This can happen when, for instance you mix legacy and code using generics. (http://java.sun.com/docs/books/tutorial/java/generics/erasure.html) If you what the message says you will get more information - like the line(s) where the problem occurs and often a suggested alternative.
By "recompile with -Xlint:unchecked" they mean you should add that switch to the command you use for compiling like:javac -Xlint:unchecked -cp . MyClass.java
This may be because you are using some generic classes and havent declared their type properly.
like compiling the below code will give the same error(assuming that byte[ ] bytes is prepopulated)
Vector vect= new Vector();
for (int j=0;j<size;j++){
vect.add(bytes[j]);
}
Here Vector is a generic Class that you are using to store bytes.
so to avoid the error you need to write the code as following
Vector><Byte> vect= new Vector<Byte> ();
for (int j=0;j<size;j++){
vect.add(bytes[j]);
}>