Offer31 array prove safety seeking the maximum and successive subarray

public class Test{
public static Integer findSum(int[]arr) {
if(arr==null||arr.length<=0)return null;
int sum=0;
int bigSum=Integer.MIN_VALUE;
for(int i=0;i<arr.length;i++) {
if(sum<=0) {
sum=arr[i];
}else {
sum+=arr[i];
}
if(sum>bigSum) {
bigSum=sum;
}
}
return bigSum;
}
public static void main(String[] args) {
int[]arr= {2,-3,4,5,-7,3,-2,4,5,-6};
System.out.println(findSum(arr));
}
}