Final (Java): Difference between revisions

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The '''blank final''', which was introduced in Java 1.1, is a final variable whose declaration lacks an initializer.<ref>{{cite book|last=Flanagan|first=David |title=Java in a Nutshell|url=http://web.deu.edu.tr/doc/oreily/java/javanut/ch05_06.htm|edition=2nd Edition|date=May 1997 |publisher=O'Reilly|isbn=1-56592-262-X|chapter=Chapter 5 Inner Classes and Other New Language Features:5.6 Other New Features of Java 1.1}}</ref><ref>{{cite web|url=http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html|title=Types, Values, and Variables|year=2000|work=Java Language Specification Chapter 4|publisher=Sun Microsystems, Inc|accessdate=23 July 2010}}</ref> A blank final can only be assigned once and must be unassigned when an assignment occurs. In order to do this, a Java compiler runs a flow analysis to ensure that, for every assignment to a blank final variable, the variable is definitely unassigned before the assignment; otherwise a compile-time error occurs.<ref>{{cite web|url=http://java.sun.com/docs/books/jls/second_edition/html/defAssign.doc.html|title=Definite Assignment|year=2000|work=Java Language Specification|publisher=Sun Microsystems, Inc.|accessdate=23 July 2010}}</ref>

==C++ analog==

==Difference from the C++ <code>const</code> type qualifier==

{{details|const (computer programming)}}

In [[C (programming language)|C]] and [[C++]], the analogous construct is the <code>[[const (computer programming)|const]]</code> [[keyword (computer programming)|keyword]]. This differs substantially from <code>final</code> in Java, most basically in being a [[type qualifier]]: <code>const</code> is part of the ''[[data type|type]],'' not only part of the identifier (variable). This also means that the constancy of a value can be changed by casting (explicit type conversion), in this case known as "const casting".

AnotherFurther, because C and C++ parallelexpose pointers and references directly, there is a distinction between whether the pointer itself is constant, and whether the data pointed to by the pointer is constant. Applying <code>const</code> to a pointer itself, as in <code>SomeClass * const ptr</code>, means wherethat the contents being referenced can be modified, but the reference itself cannot (without casting). ThisBy iscontrast, differentwhen applying const fromto the C++referenced data only, as in <code>const SomeClass * ptr</code> where, the contents cannot be modified (without casting), but the reference itself can.

The [[C++]] equivalent would be a reference that cannot be changed after initialization, though the data it points to may be.

Due to casting, C++ const is a soft guideline and it can easily be overridden by the programmer; the programmer can easily cast a const reference to an non-const reference. Java's final is a strict rule such that it is impossible to compile code that breaks or bypasses the final restrictions.

Another C++ parallel is <code>SomeClass * const ptr</code> where the contents being referenced can be modified, but the reference itself cannot without casting. This is different from the C++ <code>const SomeClass * ptr</code> where the contents cannot be modified without casting, but the reference itself can.

C++ const is a soft guideline and it can easily be overridden by the programmer; the programmer can easily cast a const reference to an non-const reference. Java's final is a strict rule such that it is impossible to compile code that breaks or bypasses the final restrictions.

==References==