android 官方推薦除了ArrayList耻煤，別的collections 使用增強LOOP ，也就是foreach
ArrayList 使用手寫計數(shù)loop without size
以下為官方原文
static class Foo {
int mSplat;
}
Foo[] mArray = ...
public void zero() {
int sum = 0;
for (int i = 0; i < mArray.length; ++i) {
sum += mArray[i].mSplat;
}
}
public void one() {
int sum = 0;
Foo[] localArray = mArray;
int len = localArray.length;
for (int i = 0; i < len; ++i) {
sum += localArray[i].mSplat;
}
}
public void two() {
int sum = 0;
for (Foo a : mArray) {
sum += a.mSplat;
}
}
zero() is slowest, because the JIT can't yet optimize away the cost of getting the array length once for every iteration through the loop.
one() is faster. It pulls everything out into local variables, avoiding the lookups. Only the array length offers a performance benefit.
two() is fastest for devices without a JIT, and indistinguishable from one() for devices with a JIT. It uses the enhanced for loop syntax introduced in version 1.5 of the Java programming language.
So, you should use the enhanced for loop by default, but consider a hand-written counted loop for performance-critical [ArrayList](https://developer.android.google.cn/reference/java/util/ArrayList.html) iteration.
