make decision while importing packages

I am facing a problem , i want to make decision while
importing the packages ... i.e. i want if package A is not found then import
package B ...

I know C/C++, and in these languages it can done very easily ... i.e. we can
make decision while including header file ... whether we want to include A
header file or B header file ...

I need your urgent response ... please give me the solution of this problem
or guide me how to come out of this problem ...

Easy. Let's assume class A is in the package com.someco.project, and
class B is there, too. Then, you can import either one or both:

u can import both classes and not the
whole package.
ie.: