PROPOSAL: Conditional Statement (original) (raw)

Matt Mastracci matthew at mastracci.com
Sun Mar 29 14:41:10 PDT 2009

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.

  1. 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

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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