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class Math {
static double abs(double num) {
if (num < 0) {
return -num;
} else {
return num;
}
class System {
//todo：后面再检查这个方法是否正确
// src:源数组；
// srcPos:源数组要复制的起始位置；
// dest:目的数组；
// destPos:目的数组放置的起始位置；
// length:复制的长度。
static void arraycopy(List<String> src, int srcPos, List<String> dest,
int destPos, int length) {
List.copyRange(dest, destPos, src, srcPos, srcPos+length);
