CHAPTER 14: Blocks and Statements

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14.4 Statements

There are many kinds of statements in the Java language. Most correspond to statements in the C and C++ languages, but some are unique to Java.

As in C and C++, the Java if statement suffers from the so-called "dangling else problem," illustrated by this misleadingly formatted example:

if (door.isOpen())
	if (resident.isVisible())
		resident.greet("Hello!");
else door.bell.ring();								// A "dangling else"

The problem is that both the outer if statement and the inner if statement might conceivably own the else clause. In this example, one might surmise that the programmer intended the else clause to belong to the outer if statement. The Java language, like C and C++ and many languages before them, arbitrarily decree that an else clause belongs to the innermost if to which it might possibly belong. This rule is captured by the following grammar:

Statement:

	StatementWithoutTrailingSubstatement

	LabeledStatement

	IfThenStatement

	IfThenElseStatement

	WhileStatement

	ForStatement

StatementNoShortIf:

	StatementWithoutTrailingSubstatement

	LabeledStatementNoShortIf

	IfThenElseStatementNoShortIf

	WhileStatementNoShortIf

	ForStatementNoShortIf

StatementWithoutTrailingSubstatement:

	Block

	EmptyStatement

	ExpressionStatement

	SwitchStatement

	DoStatement

	BreakStatement

	ContinueStatement

	ReturnStatement

	SynchronizedStatement

	ThrowStatement

	TryStatement

The following are repeated from S14.8 to make the presentation here clearer:

IfThenStatement:

	if ( Expression ) Statement

IfThenElseStatement:

	if ( Expression ) StatementNoShortIf else Statement

IfThenElseStatementNoShortIf:

	if ( Expression ) StatementNoShortIf else StatementNoShortIf

Statements are thus grammatically divided into two categories: those that might end in an if statement that has no else clause (a "short if statement") and those that definitely do not. Only statements that definitely do not end in a short if statement may appear as an immediate substatement before the keyword else in an if statement that does have an else clause. This simple rule prevents the "dangling else " problem. The execution behavior of a statement with the "no short if " restriction is identical to the execution behavior of the same kind of statement without the "no short if " restriction; the distinction is drawn purely to resolve the syntactic difficulty.