Not sure I understand the question. But you have not implemented the push and pop methods the usual way.
The push method takes a method parameter and adds it as the top member of the stack; the stack size is not one larger. Push doesn't usually return anything; you could return "true" if it worked, otherwise "false" if you so wish.
The pop method doesn't usually take a parameter; it takes the top member off the stack, sets that previous location on the stack to null, reduces the size by 1 then returns what was taken off the stack.
It is also odd design to have a stack inside another class. The stack is independent of the Volume class; you should implement a VolumeStack class. Also implement the isEmpty() and peek() methods. The peek() method returns the top element, without removing it from the stack.
I know of no way to print the names of variables other than writing literal Strings. You can use a Formatter to format to System.out, but that is little different from System.out.print. Not sure I understand that part of the problem. You can have a print() method in the Volume class,