I didn't look deeply in your code but what I understand is that you want to show value of Purchase object and you want a Display method in Purchase Class.
You should declare a Display() method without any argument list like public void Display(). You can call this method from your CreatePurchase class with Purchase class object like onePurch.display();
Inside this method just place the System.out.print statement to print the values of the object like System.out.print("Value of invoice is: " +invNumber);
Hope this help.