Array Implementation of Stack
Good  


Naming of parameters and methods follow the naming convention  


Bad  


not checking input parameter for constructor  
not listening to previous answers ( pop() returning nothing )  
not using braces {} for every if..else statement  


Additional  


int size should be final as it won't be changed.  
Instead of display() you can override toString() as this is more common.  
Instead of writing to System.out you should throw an exception.  
give your variables some space to breathe. E.g top=top+1; would be more readable if written like so top = top + 1;