13.4.8

final

Fields and Constants

BINARY COMPATIBILITY

public static void main(String[] args) {

if (Flags.debug)

System.out.println("debug is true");

}

}

(One reason for requiring inlining of primitive constants is that Java

switch

 state 

ments require constants on each

case

, and no two such constant values may be

the same. Java checks for duplicate constant values in a

switch

 statement at com 

pile time; the

class

 file format does not do symbolic linkage of

case

 values.)

The best way to avoid problems with  inconstant constants  in widely distrib 

uted code is to declare as primitive constants only values which truly are unlikely

ever to change. Many primitive constants in interfaces are small integer values

replacing enumerated types, which Java does not support; these small values can

be chosen arbitrarily, and should not need to be changed. Other than for true math 

ematical constants, we recommend that Java 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. Thus we recommend:

private static int N;

public static int getN() { return N; }

rather than:

public static final int N = ...;

There is no problem with:

public static int N = ...;

if

N

 need not be read only. We also recommend, as a general rule, that only truly

constant values be declared in interfaces. We note, but do not recommend, that if a

field of primitive type of an interface may change, its value may be expressed idi 

omatically as in:

interface Flags {

boolean debug = new Boolean(true).booleanValue();

}

insuring that this value is not a constant. Similar idioms exist for the other primi 

tive types.

One other thing to note is that

static final

 fields that have constant values

(whether of primitive or

String

 type) must never appear to have the default initial

value for their type ( 4.5.4). This means that all such fields appear to be initialized

first during class initialization ( 8.3.2.1,  9.3.1,  12.4.2).

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