Hey! i have got a java 2d game that i just made..although at the moment its not a real game because all it is, is a character that walks left and right...There is two things i want to create..

(1) I want to make a floor and objects/walls.. I want to make a mario game although at the moment the floor is determined by the co-ords...i would like to make it so the Program knows if there is a block underneath me and if there is..the program will stop moving the character (if that makes sense)...so if anyone know how i do this, can u point me in the correct direction?

(2)I want to make monsters...in order to make these i would need help with the first bit (so i know when contact is made with me and monster) and also i THINK i need to know a bit more about threads? although im not sure if thats what i need to know..

so, if anyone can help out..thanks!!

P.S...
im trying to make a mario game, so thats the kind of techniques i need to use :d

-thanks