uvm help needed

 I am a newbee for uvm. Looking for some understaning on using uvm_tes/uvm_env  under module.

IUS allows me using uvm_env under module but not uvm_test or uvm-agent ,  throwing below error  -

" class my_test extends uvm_test;
                               |
ncvlog: *E,FAABP1 (my_tb.sv,1|31): task, function, or assertion instance does not specify all required formal arguments [10.2.2][10.3(IEEE)]."

Follwing is the code - 

module top;

class my_env extends uvm_env; //works fine

//class my_test extends uvm_test; // Gives error

// class field,function

endclass

endmodule

What should be the reason?

thanks

joys