Beginning with JDK 5, there are two forms of theforloop. The first is the traditional form
that has been in use since the original version of Java. The second is the new “for-each” form.
Both types offorloops are discussed here, beginning with the traditional form.
Here is the general form of the traditionalforstatement:
for(initialization;condition;iteration) {
// body
}If only one statement is being repeated, there is no need for the curly braces.
Theforloop operates as follows. When the loop first starts, theinitializationportion of
the loop is executed. Generally, this is an expression that sets the value of theloop control
variable, which acts as a counter that controls the loop. It is important to understand that
the initialization expression is only executed once. Next,conditionis evaluated. This must be
a Boolean expression. It usually tests the loop control variable against a target value. If this
expression is true, then the body of the loop is executed. If it is false, the loop terminates.
Next, theiterationportion of the loop is executed. This is usually an expression that increments
or decrements the loop control variable. The loop then iterates, first evaluating the conditional
expression, then executing the body of the loop, and then executing the iteration expression
with each pass. This process repeats until the controlling expression is false.
Here is a version of the “tick” program that uses aforloop:
// Demonstrate the for loop.
class ForTick {
public static void main(String args[]) {
int n;
for(n=10; n>0; n--)
System.out.println("tick " + n);
}
}
Declaring Loop Control Variables Inside the for Loop
Often the variable that controls aforloop is only needed for the purposes of the loop and
is not used elsewhere. When this is the case, it is possible to declare the variable inside the
initialization portion of thefor. For example, here is the preceding program recoded so that
the loop control variablenis declared as anintinside thefor:
// Declare a loop control variable inside the for.
class ForTick {
public static void main(String args[]) {
// here, n is declared inside of the for loop
for(int n=10; n>0; n--)
System.out.println("tick " + n);
}
}
When you declare a variable inside aforloop, there is one important point to remember:
the scope of that variable ends when theforstatement does. (That is, the scope of the variable
is limited to theforloop.) Outside theforloop, the variable will cease to exist. If you need
Chapter 5: Control Statements 89