The document discusses a common myth about if-then-else statements in Java. It explains that in an if-then-else statement with nested if conditions, if the outer if condition evaluates to false, neither the inner if statement nor the else block will execute due to short-circuit evaluation. This can surprise developers who are not aware of how if-then-else statements are defined in the Java Language Specification. The document provides code examples and references the JLS to clarify the actual behavior.

1. JLS myths
~ if-then-else statement ~
HASUNUMA Kenji
k.hasunuma@coppermine.jp
GlassFish Users Group Japan

2. if-else statement
if ( someObject.eval() )
if ( otherObject.eval() )
func1();
else
func2();
assume that someObject.eval() is false.
which is run, func1 or func2?

3. if-else statement
if ( someObject.eval() )
if ( otherObject.eval() )
func1();
else
func2();
assume that someObject.eval() is false.
Neither func1 nor func2 is run.

4. if-else statement
if ( someObject.eval() )
if ( otherObject.eval() )
func1();
else
func2();
It's if-else's short-circuit.
Don't be misled by source code format!

5. From JLS (Java SE 8)
IfThenStatement:
if ( Expression ) Statement
IfThenElseStatement:
if ( Expression ) StatementNoShortIf
else Statement
IfThenElseStatementNoShortIf:
if ( Expression ) StatementNoShortIf
else StatementNoShortIf

6. Statement
Statement:
StatementWithoutTrailingSubstatement
LabeledStatement
IfThenStatement
IfThenElseStatement
WhileStatement
ForStatement

7. StatementNoShortIf
StatementNoShortIf:
StatementWithoutTrailingSubstatement
LabeledStatementNoShortIf
IfThenElseStatementNoShortIf
WhileStatementNoShortIf
ForStatementNoShortIf
*** 'IfThenStatementNoShortIf' don't exist ***

8. if-else statement (fixed)
if ( someObject.eval() ) {
if ( otherObject.eval() )
func1();
} else
func2();
assume that someObject.eval() is false.
which is run, func1 or func2?

9. if-else statement (fixed)
if ( someObject.eval() ) {
if ( otherObject.eval() )
func1();
} else
func2();
func2 is run!
because it use block as statement in outer
if statement

10. JLS myths
~ If-then-else statement ~
HASUNUMA Kenji
k.hasunuma@coppermine.jp
GlassFish Users Group Japan