cases we should restrict to usage of static classes. That cases are:
When the create an object is meaningless. Like methods of java.lang.Math. We can use the class like an object. Because the behavior of Math class methods doesn't depend on the state of the objects to be created in this class.
Codes to be used jointly by more than one object method, the codes that do not reach the object's variables and are likely to be closed out can be static methods
Another important thing is singleton is extensible. Singleton can be extended. In the Math class, using final methods, the creation and extension of the object of this class has been avoided. The same is true for the java.lang.System class. However, the Runtime class is a single object, not a static method. In this case you can override the inheritance methods of the Runtime class for different purposes.
You can delay the creation of a Singleton object until it is needed (lazy loading). However, for static method classes, there is no such thing as a condition. If you reach any static member of the class, the class will be loaded into memory.
As a result, the most basic benefit to the static method class is that you do not have to create an object, but when used improperly, it will remove your code from being object-oriented.
The difference is language independent. Singleton is by definition: "Ensure a class has only one instance and provide a global point of access to it. " a class filled with only static fields is not same as singleton but perhaps in your usage scenario they provide the same functionality. But as JRL said lazy initiation is one difference.
At least you can more easily replace it by a mock or a stub for unit testing. But I am not a big fan of singletons for exactly the reason you are describing : it are global variables in disguise.
A singleton class will have an instance which generally is one and only one per classloader. So it can have regular methods(non static) ones and they can be invoked on that particular instance.
While a Class with only static methods, there is really no need in creating an instance(for this reason most of the people/frameworks make these kind of Util classes abstract). You will just invoke the methods on class directly.
The first thing that comes to mind is that if you want to use a class with only static methods and attributes instead of a singleton you will have to use the static initializer to properly initialise certain attributes. Example:
class NoSingleton {
static {
//initialize foo with something complex that can't be done otherwise
}
static private foo;
}
This will then execute at class load time which is probably not what you want. You have more control over this whole shebang if you implement it as a singleton. However I think using singletons is not a good idea in any case.
A singleton is a class with just one instance, enforced. That class may have state (yes I know static variables hold state), not all of the member variables or methods need be static.
A variation would be a small pool of these objects, which would be impossible if all of the methods were static.
NOTE: The examples are in C#, as that is what I am more familiar with, but the concept should apply to Java just the same.
Ignoring the debate on when it is appropriate to use Singleton objects, one primary difference that I am aware of is that a Singleton object has an instance that you can pass around.
If you use a static class, you hard-wire yourself to a particular implementation, and there's no way to alter its behavior at run-time.
Poor design using static class:
public class MyClass
{
public void SomeMethod(string filename)
{
if (File.Exists(filename))
// do something
}
}
Alternatively, you could have your constructor take in an instance of a particular interface instead. In production, you could use a Singleton implementation of that interface, but in unit tests, you can simply mock the interface and alter its behavior to satisfy your needs (making it thrown some obscure exception, for example).
public class MyClass
{
private IFileSystem m_fileSystem;
public MyClass(IFileSystem fileSystem)
{
m_fileSystem = fileSystem;
}
public void SomeMethod(string filename)
{
if (m_fileSystem.FileExists(filename))
// do something
}
}
This is not to say that static classes are ALWAYS bad, just not a great candidate for things like file systems, database connections, and other lower layer dependencies.
One of the main advantages of singletons is that you can implement interfaces and inherit from other classes. Sometimes you have a group of singletons that all provide similar functionality that you want to implement a common interface but are responsible for a different resource.
Singleton Class :
Singleton Class is class of which only single instance can exists per classloader.
Helper Class (Class with only static fields/methods) :
No instance of this class exists. Only fields and methods can be directly accessed as constants or helper methods.
These few lines from this blog describes it nicely:
Firstly the Singleton pattern is very
useful if you want to create one
instance of a class. For my helper
class we don't really want to
instantiate any copy's of the class.
The reason why you shouldn't use a
Singleton class is because for this
helper class we don't use any
variables. The singleton class would
be useful if it contained a set of
variables that we wanted only one set
of and the methods used those
variables but in our helper class we
don't use any variables apart from the
ones passed in (which we make final).
For this reason I don't believe we
want a singleton Instance because we
do not want any variables and we don't
want anyone instantianting this class.
So if you don't want anyone
instantiating the class, which is
normally if you have some kind of
helper/utils class then I use the what
I call the static class, a class with
a private constructor and only
consists of Static methods without any
any variables.