when exactly are we supposed to use public static final String

Question

I have seen much code where people write public static final String mystring       and then just use a value   Why do they have to do that  Why do they have to initialize the value as final prior to using it   UPDATE  Ok  thanks all for all your answers  I understand the meaning of those key  public static final   What I dont understand is why people use that even if the constant will be used only in one place and only in the same class  why declaring it  why dont we just use the variable

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final indicates that the value cannot be changed once set   static allows you to set the value  and that value will be the same for ALL instances of the class which utilize it   Also  you may access the value of a public static string w o having an instance of a class

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final indicates that the value of the variable won t change - in other words  a constant whose value can t be modified after it is declared  Use public final static String when you want to create a String that   belongs to the class  static  no instance necessary to use it   and that won t change  final   for instance when you want to define a String constant that will be available to all instances of the class  and to other objects using the class   Example  public final static String MY CONSTANT    quot SomeValue quot           in some other code  possibly in another object  use the constant  if  input equals MyClass MY CONSTANT   Similarly  public static final int ERROR CODE   127   It isn t required to use final  but it keeps a constant from being changed inadvertently during program execution  and serves as an indicator that the variable is a constant  Even if the constant will only be used - read - in the current class and or in only one place  it s good practice to declare all constants as final  it s clearer  and during the lifetime of the code the constant may end up being used in more than one place  Furthermore using final may allow the implementation to perform some optimization  e g  by inlining an actual value where the constant is used

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Why do people use constants in classes instead of a variable   readability and maintainability     having some number like 40 023 in your code doesn t say much about what the number represents  so we replace it by a word in capitals like  USER AGE YEARS   Later when we look at the code its clear what that number represents    Why do we not just use a variable   Well we would if we knew the number would change  but if its some number that wont change  like 3 14159   we make it final   But what if its not a number like a String   In that case its mostly for maintainability  if you are using a String multiple times in your code   and it wont be changing at runtime  it is convenient to have it as a final string at the top of the class   That way when you want to change it  there is only one place to change it rather than many     For example if you have an error message that get printed many times in your code  having final String ERROR MESSAGE    Something went bad   is easier to maintain   if you want to change it from  Something went bad   to  It s too late jim he s already dead    you would only need to change that one line  rather than all the places you would use that comment

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It is kind of standard best practice  There are already answers listing scenarios  but for your second question      Why do they have to do that  Why do they have to initialize the value as final prior to using it    Public constants and fields initialized at declaration should be  static final  rather than merely  final   These are some of the reasons why it should be like this    Making a public constant just final as opposed to static final leads to duplicating its value for every instance of the class  uselessly increasing the amount of memory required to execute the application  Further  when a non-public  final field isn t also static  it implies that different instances can have different values  However  initializing a non-static final field in its declaration forces every instance to have the same value owing to the behavior of the final field

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You do not have to use final  but the final is making clear to everyone else - including the compiler - that this is a constant  and that s the good practice in it   Why people doe that even if the constant will be used only in one place and only in the same class  Because in many cases it still makes sense  If you for example know it will be final during program run  but you intend to change the value later and recompile  easier to find   and also might use it more often later-on  It is also informing other programmers about the core values in the program flow at a prominent and combined place   An aspect the other answers are missing out unfortunately  is that using the combination of public final needs to be done very carefully  especially if other classes or packages will use your class  which can be assumed because it is public    Here s why    Because it is declared as final  the compiler will inline this field during compile time into any compilation unit reading this field  So far  so good  What people tend to forget is  because the field is also declared public  the compiler will also inline this value into any other compile unit  That means other classes using this field    What are the consequences   Imagine you have this   class Foo     public static final String VERSION    1 0      class Bar     public static void main String   args        System out println  I am using version     Foo VERSION           After compiling and running Bar  you ll get   I am using version 1 0   Now  you improve Foo and change the version to  1 1   After recompiling Foo  you run Bar and get this wrong output   I am using version 1 0   This happens  because VERSION is declared final  so the actual value of it was already in-lined in Bar during the first compile run  As a consequence  to let the example of a public static final     field propagate properly after actually changing what was declared final  you lied     you d need to recompile every class using it   I ve seen this a couple of times and it is really hard to debug   If by final you mean a constant that might change in later versions of your program  a better solution would be this   class Foo     private static String version    1 0     public static final String getVersion         return version          The performance penalty of this is negligible  since JIT code generator will inline it at run-time

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This is related to the semantics of the code  By naming the value assigning it to a variable that has a meaningful name  even if it is used only at one place  you give it a meaning  When somebody is reading the code that person will know what that value means  In general is not a good practice to use constant values across the code  Imagine a code full of string  integer  etc  values  After a time nobody will know what those constants are  Also a typo in a value can be a problem when the value is used on more than one place

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The keyword final means that the value is constant it cannot be changed   It is analogous to const in C   And you can treat static as a global variable which has scope  It basically means if you change it for one object it will be changed for all just like a global variable limited by scope    Hope it helps

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public makes it accessible across the other classes  You can use it without instantiate of the class or using any object       static makes it uniform value across all the class instances  It ensures that you don t waste memory creating many of the same thing if it will be the same value for all the objects       final makes it non-modifiable value  It s a  constant  value which is same across all the class instances and cannot be modified

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static means that the object will only be created once  and does not have an instance object containing it  The way you have written is best used when you have something that is common for all objects of the class and will never change  It even could be used without creating an object at all   Usually it s best to use final when you expect it to be final so that the compiler will enforce that rule and you know for sure  static ensures that you don t waste memory creating many of the same thing if it will be the same value for all objects

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public makes it accessible across other classes  static makes it uniform value across all the class instances  final makes it non-modifiable value  So basically it s a  constant  value which is same across all the class instances and which cannot be modified  With respect to your concern  What I don t understand is why people use that even if the constant will be used only in one place and only in the same class  Why declaring it  Why don t we just use the variable   I would say since it is a public field the constant value can also be used elsewhere in some other class using ClassName value  eg  a class named Math may have PI as final static long value which can be accessed as Math PI

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Static means  You can use it without instantiate of the class or using any object  final  It is a keyword which is used for make the string constant  You can not change the value of that string  Look at the example below     public class StringTest               static final String str    Hello      public static void main String args                     str    world      gives error             System out println str      called without the help of an object                                   System out println StringTest str     called with class name                                  Thanks

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Usually for defining constants  that you reuse at many places making it single point for change  used within single class or shared across packages  Making a variable final avoid accidental changes

[java] when exactly are we supposed to use "public static final String"?

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