As you have probably figured out the answer is no.
But you can force the client of your class  to not
change the underlying object that you return. For
example :

class ConstTest_C{
private String s;

public String getString()
{
if ( this.s == null )
return null;
else
return this.s.clone();
}

public void setString( String s ) { this.s = s; }
}

The user of your class can do anything they want with
the String that is returned by getString(), but it
will not change the class's String object.