Related
This question already has answers here:
In laymans terms, what does 'static' mean in Java? [duplicate]
(6 answers)
Closed 7 years ago.
As I understand it:
A static class only applies to nested classes, and it means that the nested class doesn't have references to the outer class.
A static field is kind of like a global variable, in that there is only one instance of it, and it is shared by other members of the same class.
A static method means that it can be called even if the object hasn't been instantiated yet.
I am taking an introduction to Java course and am trying to cement my knowledge, as well as trying to figure out why different keywords weren't used to signify different meanings.
Your examples are all correct, however, they all share a common feature. The word static means that an enclosing instance is not necessary.
Only a static inner class can exist without an enclosing instance. For example, if you have a class Foo and a non-static inner class Bar then you cannot create an instance of Bar outside an instance of Foo.
A static method means you do not need an instance of the class to call the method. You can call String.format without an actual String instance for example.
A static field will exist even without an instance of the class. If your Foo class has a counter field that is static you can access it without ever instantiating an instance of the Foo class.
Consider, as a clarifying point, that an interface can have static classes, static fields, and static methods. However, it cannot have the non-static version of any of those things (ignoring default methods which are sort of ad-hoc'd into the concept). This is because you can never create an instance of an interface so there could never be an enclosing instance.
You can also declare inner interfaces, annotations, and enums to be static although the keyword in that case is entirely redundant (e.g. similar to declaring an interface method abstract). Interfaces, annotations, and enums have no relationship to an enclosing class to begin with so static can't really take that away.
One last byzantine point. If you do a static import (import static pack.age.Foo.*) you will be able to make unqualified references to any static items in a class (including interfaces, annotations, and enums regardless of whether or not they are redundantly marked static).
Why does static have different meanings depending on the context? Why
didn't aren't different key words used?
It doesn't really have different meanings.
You can take the static keyword to indicate the following wherever it may be encountered:
"without regard or relationship to any particular instance"
A static field is one which belongs to the class rather than to any particular instance.
A static method is defined on the class and has no notion whatsoever of this. Such a method can access no instance fields in any particular instance, except when an instance is passed to it.
A static member class is a nested class that has no notion of its enclosing class and has no relationship to any particular instance of its enclosing class unless such an instance is passed to it (such as an argument to its constructor).
From Core Java by Cay Horstmann:
The term “static” has a curious history. At first, the keyword static was introduced in C to
denote local variables that don’t go away when a block is exited. In that context, the term
“static” makes sense: The variable stays around and is still there when the block is entered
again. Then static got a second meaning in C, to denote global variables and functions
that cannot be accessed from other files. The keyword static was simply reused, to avoid
introducing a new keyword. Finally, C++ reused the keyword for a third, unrelated,
interpretation—to denote variables and functions that belong to a class but not to any
particular object of the class. That is the same meaning the keyword has in Java.
Java inherits from C++ and C. In those languages, static has two additional meanings. A local variable (function scope) qualified as static has meaning somewhat similar to that of a static field in a class. Java however does not support this context of "static". Qualifying a variable or function as static in C or C++ at file scope means "Ssh! Don't tell the linker!". Java does not support this mean of static, either.
In English, the same word can have multiple meanings, depending on context. Look up any commonly-used word in the dictionary and you will find multiple definitions of that word. Some words not only have multiple meanings, they have multiple parts of speech. "Counter", for example, can be a noun, a verb, an adjective, or an adverb, depending on context. Other words can have contradictory meanings, depending on context. "Apology" can mean "I'm so sorry!" or it can mean "I am not sorry at all!" A premier example of the latter is "A Mathematician's Apology" by G. H. Hardy. English is not at all unique in this regard; the same applies to any language humans use to communicate with one another. As humans, we are quite used to words having different meanings depending on context.
There's an inherent conflict between having too few keywords and too many in a computer language. Lisp, forth, and smalltalk are very beautiful languages with very few, if any, keywords. They have a few special characters, e.g., open and close parentheses in lisp. (Full disclosure: I've programmed in all three of those languages, and I loved it.) There's a problem here: Good luck reading the code you yourself wrote six months after the fact. Even better luck turning that code over to someone else. As a result, these languages also have a rather limited number of adherents. Other languages go over the top and reserve a huge number of words as "keywords." (Full disclosure: I've been forced to program in those languages as well, and I hated it.)
Too few or too many keywords in a computer language results in cognitive dissonance. Having the same keyword have different contexts in different does not, because as humans, we are quite used to that.
Java Tutorial says
As with class methods and variables, a static nested class is
associated with its outer class. And like static class methods, a
static nested class cannot refer directly to instance variables or
methods defined in its enclosing class: it can use them only through
an object reference.
Basically "static" means that the entity marked with it is divorced from the instances of a class. Static method doesn't have an instance associated with it. Static field is shared between all instances (essentially exist in the Class, not in the instance).
static nested classes are divorced from the enclosing instance. You are right that it is a bit confusing because you can have an instance of a static nested class with non-static methods and fields inside of it.
Think of the word static as saying "I am declaring an entity, a field, a method or an inner class, which is going to have no relationship to the enclosing instance"
All the mentioned uses of static have some commonality as I see it - in all cases they mean that the class/field/method is less tied to the class instance than would be the case without static. Certainly the equivalence between static fields and static methods in particular should be clear: they are the way to declare singleton (per-classloader) fields and methods that work on those fields, in a similar way to global objects in other languages.
Perhaps then the use of static for nested classes isn't as obviously in the same spirit, but it does share the aspect that you don't need an instance of the containing class to use this construct.
So I don't see these as being particularly inconsistent.
One answer to the more general question of why keywords are re-used for apparently different purposes in a programming language is that often features are introduced as a language evolves - but it is difficult to add new keywords since often break existing programs that may have used that as an identifier. Java, for example, actually reserves the keyword const even though it is unused in the language, perhaps to allow for future expansion!
This reluctance to add new keywords often leads to overloading old ones.
I'm writing an explanation for some code for a course, and have been accidentally using the words method and function interchangeably. I decided to go back over and fix the wording, but ran into a hole in my understanding.
From what I understand, a subroutine is a function if it doesn't act on an instance of a class (its effect is restricted to its explicit input/output), and is a method if it operates on an instance of a class (it may carry out side effects on the instance that make it impure).
There's a good discussion here on the topic. Note that by the accepted answer's definitions, a static method should actually be a function because an instance is never implicitly passed, and it doesn't have access to any instance's members.
With this is mind though, shouldn't static methods actually be functions?
By their definition they don't act on particular instances of a class; they're only "tied" to the class because of relation. I've seen a few good looking sites that refer to static subroutines as "methods" though (Oracle, Fredosaurus, ProgrammingSimplified), so either they're all overlooking the terminology, or I'm missing something (my guess is the latter).
I'd like to make sure I am using the correct wording.
Can anybody clear this up?
This quote from 8.4.3.2 may help:
A method that is declared static is called a class method.
A method that is not declared static is called an instance method [...].
Class methods: associated with a class.
Instance methods: associated with an instance.
Java just wants you to "think object-oriented". Also, static methods have access to a surrounding scope which may include state. In a way, the class is like an object itself.
The simple answer is that when Java decided to call everything a "method", they didn't care about the distinction between a function and a method in theoretical computer science.
Static methods are not exactly functions, the difference is subtle, but important.
A static method using only given input parameters is essentially a function.
But static methods may access static variables and other static functions (also using static variables) so static methods may have a state which is fundamentally different to a function which are by definition stateless.
(ADDENDUM: While programmers are often not so strict with using "function" as definition, a strict function in computer science can access only input parameters). So defining this case of accessing static fields it is not valid to say that static methods are always functions.
Another difference which justifies the usage of "static method" is that you can define in C derivates global functions and global variables which can be accessed everywhere. If you cannot access the class which contain static methods, the methods are inaccessible, too. So "static methods" are limited in their scope by design in contrast to global functions.
In Java, a user-defined class is actually an instance of a subclass of java.lang.Class.
In this sense, static methods are attached to an instance of a conceptual class: they are attached to an instance of a subclass of java.lang.Class.
With this in mind, the term "class method" (an alternate name for Java's static methods) begins to make sense. And the term "class method" can be found in many places: Objective C, Smalltalk, and the JLS -- to name just a few.
In computer science function clearly maps to a static method. But "method" of a class is a bit generic, like "member" (field member, method member). There are wordings like
Data members and method members have two separate name spaces: .x and .x() can coexist.
So the reason is, that as the philosoph Ludwig Wittgenstein said, Language is a tool with different contexts. "Method" is a nice moniker in the citation above to categorize a "member".
Your thinking is right and it makes sense. It's just not established terminology in the Java community. Let me explain some internals that can help understand why the terminology subsists.
Java is a class based object oriented language. A method is always member of a class or instance (This is a general statement valid for other programming languages too). We think of class and instance being both objects.
Instance method (dynamic)
You cannot invoke this method from a class directly, you have to create an instance. Each instance references that method. You can overwrite a method definition with the exact same method signature (when subclassing), i.e. the reference points to a different method (which has the same signature, but can have a different method body). The method is dynamic.
Class method (static)
You only can invoke this method from the class directly, i.e. you don't need to create an instance of that class. There is only one global definition of that method in the whole program. You cannot overwrite the exact same method signature when the method is declared static, because there is only one definition valid for the whole program. Note that the method is member of the class object itself, the instances have all the same unique (and fix) reference to that method.
