Proposal: Improved declaration using final keyword of internal method variables and for-loop statement (original) (raw)

Andrej Navodnik andrejnavodnik at yahoo.com
Tue Mar 31 02:04:03 PDT 2009

Hi Project Coin members,

here is my little proposal for improved declaration using final keyword of internal method variables and for-loop statement.

Is it too late? Maybe not. At the time I'm sending this e-mail, in Honolulu, Hawaii, it is still Monday, 30 March 2009.

Kind regards, Andrei

PROJECT COIN SMALL LANGUAGE CHANGE PROPOSAL FORM v1.0

TITLE: Improved declaration using final keyword of internal method variables and for-loop statement

AUTHOR(S): Andrei Navodnik (andrejnavodnik at yahoo.com)

OVERVIEW

FEATURE SUMMARY: Allow declaration of mixed final and non-final internal method variables in one statement and enable final keyword in for-loop declaration for variables other than index variable.

MAJOR ADVANTAGE: Suppose that the developer wants to have as much variables defined as final then if this feature were accepted in the language one major advantage would be to allow shorter syntax of internal method variables. Mixing of final and non-final variables in one declaration statement would be allowed which is prohibited with the current state of the compiler. The compiler either allows all variables to be final or all variables to be non-final.

As far as for-loop declaration is concerned, this feature would allow syntax that is more compact (no pollution of scope outside of the for-loop) and "safer" (the size of the list would be assigned to final variable and this value can not be changed inside of the loop).

MAJOR BENEFIT: Suppose that the majority of code is written as

for (int i = 0; i < values.size(); i++ ) {
    // ...
}

then this feature would allow easier refactoring to the optimized style of for-loops

for (int i = 0, final n = values.size(); i < n; i++) {
    // ...
}

because variable n can not be already defined in the scope either outside or inside of the for-loop. Any such violation would make program uncompilable. According to my benchmarks the later style of for-loops is also slightly faster in client VM.

MAJOR DISADVANTAGE: If the feature were accepted then the major disadvantage in my opinion would be slightly more complex grammar which would have to take care of the compatibility with the existing programs.

ALTERNATIVES: One of the alternatives to the proposed syntax (see simple example bellow) would be to declare internal method variables in separate statements, for example one statement for all non-final variables and one statement for all final variables. Someone might even argue that syntax where each variable is defined on separate statements is better comparing to the syntax where all variables are defined in one statement.

int a, c = 1;
final b, d = 4;

But, if developer's strategy is to define all read-only variables as final then the alternatives to the proposed syntax for the for-loop are not very nice. One is to declare variable of the size of the list outside of the for-loop declaration, for example

final n = values.size();
for (int i = 0; i < n; i++) {
    // ...
}

or to declare index variable outside of the for-loop.

int i = 0;
for (final int n = values.size(); i < n; i++ ) {
    // ...
}

EXAMPLES

SIMPLE EXAMPLE: If the proposal were accepted then the code below should be valid. Please, compare the proposed syntax with the alternatives presented above.

int a, final b, c = 1, final d = 4;
for (int i = 0, final n = values.size(); i < n; i++) {
    // ...
}

ADVANCED EXAMPLE: No advanced example.

DETAILS

SPECIFICATION: To support mixed final and non-final internal method variables the grammar should be changed as follows: 

LocalVariableDeclarationStatement:
    AllVariablesFinalDeclarationStatement 
    SomeVariablesFinalDeclarationStatement
    
AllVariablesFinalDeclarationStatement:
    final Type VariableDeclarators ;

SomeVariablesFinalDeclarationStatement:
    Type SomeVariablesFinalDeclarators ;
    
SomeVariablesFinalDeclarators:        
    VariableDeclarator { , VariableMaybeFinalDeclarator }

VariableMaybeFinalDeclarator:
     [final] Identifier SomeVariablesFinalDeclaratorsRest        

SomeVariablesFinalDeclaratorsRest:
    SomeVariablesFinalDeclaratorsRest { , VariableMaybeFinalDeclarator }

To better support final keyword in the for-loop declaration the grammar should be changed as follows: 

ForVarControl:
    AllVariablesFinalForVarControl
    SomeVariablesFinalForVarControl

AllVariablesFinalForVarControl:
    final [Annotations] Type Identifier ForVarControlRest

SomeVariablesFinalForVarControl:
     [Annotations] Type Identifier SomeVariablesFinalForVarControlRest
     
SomeVariablesFinalForVarControlRest:
    SomeVariablesFinalDeclaratorsRest;   [Expression]   ;   [ForUpdate]
    : Expression

TESTING: No specific testing is needed to test this feature.

LIBRARY SUPPORT: The proposed language feature needs no additional library support.

REFLECTIVE APIS: The feature does not affect reflective API's.

OTHER CHANGES: The feature does not need other changes.

MIGRATION: Existing programs should work without any change.

COMPATIBILITY: All existing programs are compatible.

BREAKING CHANGES: None.

EXISTING PROGRAMS: There should be no impact on existing programs.

REFERENCES

EXISTING BUGS: RFE has been already filed for this feature, please check internal review ID 1200191.

URL FOR PROTOTYPE: There is no prototype for this feature.

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