java - EmptyStackException when trying to implement Comparator -

i'm new here , have problem. i'm trying implement comparator compare 2 stacks top. code looks this

class comp implements comparator<stack<integer>> {     @override     public int compare(stack<integer> st1,stack <integer> st2) {         return st1.peek()-st2.peek();     } } 

i got java.util.emptystackexception @ st1.peek()-st2.peek(); , don't know why. maybe me better implementation problem. thanks!

stack.peek throws emptystackexception when stack empty. need check if stack empty before calling peek on it, example, if want empty stacks come before non-empty ones:

@override public int compare(stack<integer> st1, stack<integer> st2) {   if (st1.isempty() && st2.isempty()) {      return 0;   }   if (st1.isempty()) {     return -1;   }   if (st2.isempty()) {     return 1;   }   return st1.peek() - st2.peek(); } 

or if want empty stacks come after non-empty ones:

@override public int compare(stack<integer> st1, stack<integer> st2) {   if (st1.isempty() && st2.isempty()) {      return 0;   }   if (st1.isempty()) {     return 1;   }   if (st2.isempty()) {     return -1;   }   return st1.peek() - st2.peek(); }