A variable is declared as static to get the latest and single copy of its value; it means the value is going to be changed somewhere. But why should the same variable be declared as final, which will not allow the variable to be changed else where (constant value)?
static so that the variable or method can be accessed without creating a class instance, and there is only one variable for the class instead of one for each instance.
A final class cannot be extended. A final variable cannot have its value changed, it behaves as a constant. And a final method cannot be over-ridden.
The minute a variable is defined as final, it should probably not be referred to as "variable", since it no longer "varies" :)
A static variable is not tied to any particular instance of a class -- it is only tied to the class itself and only from a scoping standpoint.
So there you are -- a static and final variable is actually a value that is not tied to any particular instance of class and does not vary. It is a constant value, to be referenced from anywhere in your Java code.
At some point, when you should decide to change the value of this constant, it only takes one change to propagate this change correctly to all other classes that use this constant.
A variable declared as static means that its value is shared by all instances of this class. Declaring a variable as final gives a slightly better performance and makes your code better readable.
local variables are on the stack and are not static.
You can have a static field which may or may not be final. You would make the field final if it is not going to change.
static fields can be modified (e.g. public static fields can be modified by any class). static final fields cannot be modified after initialization.
Like you mention yourself, this is done to create constants. You create a single field to hold a value with a specific meaning. This way you don't have to declare that value everywhere, but instead you can reference the static.
Static has nothing to do with getting the latest and single copy unless "single copy" here means one and the same value for all the instances of a class (however, I think you may be confusing it with volatile). Static means class variable. You make it final when you want that to be a constant (that's actually the way Java constants are declared: static final).
static final is used in Java to express constants. Static is used to express class variables, so that there is no need to instantiate an object for that class in order to access that variable.
Final methods can't be overriden and final variables can only be initialised once.
If you only use the static keyword, that value will not be a constant as it can be initialised again.
May be to provide something similar to constants.
Variables should be declared as fields only if they’re required for use in more than one method of the class or if the program should save their values between calls to the class’s methods.
for example user.lastName lastName should be field because it is needed during object lifecycle
Variables should be declared as static only if they’re not required for use in more than one method of the class or if the program should not save their values between calls to the class’s methods.
for example Math.max(num1,num2) Im not intristed in num1 and num2 after compleating this operation
Final stops any classes inheriting from it
You create static final variable to make its value accessible without instantiating an object.
E.G.:
public class MyClass
{
public static final String endpoint= "http://localhost:8080/myClass":
/* ...*/
}
Then you can access to the data using this line:
MyClass.endpoint
Related
Why can we access a static variable via an object reference in Java, like the code below?
public class Static {
private static String x = "Static variable";
public String getX() {
return this.x; // Case #1
}
public static void main(String[] args) {
Static member = new Static();
System.out.println(member.x); // Case #2
}
}
Generally, public variables can be accessed by everybody, and private variables can only be accessed from within the current instance of the class. In your example you're allowed to access the x variable from the main method, because that method is within the Static class.
If you're wondering why you're allowed to access it from another instance of Static class than the one you're currently in (which generally isn't allowed for private variables), it's simply because static variables don't exist on a per-instance basis, but on a per class basis. This means that the same static variable of A can be accessed from all instances of A.
If this wasn't the case, nobody would be able to access the private static variable at all, since it doesn't belong to one instance, but them all.
The reason that it is allowed is that the JLS says it is. The specific sections that allows this are JLS 6.5.6.2 (for the member.x cases) and JLS 15.11.1 (in both cases). The latter says:
If the field is static:
If the field is a non-blank final field, then the result is the value of the specified class variable in the class or interface that is the type of the Primary expression.
If the field is not final, or is a blank final and the field access occurs in a class variable initializer (§8.3.2) or static initializer (§8.7), then the result is a variable, namely, the specified class variable in the class that is the type of the Primary expression.
Why are these allowed by the JLS?
Frankly, I don't know. I can't think of any good reasons to allow them.
Either way, using a reference or this to access a static variable is a bad idea because most programmers are likely to be mislead into thinking that you are using an instance field. That is a strong reason to not use this feature of Java.
In your first and second cases you should reference the variable as x or Static.x rather than member.x. (I prefer Static.x.)
It is not best practice to reference a static variable in that way.
However your question was why is it allowed? I would guess the answer is to that a developer can change an instance member (field or variable) to a static member without having to change all the references to that member.
This is especially true in multi-developer environments. Otherwise your code may fail to compile just because your partner changed some instance variables to static variables.
static variables are otherwise called as class variables, because they are available to each object of that class.
As member is an object of the class Static, so you can access all static as wll as non static variables of Static class through member object.
The non-static member is instance member. The static member(class wide) could not access instance members because, there are no way to determine which instance owns any specific non-static members.
The instance object could always refers to static members as it belongs to class which global(shared) to its instances.
