练习一

int[] ns = {1, 4, 9, 16, 25};    for(int i=ns.length-1;i>-1;i--)    {      System.out.println(ns[i]);            }

练习二

int[] ns = {1, 4, 9, 16, 25};int sum = 0;for (int n : ns) {    sum += n;}System.out.println(sum); // 55

练习三

double pi = 0;
int j = 1;
for (int i = 1; i < 9999999; i = i + 2) {
pi += ((1.0 / i) * j);
j = j * (-1);
}
pi = pi * 4;
System.out.println(pi);