I wrote same to this.dest the sortStr is better than mine.

Length of an array is a public final int, so no calculation is done.

see here

http://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.7

really?learn it!

That is not true.

i<length

and

i<arr.length

have exactly the same cost.

The difference would be, when comparing to collection size, such as ArrayList and using .size() method, but here it doesn't matter.

for (int i=0; i<arr.length; i++)

Calculate the length in the cycle, is not efficient

int sum = 0;
int length = arr.length;
for (int i=0; i<length; i++) {
sum += (i % 2 == 0) ? arr[i] : arr[i] * arr[i];
}
return sum;