Java Reference
In-Depth Information
For the
while
,
do
, and
for
loops, we've shown an example that prints 10 numbers.
The foreach loop can do this too, but it needs a collection to iterate over. In order to
loop 10 times (to print out 10 numbers), we need an array or other collection with
10 elements. Here's code we can use:
// These are the numbers we want to print
int
[]
primes
=
new
int
[]
{
2
,
3
,
5
,
7
,
11
,
13
,
17
,
19
,
23
,
29
};
// This is the loop that prints them
for
(
int
n
:
primes
)
System
.
out
.
println
(
n
);
a
x
What foreach cannot do
Foreach is different from the
while
,
for
, or
do
loops, because it hides the loop
counter or
Iterator
from you. This is a very powerful idea, as we'll see when we
discuss lambda expressions, but there are some algorithms that cannot be expressed
very naturally with a foreach loop.
For example, suppose you want to print the elements of an array as a comma-
separated list. To do this, you need to print a comma after every element of the array
except the last, or equivalently, before every element of the array except the first.
With a traditional
for
loop, the code might look like this:
for
(
int
i
=
0
;
i
<
words
.
length
;
i
++)
{
if
(
i
>
0
)
System
.
out
.
print
(
", "
);
System
.
out
.
print
(
words
[
i
]);
}
This is a very straightforward task, but you simply cannot do it with foreach. The
problem is that the foreach loop doesn't give you a loop counter or any other way to
tell if you're on the first iteration, the last iteration, or somewhere in between.
A similar issue exists when using foreach to iterate through
the elements of a collection. Just as a foreach loop over an
array has no way to obtain the array index of the current ele‐
ment, a foreach loop over a collection has no way to obtain the
Iterator
object that is being used to itemize the elements of
the collection.
Here are some other things you cannot do with a foreach style loop:
• Iterate backward through the elements of an array or
List
.
• Use a single loop counter to access the same-numbered elements of two dis‐
tinct arrays.
• Iterate through the elements of a
List
using calls to its
get()
method rather
than calls to its iterator.
