Chapter 7. Loops
Computers are often used to automate repetitive tasks. Repeating tasks without making errors is something that computers do well and people do poorly.
Running the same code multiple times is called iteration. We have seen methods, like countdown
and factorial
, that use recursion to iterate. Although recursion is elegant and powerful, it takes some getting used to. Java provides language features that make iteration much easier: the while
and for
statements.
The while Statement
Using a while
statement, we can rewrite countdown
like this:
public
static
void
countdown
(
int
n
)
{
while
(
n
>
0
)
{
System
.
out
.
println
(
n
);
n
=
n
-
1
;
}
System
.
out
.
println
(
"Blastoff!"
);
}
You can almost read the while
statement like English: “While n
is greater than zero, print the value of n
and then reduce the value of n
by 1. When you get to zero, print Blastoff!”
The expression in parentheses is called the condition. The statements in braces are called the body. The flow of execution for a while
statement is:
Evaluate the condition, yielding
true
orfalse
.If the condition is
false
, skip the body and go to the next statement.If the condition is
true
, execute the body and go back to step 1.
This type of flow is called a loop, because the last step loops back around to the first.
The body of the loop should change the value of one or more variables so that, eventually, the condition becomes false
and the loop terminates. Otherwise the loop will repeat forever, which is called an infinite loop ...
Get Think Java now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.