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The final keyword can be used to specify constant values (that is, values that cannot change during program execution). However, constants that can change over the lifetime of a program should not be declared public final. The Java Language Specification  (JLS) [JLS 20112013] allows implementations to insert the values value of any public final fields field inline in any compilation unit that reads the field. Consequently, if the declaring class is edited so that the new version gives a different value for the field, compilation units that read the public final field could still see the old value until they are recompiled. This problem may occur, for example, when a third-party library is updated to the latest version but the referencing code is not recompiled.

A related error can arise when a programmer declares a static final reference to a mutable object; see OBJ50-JG 73. Never confuse the immutability of a reference with that of the referenced object for additional information.

...

In this noncompliant code example, class Foo in Foo.java declares a field whose value represents the version of the software. The field is subsequently accessed by class Bar from a separate compilation unit.Foo.java:

class Foo {
  public static final int VERSION = 1;
  // ...
}

The field is subsequently accessed by class Bar from a separate compilation unit (Bar.java):

class Bar {
  public static void main(String[] args) {
    System.out.println("You are using version " + Foo.VERSION);
  }
}

...

    You are using version 1

But if a developer changes were to change the value of VERSION to 2 by modifying Foo.java and subsequently recompiles recompile Foo.java but fails while failing to recompile Bar.java, the software would incorrectly printsprint

    You are using version 1

...

According to §13.4.9, "final Fields and Constants," of the Java Language Specification JLS [JLS 20112013],

Other than for true mathematical constants, we recommend that source code make very sparing use of class variables that are declared static and final. If the read-only nature of final is required, a better choice is to declare a private static variable and a suitable accessor method to get its value.

Consequently, a compliant solution is as follows:

Foo.javaIn this compliant solution, the version field in Foo.java is declared private static and accessed by the getVersion() method:

class Foo {
  private static int version = 1;
  public static final int getVersion() {
    return version;
  }

  // ...
}

The Bar class in Bar.java is modified to invoke the getVersion() accessor method to retrieve the version field from Foo.java:

class Bar {
  public static void main(String[] args) {
    System.out.println(
      "You are using version " + Foo.getVersion());
  }
}

In this solution, the private version value cannot be copied into the Bar class when it is compiled, consequently preventing the bug. Note that this transformation imposes little or no performance penalty because most just-in-time (JIT) code generators can inline the getVersion() method at runtime, so little or no performance penalty is incurred.

Applicability

Declaring as final a value that changes over the lifetime of the software as final may lead to unexpected results.

According to §9.3, "Field (Constant) Declarations," of the Java Language Specification JLS [JLS 20112013], "Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields." Therefore, this guideline does not apply to fields defined in interfaces. Clearly, if the value of a field in an interface changes, every class that implements or uses the interface must be recompiled. See 35. Carefully design interfaces before releasing them for more information.

Constants declared using the enum type are permitted to violate this guideline.

Constants whose value never changes throughout the entire lifetime of the software may be declared as final. For instance, the Java Language Specification JLS recommends that mathematical constants be declared final.

Bibliography

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