SelectorModel(int n){
  if (n <= 0) {
    throw new IllegalArgumentException("Can not create a model with " + n + " elements");
  }
  this.n=n;
}

SelectorModel ( int n ) { if ( n <= 0 ) { throw new IllegalArgumentException ( "Can not create a model with " + n + " elements" ) ; } this . n = n ; }
create a new selectormodel . valid indices are 0 to n - 1 .