This logically makes sense although it is not interesting practice. Static variable is usually for enforcing single declaration of variable during instantiation. Object is a new copy of Class with other name. Even though object is new copy of class it is still with characteristics of the (uninstantiated) Class (first invisible instance). Therefore new object also has that static members pointing to the original copy. Thing to note is: New instance of StackOverflow is also StackOverflow.
when we use final keyword that variable has been declared as constant then what is the necessity of using static ?
I have seen in most of the places that we use
public static final int nVar = 12
for constant data member .
final means the variable may not be reassigned to another object/primitive.
static means that all code running in the JVM shares the same variable.
A final non-static variable can not be reassigned, but each instance has its own copy.
static final fields (accessible without creating an instance) are generally called "Constants"
final (non static) instance variables are generally called "Immutable fields"
final means that, once assigned the value of variable cannot be modified.
static means "associated with the class"; without it, the variable is associated with each instance of the class. if not, you'll have one for each instance you create. static means the variable will remain in memory for as long as the class is loaded
There is no point in declaring a variable like this.
public final int nVar = 12;
If this is not meant to be modified, why have one copy per instance.
Hence, Class constants need to be declared as static final where as the variables which you want to be immutable on per instance basis, you declare them as final
declaring it static enables you accessing the variale without creating an ojbect of the type .
The "final" identifier means that the variable's value cannot be changed.
"static" however means that there is one instance of the variable within the class that it is defined.
So final and static are 2 different things, but you often see them used together to define constants for a class.
One reason could be that you do not need to create an object of the class to access that constant that is why you should/may declarer it as static. As you can access it with class name as it is static.
Check this code
public class A{
public static final int aa = 1;
}
public class B{
public final int bb = 1;
}
public class Testing{
SOP(A.aa);// a can be accessed with class name
B b = new B();// where to access the constant bb we need to create class object
SOP(b.bb);
}
A static variable means it is available at class level.Only one instance of that variable will be available for all objects of that class.Static variable can be modified, but the change will reflect in all objects of that class.
A final variable means its value cannot be change after initialization.
A final variable with static modifier means the variable is available at class level.In short, it will act as a constant for all objects of that class
I have used final and static variables as well. what i found about these variables is,
final variable
A final variable can only be initialized once, either via an initializer or an assignment statement.
Unlike the value of a constant, the value of a final variable is not necessarily known at compile time.
what variables should i declare as final-
Most often i use those variables whose value is constant universally and can never changed, such as the value of PI.
public static final double PI = 3.141592653589793;
static variables
These are those variables which belongs to the class and not to object(instance).
Static variables are initialized only once , at the start of the execution .
A single copy to be shared by all instances of the class
A static variable can be accessed directly by the class name and doesn’t need any object.
what variables should i declare as final-
Most of the time, i use those variables which i want to initialize only once and use them in the enitre class.
When to use final static variable
Now, i came across a term final static in one of my database project. I found that some of the database objects or even database version were declared as statci final.
static final String DATA_BASE = "BackUpDatabase.db";
static final int DATA_BASE_VERSION = 1;
Now, my question is what variables should we declare as final or static or final static, as using either of them could have solved the issue, then wyh to use both together.
static - Only use when a variable which is used globally
final - Only use when you need to declare a value as constant
static final - Only use when a value is globally used and it is a constant.
: - Here global means across all the instances of a java class
Variables declared as static final (or vice versa) are understood to be meaningful constants, and are named in all upper-case with underscores for spaces.
An example of a commonly encountered constant is Integer.MAX_VALUE, or Math.PI.
final only says that value once initialized can't be changed; static says that the attribute belongs to Class and NOT objects.
So when you say final static; this means there is just one copy of variable and it can't be changed.
- static in java means Class's member. Its shared by all the instances of the class.
- final keyword in java means, constant, but has different interpretation depending on what its being applied.
- When we use static final on a field, consider it as a Global variable.
- PI is static variable of Math Class and its directly accessed using the class name, as Math.PI.
- Use all letters in caps to define a static final variable.
final's interpretation:
final variable : Its value canNot be changed
final method : It canNot be overridden
final class : It canNot be extended
final Parameter : Its value canNot be changed which it receives from caller's argument
final Object Reference Variable : It canNot refer to any other object, other than the one its currently referring to
Declaring variables only as static can lead to change in their values by one or more instances of a class in which it is declared.
Declaring them as static final will help you to create a CONSTANT as #Vulcan told. Only one copy exists which can be accessed anywhere.
Static Variable
You can change the static variable value by calling static method
present in same class .
Static variable value will be same for all object created from this
class . if we change the value then all object of that class will get
new value ,old value will be lost.
Value can be changed multiple times.
Final variable
This variable value can be initialized by two way:
At the time of declaring the variable.
