Use Enhanced For Loop Syntax With Caution
The enhanced for loop (also sometimes known as "for-each" loop) can be used for collections that implement the Iterable interface.
With these objects, an iterator is allocated to make interface calls
to hasNext() and next(). With an ArrayList, you're better off walking through
it directly, but for other collections the enhanced for loop syntax will be equivalent
to explicit iterator usage.
Nevertheless, the following code shows an acceptable use of the enhanced for loop:
public class Foo {
int mSplat;
static Foo mArray[] = new Foo[27];
public static void zero() {
int sum = 0;
for (int i = 0; i < mArray.length; i++) {
sum += mArray[i].mSplat;
}
}
public static void one() {
int sum = 0;
Foo[] localArray = mArray;
int len = localArray.length;
for (int i = 0; i < len; i++) {
sum += localArray[i].mSplat;
}
}
public static void two() {
int sum = 0;
for (Foo a: mArray) {
sum += a.mSplat;
}
}
}
zero() retrieves the static field twice and gets the array
length once for every iteration through the loop.
one() pulls everything out into local variables, avoiding
the lookups.
two() uses the enhanced for loop syntax introduced in version 1.5 of
the Java programming language. The code generated by the compiler takes care
of copying the array reference and the array length to local variables, making
it a good choice for walking through all elements of an array. It does
generate an extra local load/store in the main loop (apparently preserving
"a"), making it a teensy bit slower and 4 bytes longer than one().
To summarize all that a bit more clearly: enhanced for loop syntax performs well
with arrays, but be cautious when using it with Iterable objects since there is
additional object creation.
