Why is there no Constant feature in Java

Question

I was trying to identify the reason behind constants in Java I have learned that Java allows us to declare constants by using final keyword   My question is why didn t Java introduce a Constant  const  feature  Since many people say it has come from C    in C   we have const keyword   Please share your thoughts

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There is a way to create  const  variables in Java  but only for specific classes  Just define a class with final properties and subclass it  Then use the base class where you would want to use  const   Likewise  if you need to use  const  methods  add them to the base class  The compiler will not allow you to modify what it thinks is the final methods of the base class  but it will read and call methods on the subclass

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const in C   does not mean that a value is a constant   const in C   implies that the client of a contract undertakes not to alter its value    Whether the value of a const expression changes becomes more evident if you are in an environment which supports thread based concurrency   As Java was designed from the start to support thread and lock concurrency  it didn t add to confusion by overloading the term to have the semantics that final has    eg    include  lt iostream gt   int main          volatile const int x   42       std  cout  lt  lt  x  lt  lt  std  endl        const cast lt int  gt   amp x    7       std  cout  lt  lt  x  lt  lt  std  endl       return 0      outputs 42 then 7   Although x marked as const  as a non-const alias is created  x is not a constant  Not every compiler requires volatile for this behaviour  though every compiler is permitted to inline the constant   With more complicated systems you get const non-const aliases without use of const cast  so getting into the habit of thinking that const means something won t change becomes more and more dangerous  const merely means that your code can t change it without a cast  not that the value is constant

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What does const mean First  realize that the semantics of a  const  keyword means different things to different people    read-only reference - Java final semantics - reference variable itself cannot be reassigned to point to another instance  memory location   but the instance itself is modifiable readable-only reference - C const pointer reference semantics - means this reference cannot be used to modify the instance  e g  cannot assign to instance variables  cannot invoke mutable methods  - affects the reference variable only  so a non-const reference pointing to the same instance could modify the instance immutable object - means the instance itself cannot be modified - applies to instance  so any non-const reference would not be allowed or could not be used to modify the instance some combination of the the above  others    Why or Why Not const Second  if you really want to dig into some of the  pro  vs  con  arguments  see the discussion under this request for enhancement  RFE   bug    This RFE requests a  readable-only reference -type  const  feature  Opened in 1999 and then closed rejected by Sun in 2005  the  const  topic was vigorously debated   http   bugs sun com bugdatabase view bug do bug id 4211070  While there are a lot of good arguments on both sides  some of the oft-cited  but not necessarily compelling or clear-cut  reasons against const include    may have confusing semantics that may be misused and or abused  see the What does const mean above  may duplicate capability otherwise available  e g  designing an immutable class  using an immutable interface  may be feature creep  leading to a need for other semantic changes such as support for passing objects by value     Before anyone tries to debate me about whether these are good or bad reasons  note that these are not my reasons  They are simply the  gist  of some of the reasons I gleaned from skimming the RFE discussion   I don t necessarily agree with them myself - I m simply trying to cite why some people  not me  may feel a const keyword may not be a good idea   Personally  I d love more  const  semantics to be introduced to the language in an unambiguous manner

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This is a bit of an old question  but I thought I would contribute my 2 cents anyway since this thread came up in conversation today   This doesn t exactly answer why is there no const  but how to make your classes immutable   Unfortunately I have not enough reputation yet to post as a comment to the accepted answer   The way to guarantee immutability on an object is to design your classes more carefully to be immutable  This requires a bit more care than a mutable class   This goes back to Josh Bloch s Effective Java Item 15 - Minimize Mutability  If you haven t read the book  pick up a copy and read it over a few times I guarantee it will up your figurative  java game    In item 15 Bloch suggest that you should limit the mutability of classes to ensure the object s state   To quote the book directly      An immutable class is simply a class whose instances cannot be modified  All of the information contained in each instance is provided when it is created and is fixed for the lifetime of the object  The Java platform libraries contain many immutable classes  including String  the boxed primitive classes  and BigInte- ger and BigDecimal  There are many good reasons for this  Immutable classes are easier to design  implement  and use than mutable classes  They are less prone to error and are more secure    Bloch then describes how to make your classes immutable  by following 5 simple rules    Don   t provide any methods that modify the object   s state  i e   setters  aka mutators  Ensure that the class can   t be extended  this means declaring the class itself as final   Make all fields final  Make all fields private   Ensure exclusive access to any mutable components   by making defensive copies of the objects    For more details I highly recommend picking up a copy of the book

User · Answer

The C   semantics of const are very different from Java final   If the designers had used const it would have been unnecessarily confusing   The fact that const is a reserved word suggests that the designers had ideas for implementing const  but they have since decided against it  see this closed bug   The stated reasons include that adding support for C   style const would cause compatibility problems

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You can use static final to create something that works similar to Const  I have used this in the past   protected static final int cOTHER   0  protected static final int cRPM   1  protected static final int cSPEED   2  protected static final int cTPS   3  protected int DataItemEnum   0   public static final int INVALID PIN   -1  public static final int LED PIN   0

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Every time I go from heavy C   coding to Java  it takes me a little while to adapt to the lack of const-correctness in Java   This usage of const in C   is much different than just declaring constant variables  if you didn t know   Essentially  it ensures that an object is immutable when accessed through a special kind of pointer called a const-pointer  When in Java  in places where I d normally want to return a const-pointer  I instead return a reference with an interface type containing only methods that shouldn t have side effects   Unfortunately  this isn t enforced by the langauge   Wikipedia offers the following information on the subject      Interestingly  the Java language specification regards const  as a reserved keyword     i e   one that cannot be used as variable identifier     but assigns no semantics to it  It is thought that the reservation of the keyword occurred to allow for an extension of the Java language to include C  -style const methods and pointer to const type  The enhancement request ticket in the Java Community Process for implementing const correctness in Java was closed in 2005  implying that const correctness will probably never find its way into the official Java specification

User · Answer

There would be two ways to define constants - const and static final  with the exact same semantics  Furthermore static final describes the behaviour better than const

[java] Why is there no Constant feature in Java?

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