At the time of creating object of that class where class constructor will have this.finalvariable = newfinalvariablevalue;
Once initialized it can't be changed by any method (Static or non-static).
static methods or classes are implicitly final .
Because there is only one copy of this method existing for all the objects which means that subclasses dont have access to modify the copy.
lets say you have a method in the parent and you dont want subclasses to change this method.. just declare the parent method as a final. Here you go.
class Parent {
final void myMethod() {
//No one can change this method from subclassess
//compiler works efficiently because it knows that this method will not change
}
}
class Child extends Parent{
//from this class I can use myMethod but I cannot override.
}
Generally, final static members especially, variables (or static final of course, they can be used in either order without overlapping the meaning) are extensively used with interfaces in Java to define a protocol behavior for the implementing class which implies that the class that implements (inherits) an interface must incorporate all of the members of that interface.
I'm unable to differentiate between a final and a final static member. The final static member is the one which is a static member declared as final or something else? In which particular situations should they be used specifically?
A static variable or a final static variable can never be declared inside a method neither inside a static method nor inside an instance method. Why?
The following segment of code accordingly, will not be compiled and an compile-time error will be issued by the compiler, if an attempt is made to compile it.
public static void main(String args[])
{
final int a=0; //ok
int b=1; //ok
static int c=2; //wrong
final static int x=0; //wrong
}
You are making a huge mix of many different concepts. Even the question in the title does not correspond to the question in the body.
Anyways, these are the concepts you are mixing up:
variables
final variables
fields
final fields
static fields
final static fields
The keyword static makes sense only for fields, but in the code you show you are trying to use it inside a function, where you cannot declare fields (fields are members of classes; variables are declared in methods).
Let's try to rapidly describe them.
variables are declared in methods, and used as some kind of mutable local storage (int x; x = 5; x++)
final variables are also declared in methods, and are used as an immutable local storage (final int y; y = 0; y++; // won't compile). They are useful to catch bugs where someone would try to modify something that should not be modified. I personally make most of my local variables and methods parameters final. Also, they are necessary when you reference them from inner, anonymous classes. In some programming languages, the only kind of variable is an immutable variable (in other languages, the "default" kind of variable is the immutable variable) -- as an exercise, try to figure out how to write a loop that would run an specified number of times when you are not allowed to change anything after initialization! (try, for example, to solve fizzbuzz with only final variables!).
fields define the mutable state of objects, and are declared in classes (class x { int myField; }).
final fields define the immutable state of objects, are declared in classes and must be initialized before the constructor finishes (class x { final int myField = 5; }). They cannot be modified. They are very useful when doing multithreading, since they have special properties related to sharing objects among threads (you are guaranteed that every thread will see the correctly initialized value of an object's final fields, if the object is shared after the constructor has finished, and even if it is shared with data races). If you want another exercise, try to solve fizzbuzz again using only final fields, and no other fields, not any variables nor method parameters (obviously, you are allowed to declare parameters in constructors, but thats all!).
static fields are shared among all instances of any class. You can think of them as some kind of global mutable storage (class x { static int globalField = 5; }). The most trivial (and usually useless) example would be to count instances of an object (ie, class x { static int count = 0; x() { count++; } }, here the constructor increments the count each time it is called, ie, each time you create an instance of x with new x()). Beware that, unlike final fields, they are not inherently thread-safe; in other words, you will most certainly get a wrong count of instances of x with the code above if you are instantiating from different threads; to make it correct, you'd have to add some synchronization mechanism or use some specialized class for this purpose, but that is another question (actually, it might be the subject of a whole book).
final static fields are global constants (class MyConstants { public static final double PI = 3.1415926535897932384626433; }).
There are many other subtle characteristics (like: compilers are free to replace references to a final static field to their values directly, which makes reflection useless on such fields; final fields might actually be modified with reflection, but this is very error prone; and so on), but I'd say you have a long way to go before digging in further.
Finally, there are also other keywords that might be used with fields, like transient, volatile and the access levels (public, protected, private). But that is another question (actually, in case you want to ask about them, many other questions, I'd say).
Static members are those which can be accessed without creating an object. This means that those are class members and nothing to do with any instances. and hence can not be defined in the method.
Final in other terms, is a constant (as in C). You can have final variable inside the method as well as at class level. If you put final as static it becomes "a class member which is constant".
I'm unable to differentiate between a final and a final static member.
The final static member is the one which is a static member declared
as final or something else? In which particular situations should they
be used specifically?
Use a final static when you want it to be static. Use a final (non-static) when you don't want it to be static.
A static variable or a final static variable can never be declared
inside a method neither inside a static method nor inside an instance
method. Why?
Design decision. There's just no way to answer that without asking James Gosling.
The following segment of code accordingly, will not be compiled and an
compile-time error will be issued by the compiler, if an attempt is
made to compile it.