Here is another take on the terminology, using Scala as a mnemonic:
In Scala you have objects, which are singleton instances of an implicitly defined class 1.
Per your definition, we can call these subroutines belonging to the object methods, as they operate on a single instance of the class.
Additionally the object will also define class A, and create all of the methods in object A as static methods on class A (for interfacing with Java) [2].
Therefore we can say that the static methods of Java class A access the same members as the Scala singleton instance, which per your definition then deserve to be called (static) methods of class A.
Of course, the main difference is - method can use static fields, not only method parameters.
But there is additional one - polymorphism!
Results of evaluation Class A.doTheSameStaticMethod() and ClassB.doTheSameStaticMehod() will be depends of class. In this case function is impotent.
Each class has an object to represent it that is an instance of a subclass of the Class class. Static methods are really instance methods on these objects that are instances of a subclass of Class. They have access to state in the form of static fields, so they are not restricted to being just (stateless) functions. They are methods.
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.
I know what static is, but just not sure when to use it.
static variable:
I only used it for constant fields. Sometimes there are tens of constants in a class, so using static constants can save lots of memory. Is there any other typical use cases?
static method:
I use it when I make a class about algorithms. For example, a class which provides different sorting algorithms. Is it against OOP design? I think it is better to maintain this way rather than implementing sorting algorithms inside each class that needs to use them. Am I wrong? What are some good use cases?
Also, are there any performance difference between using static and non-static fields/methods?
You are describing cases where you've used static, but this doesn't quite explain fundamentally why you would use static vs non-static - they are more than just keywords for constants and utility methods.
When something is not static (instance), it means that there is an instance of it for each instance of the class. Each one can change independently.
When something is static, it means there is only one copy of it for all instances of the class, so changing it from any location affects all others.
Static variables/methods typically use less memory because there is only one copy of them, regardless of how many instances of the class you have. Statics, when used appropriately, are perfectly fine in object oriented design.
If you have a method/variable that you only need one instance of (e.g. a constant or a utility method), then just make it static. Understand though that making a method static means it cannot be overridden. So if you have a method you want to override in a subclass, then don't make it static.
The general rule of thumb is - if you need only one copy of it, make it static. If you need a copy per instance, then make it non static.
Is there any other typical use cases?
Global Variables
Is it against OOP design?
Not exaclty, the point is that static methods are stateless since you don't need a particular instance of a class. My favorite approach is for utility methods (like Apache Commons). But you may be aware that some methods may be better placed as class members instead of static.
Also static methods can make class testability harder once you can't override these methods or replace by mock implementation.
Performance difference ?
There's a performance Android recommendation from Google that says "prefer static over virtual":
http://developer.android.com/training/articles/perf-tips.html#PreferStatic
I'm not sure it's true for JVM since Android uses a different VM, but it makes sense given the reasons the link points out:
If you don't need to access an object's fields, make your method static. Invocations will be about 15%-20% faster. It's also good practice, because you can tell from the method signature that calling the method can't alter the object's state."
My personal rule of thumb is that static things are "just hanging out there". They are things that (disclaimer, not entirely true) are global, but make sense to include with this one particular class.
Static fields are good if you find yourself loading some heavyweight objects repeatedly. For instance, the project I'm working on now has a toggle between two images. These are static fields that are loaded with the application and kept in memory, rather than reloading them every time and letting GC take care of the mess.
Apart from very specific situations, I use static (and final) variables for constants only. It's a totally valid to use them, of course.
I tend to avoid static utility methods, because they make it harder to write unit tests for the code (mocking the results of the method invocation). When you start developing Test Driven way, this issue becomes quite apparent. I prefer using dependency injection and singleton beans (though it depends on your needs and situation).
Static variables belong to a class, hence shared by all the objects, so memory usage is less if you really want the varible to be shared. If you declare the variable as public and static, then it is globally available for everyone.
Static methods are generally the utility methods, depending on the access modifier, those can be used within a class or across the classes. Static utility class will help to reduce the memory usage again because you need not to create the object to call those methods.
The static field has one value among all objects and they call it Class member also because it's related to the class.
You can use static filed as a utility.
an example just Assume we need to know how many instances we have :
class Counter
public class Counter {
public static int instanceCount ;
public Counter()
{
instanceCount++;
}
public int getInstanceCount()
{
return instanceCount;
}
}
After creating two instances of Counter Class. But they share the same instanceCount field because it's a static field so the value of instanceCount will become the same in firstCounter and secondCounter .
Class main
Counter firstCounter = new Counter();
// will print 1
System.out.println(co.getInstanceCount());
// will print 2
Counter secondCounter = new Counter();
System.out.println(co1.getInstanceCount());
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.