This is O(nlogn) Java solution that will use sorting as a basis. The idea is to sort and pick every pair and we deal with the minimum only.

```
public int arrayPairSum(int[] nums) {
if(nums.length==0) {
return 0;
}
int sum=0;
Arrays.sort(nums);
for(int i=1;i<nums.length;i=i+2) {
sum += nums[i-1];
}
return sum;
}
```