Because it violates the rule you just described.
final keyword simply means "this cannot be changed".It can be used with both fields and variables in a method.When a variable is declared final an attempt to change the variable will result to a compile-time error.For example if i declare a variable as final int x = 12; trying to increment x that is (++x) will produce an error.In short with primitives final makes a value a constant.
On the other hand static can only be applied with fields but not in methods.A field that is final static has only one piece of storage.final shows that it is a constant(cannot be changed), static shows it is only one.
In Java, a static variable is one that belongs to class rather than the object of a class, different instances of the same class will contain the same static variable value.
A final variable is one that once after initialized ,after the instantiation of a class (creation of an object) cannot be altered in the program. However this differ from objects if a different value is passed post creation of another object of the same class.
final static means that the variable belongs to the class as well as cannot be change once initialized. So it will be accessible to the same value throughout different instances of the same class.
Just to add a minor information to #Bruno Reis 's answer, which I sought to complete the answer, as he spoke about important condition to initialize final fields before constructor ends, final static fields must also be initialized before before static blocks' execution finishes.
You cannot declare static fields in static block, static fields can only belong to a class, hence the compiler error.
When you declare a final variable (constant) in a class, for example:
private static final int MyVar = 255;
How much memory will this require if I have 100,000 instances of the class which declared this?
Will it link the variable to the class and thus have 1*MyVar memory usage (disregarding internal pointers), or will it link to the instance of this variable and create 100,000*MyVar copies of this variable?
Unbelievably fast response! The consensus seems to be that if a variable is both static and final then it will require 1*MyVar. Thanks all!
The final keyword is irrelevant to the amount of memory used, since it only means that you can't change the value of the variable.
However, since the variable is declared static, there will be only one such variable that belongs to the class and not to a specific instance.
Taken from here:
If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized . A field that is not declared static (sometimes called a non-static field) is called an instance variable. Whenever a new instance of a class is created, a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses.
There will be only 1*MyVar memory usage because it is declared as static.
The static declaration means it will only have one instance for that class and it's subclasses (unless they override MyVar).
An int is a 32-bit signed 2's complement integer primitive, so it takes 4 bytes to hold it, if your example wasn't using static you'd just multiply that by the number of instances you have (for your example of 100,000 instances that's 0.38 of a megabyte - for the field alone, extra overhead for actual classes).
The final modifier on a field means it cannot be repointed to another value (whereas final on a class of method means it cannot be overridden).
It's static and thus class scope -> 1.
Edit: actually, it depends on the class loaders. In the general case you have one copy of the class but if you have multiple class loaders/class repositories (might be the case in application servers etc.) you could end up with more.
In addition to the fact that static fields belong to their classes, and thus there is only one instance of static varaible per class (and per classloader), it's important to understand that static final variables initialized by compile-time constant expressions are inlined into classes that use them.
JLS §13.1 The Form of a Binary:
References to fields that are constant variables (§4.12.4) are resolved at compile time to the constant value that is denoted. No reference to such a constant field should be present in the code in a binary file (except in the class or interface containing the constant field, which will have code to initialize it), and such constant fields must always appear to have been initialized; the default initial value for the type of such a field must never be observed.
So, in practice, the instance of static final variable that belong to its class is not the only instance of value of that variable - there are other instances of that value inlined into constant pools (or code) of classes that use the variable in question.
class Foo {
public static final String S = "Hello, world!";
}
class Bar {
public static void main(String[] args) {
// No real access to class Foo here
// String "Hello, world!" is inlined into the constant pool of class Bar
String s = Foo.S;
System.out.println(s);
}
}
In practice it means that if you change the value of Foo.S in class Foo, but don't recompile class Bar, class Bar will print the old value of Foo.S.
static means you will have only one instatnce
final just means, that you can't reassign that value.
The crucial part here is that you declared the variable as static because static variables are shared among all instances of the class, thus requiring only as much space as one instance of the variable. Declaring a variable final makes it immutable outside its declaration or constructor.
final makes is 1*instances memory usage.
However, static makes it simply 1.
The keyword "final" helps you to declare a constant with a specific amount of memory, where as the keyword "static" as its prefix will gives a single instance of this constant, what ever be the amount of memory consumed...!!!
Static means, one instance per class, static variable created once and can be shared between different object.
Final variable, once value is initialized it can't be changed. Final static variable use to create constant (Immutable) and refer directly without using the object.
It's static, so there will be only one instance created, so however many bytes are required to hold one int primitive will be allocated
You will have one instance per class. If you have the class loaded more than once (in different class loaders) it will be loaded once per class loader which loads it.
BTW: Memory is surprising cheap these days. Even if there was a copy per instance, the time it takes you to ask the question is worth more than the memory you save. You should make it static final for clarity rather than performance. Clearer code is easier to maintain and is often more efficient as well.