PROPOSAL: Conditional Statement

[Matt Mastracci]

Conditional Statement
=====================

Richly formatted version available at:
http://docs.google.com/Doc?id=dgthwhwr_2hs2hknhf


AUTHOR(S): Matt Mastracci

OVERVIEW

FEATURE SUMMARY: Allows an analogue of the conditional expression to  
exist at the statement level, with relaxed restrictions on the type of  
its second and third operands.

MAJOR ADVANTAGE: Ability to use conditional operator for choosing  
between two statement-level expressions (ie: two different method  
invocations).

MAJOR BENEFIT: Simplifies code that executes one of two expression  
statements given a boolean input.

MAJOR DISADVANTAGE: Turns the conditional operator into two different  
constructs in the Java language with different rules.

ALTERNATIVES: Continue using if/else (this feature is sugar only).

See http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.25 
  for the definition of the original conditional expression.


EXAMPLES

// even() and odd() are defined as "public void"
public void onEvenOrOdd(int i) {
     (i % 1 == 0) ? even() : odd();
}

// Two methods with different return types, valid
public void oneOfTwoMethodsWithDifferentReturnTypes(boolean b) {
     b ? methodReturningObject() : methodReturningVoid();
}


Examples that fail to compile:

// Invalid because operands are not valid StatementExpressions
public void invalid1() {
     b ? 1 : 2;
}

// Invalid because third operand is not a valid StatementExpression
public void invalid2() {
     b ? method() : null;
}


DETAILS

SPECIFICATION: This change requires two modifications to the Java  
Language Specification:

1.  A ConditionalStatement is added to the specification.  A  
ConditionalStatement takes a boolean expression as its first operand  
and two StatementExpressions as the second (if-true) and third (if- 
false) operands:

ConditionalStatement
    Expression ? ConditionalStatementOperand :  
ConditionalStatementOperand

ConditionalStatementOperand
    ( ConditionalStatementOperand )
    StatementExpression

There are no restrictions on the output type of the  
StatementExpression.  Any valid statement expression today is valid as  
an operand of the conditional statement.

The conditional statement is right-associative like the conditional  
expression.


2.  The definition of StatementExpression is changed to allow the  
conditional operator statement as one of its alternatives (this might  
be better to add to the StatementWithoutTrailingSubstatement block  
instead):

  StatementExpression:
          Assignment
          PreIncrementExpression
          PreDecrementExpression
          PostIncrementExpression
          PostDecrementExpression
          MethodInvocation
          ClassInstanceCreationExpression
+        ConditionalStatement

COMPILATION: The ConditionalStatement may always be desugared to a  
standard if/else construct:

boolean-expression ? statement-expression-1 : statement-expression-2

desugars to:

if (boolean-expression)
   statement-expression-1;
else
   statement-expression-2;


TESTING: Add a conditional statement to the files tested during  
complation.  Test that a conditional statement fails to compile when  
operands are not valid StatementExpressions.

LIBRARY SUPPORT: None.

REFLECTIVE APIS: No changes.

OTHER CHANGES: No changes.

MIGRATION:

An IDE may offer to automatically simplify if/else constructs with two  
expression statements:

if (condition) {
      expression-statement-1;
} else {
      expression-statement-2;
}

to:

condition ? expression-statement-1 : expression-statement-2;


COMPATIBILITY

BREAKING CHANGES: None
EXISTING PROGRAMS: No issues.


REFERENCES

EXISTING BUGS: None found.