Controlling the Flow of Your Program - Beginning Java Programming: The Object-Oriented Approach - page 136

Note If there is only a single statement following the if condition, the
curly brackets are optional. It is recommended to use the brackets, even
when unnecessary, both to make it more clear to someone reading the
code and also to improve maintainability. For instance, if you (or another
programmer) later add additional statements to the if-then statement,
you will not risk forgetting to place brackets around the entire block at
that time.
For a concrete example of if-then statements, imagine a banking program that prints a short noti-
fication at the bottom of each ATM transaction receipt according to the remaining balance on the
account. This example prints to the console for simplicity's sake.
if (accountBalance > 100) {
System.out.println("Safe balance.");
}
If the value of the variable accountBalance is greater than 100 , then output a notification of " Safe
balance." to the console. If accountBalance is not greater than 100 , nothing will happen.
The basic if-then statement can also be extended with the keyword else . This provides an alterna-
tive set of statements to be executed if the condition is not true.
if (accountBalance > 100) {
System.out.println("Safe balance.");
} else {
System.out.println("Warning: Low balance.");
}
Now, if accountBalance is greater than 100 , the same notification will be printed. However, if
accountBalance is less than or equal to 100 , a different notification will be printed.
The else keyword can also be followed by a second if statement, which is then evaluated only if
the first condition is false .
if (accountBalance > 100) {
System.out.println("Safe balance.");
} else if (accountBalance < 0){
System.out.println("ALERT: Negative balance!");
} else {
System.out.println("Warning: Low balance.");
}
The first if condition is evaluated if accountBalance is greater than 100 , at which point the "Safe
balance." notification will print and nothing further is executed. If accountBalance is not greater
than 100 , the second if condition is evaluated. If accountBalance is less than 0 , the ALERT will be
printed and nothing further will be executed. If accountBalance is not less than 0, that is, if account-
Balance is between 0 and 100 , the warning will be printed, and the end of the statement is reached.