String.valueof，Inteager.parseInt

class Solution {
    public String largestNumber(int[] nums) {

        Integer a[] = new Integer [nums.length];

        for(int i=0;i<nums.length;i++){
            a[i]=nums[i];
        }

         Arrays.sort(a,new Comparator<Integer>(){

            @Override
            public int compare(Integer x, Integer y) {
                long sx = 10, sy = 10;
                while (sx <= x) {
                    sx *= 10;
                }
                while (sy <= y) {
                    sy *= 10;
                }
                return (int) (-sy * x - y + sx * y + x);
            }
        });
        if(a[0]==0) return "0";
        String s="";
        for(int i=0;i<a.length;i++){
            s+=String.valueOf(a[i]);
        }

        return s;
    }
}