put this    "@SuppressWarnings("unchecked")" on the top of you method.

put this     @SuppressWarnings("unchecked") on the top of your method

There is no way to do what you ask.  That is, if a cast is required in
the first place to make the code compile in a 1.5+ compiler, then it is
necessarily a "possibly unsafe" cast.  You cannot rewrite the cast to
make it safe because the problem is not with the cast itself, but rather
with the expression being typecast and the variable to which you are
assigning it.

You do not provide the signature of the getAppSetting() method, but it
is probably this:

    Set getAppSetting(String s);

What you appear to want instead is this:

    Set<String> getAppSetting(String s);

If the method signature were the latter then no cast would be required
at all, and certainly not an unsafe one.  (Some compilers would in fact
emit an "unnecessary cast" warning if you used a cast with it anyway.)

It's not so simple and straightfoward at all, since you have oversimplified
details.
One can only speculate at what your getAppSetting() method returns - a Set
or Set<?> is my guess.
You cannot perform the cast without a compile-time warning for reasons which
are covered explicitly in the generics tutorial.
You might decide to change your reference to type Set<?> or Set<? super
String> or even change the return type of the method.
http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

You need to change the signature for getAppSetting to return a
Set<String> or Set<T>. Then the compiler can check that you have not
cheated.

There is not enough runtime information to check the cast. So you should
find a different way.

You can introduce specialist class to represent the set of strings:

class ShapeSettings {
    private final Set<String> shapes;
    public ShapeSettings(Set<String> shapes) {
        this.shapes = Collections.unmodifiableSet(
            new HashSet<String>(shapes)
        );
    }
    public Set<Shape> getShapes() {
        return shapes;
    }
}
...
    ShapeSettings shapeSettings = (ShapeSettings)
        settings.getAppSettings(SettingKeys.SHAPE_TYPES);
    Set<String> shapes = shapeSettings.getShapes();

Tom Hawtin