Flow Control - SCJP Sun Certified Programmer for Java Platform SE6 - page 189

FIGURE 3.1 The syntax of an if statement

The if keyword

Parentheses (required)

if( boolean_expression ) {

//if block

Executes when boolean_expression is true.

} else {

The else statement and subsequent
else block is optional.

//else block

}

Executes when boolean_expression is false.

The following rules apply to an if-else statement:
The expression in parentheses must evaluate to a boolean . Otherwise, a compiler error
is generated.
If the boolean expression evaluates to true , the block of code following the if executes.
If the boolean expression evaluates to false , the else block executes.
The else block is optional.
The curly braces are not required in either the if or else block if the block of code is a
single statement. However, for readability it is a good idea to always use the curly braces.
An else block can contain an additional if statement.

The following simple example of an if statement demonstrates the syntax:

8. int x = (int) (Math.random() * 10 + 1);
9. if(x <= 5) {
10. System.out.println(“Under five”);
11. }

The value of x is assigned a random number between 1 and 10 . If the value of x is less
than or equal to 5 , then Under five displays on line 10. If x is greater than 5 , the block of
code that contains line 10 is skipped.
An else can be added to any if statement. The following if-then-else statement
outputs either

Under five or Over five :

8. int x = (int) (Math.random() * 10 + 1);
9. if(x <= 5) {
10. System.out.println(“Under five”);

11. } else {
12. System.out.println(“Over five”);
13. }