Effective Java

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ITEM 57: MINIMIZE THE SCOPE OF LOCAL VARIABLES
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code compiles without error and runs without throwing an exception, but it does
the wrong thing. Instead of iterating over 
c2
, the second loop terminates immedi-
ately, giving the false impression that 
c2
is empty. Because the program errs
silently, the error can remain undetected for a long time.
If a similar copy-and-paste error were made in conjunction with either of the
for
loops (for-each or traditional), the resulting code wouldn’t even compile. The
element (or iterator) variable from the first loop would not be in scope in the
second loop. Here’s how it looks with the traditional 
for
loop:
for (Iterator i = c.iterator(); i.hasNext(); ) {
Element e = i.next();
... // Do something with e and i
}
...
// Compile-time error - cannot find symbol i
for (Iterator i2 = c2.iterator(); 
i
.hasNext(); ) {
Element e2 = i2.next();
... // Do something with e2 and i2
}
Moreover, if you use a 
for
loop, it’s much less likely that you’ll make the
copy-and-paste error because there’s no incentive to use different variable names
in the two loops. The loops are completely independent, so there’s no harm in
reusing the element (or iterator) variable name. In fact, it’s often stylish to do so. 
The 
for
loop has one more advantage over the 
while
loop: it is shorter, which
enhances readability.
Here is another loop idiom that minimizes the scope of local variables:
for (int i = 0, n = expensiveComputation(); i < n; i++) {
... // Do something with i;
}
The important thing to notice about this idiom is that it has 
two
loop variables, 
i
and 
n
, both of which have exactly the right scope. The second variable, 
n
, is used
to store the limit of the first, thus avoiding the cost of a redundant computation in
every iteration. As a rule, you should use this idiom if the loop test involves a
method invocation that is guaranteed to return the same result on each iteration.
A final technique to minimize the scope of local variables is to