PROPOSAL: Conditional Statement (original) (raw)

Neal Gafter neal at gafter.com
Sun Mar 29 18:54:27 PDT 2009

I don't understand the benefit over the existing if statement.

On Sun, Mar 29, 2009 at 2:41 PM, Matt Mastracci <matthew at mastracci.com> wrote:

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

Richly formatted version available at: http://docs.google.com/Doc?id=dgthwhwr2hs2hknhf

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/thirdedition/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.

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