算法题，栈解决方式，大佬帮帮忙看看

 给定一个无序数组，输出无重复元素
public class Wc
 { public static void main(String[] args) 
{ int [] b = {10,20,20,10,30,10,40};   wc(b);  } public static void wc( int [] a){ int [] c = a;  Stack<Integer> stack = new Stack<>();  Arrays.sort(c);  for (int i = 0;i<c.length;i++)
{ if (stack.peek()==null){
                    stack.push(c[i]);  } else if (stack.peek()==c[i]){
                stack.pop();  } else {
                stack.push(c[i]);  }
        }

        System.out.println(stack.pop());   }
}

为什么出现Exception in thread "main" java.util.EmptyStackException
    at java.util.Stack.peek(Stack.java:102)
    at java.util.Stack.pop(Stack.java:84)
    at com.company.Wc.wc(Wc.java:33)
    at com.company.Wc.main(Wc.java:10)

Process finished with exit code 1

	

这个异常