Category: IT

{
  final InputFilter filter = new InputFilter() { 
        @Override
        public CharSequence filter(CharSequence source,
                int start, int end, Spanned dest, int dstart,
                int dend)
        {
              for (int i = start; i < end; i++)
              { 
                   if (Character.isDigit(source.charAt(i))||Character.isLetter(source.charAt(i)))
                   { 
                           return ""; 
                   } 
           } 
            return null;
        } 
};
answerTXT.setFilters(new InputFilter[]{filter});

ActivityThread casts Application.getBaseContext() to ContextImpl

static ContextImpl getImpl(Context context) {
        Context nextContext;
        while ((context instanceof ContextWrapper) &&
                (nextContext=((ContextWrapper)context).getBaseContext()) != null) {
            context = nextContext;
        }
        return (ContextImpl)context;
    }

Potential crash when !(context instanceof Activity)

public static Activity unwrap(Context context) {
    while (!(context instanceof Activity)) {
        ContextWrapper wrapper = (ContextWrapper) context;
        context = wrapper.getBaseContext();
    }
    return context;
}