Why there is error of Stack?

As I ran my code, the error of educative console pops out saying sth wrong with ‘Stack’.

Hello Isobel,

It would help a lot if you could please share the screenshot of the exact error shown by the console.

It could be that you need to implement the pair class first which is going to be used by the stack in the solution, but it’s hard to be sure without looking at the exact error returned by the Java compiler.