On creating a new Android Application Project, MainActivity.java under its package name should be generated automatically in src folder &
I solved problem by changing method while creating new project... Don't know why but after updating ADT to latest version, when we create "blank activity", it generates empty src and res folders but if we create new "Empty activity", it generates default hello world program like in previous versions of ADT.... SO just click on create empty activity when new android project is to be added