Get uncheck or unsafe operation warning

Question

Hi
I define a class with following field:
Stack<Character> st=new Stack<Character>();
HashMap puhm=new HashMap();
HashMap pohm=new HashMap();

And following methods:
pushStack(char c)
pophStack(char c)

when i no use each of above method on Main Class i got :
Note: .\StackWithMemory.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

but when i use they on my class is no warning compile
why?