Question

I’m trying to make an ArrayList of Books whose price is less than a given number and
I keep getting this compiler error:

   JAmos_Chapter09_exercise_94Arrays.java:86: error: double cannot be dereferenced
   if ( ( currentJAmos_Chapter09_exercise_94.getPrice( ) ).indexOf( searchString ) != -1 )
                                                               ^
1 error

after compiling this method in my array list file’s code:

 public ArrayList<JAmos_Chapter09_exercise_94> searchForPrice( String searchString )
 {
  ArrayList<JAmos_Chapter09_exercise_94> searchResult = new ArrayList<JAmos_Chapter09_exercise_94>( );
  for ( JAmos_Chapter09_exercise_94 currentJAmos_Chapter09_exercise_94 : library )
  {
   if ( ( currentJAmos_Chapter09_exercise_94.getPrice( ) ).indexOf( searchString ) != -1 )
        searchResult.add( currentJAmos_Chapter09_exercise_94 );
  }
  searchResult.trimToSize( );
  return searchResult;
 }

the getPrice part of my method code gets a double from this class file:

 /** default constructor
 */
 public JAmos_Chapter09_exercise_94( )
 {
  title = "";
  author = "";
  price  = 0.0;
 }

 /** overloaded constructor
 *  @param newTitle   the value to assign to title
 *  @param newAuthor  the value to assign to author
 *  @param newPrice   the value to assign to price
 */
 public JustinAmos_Chapter09_exercise_94( String newTitle, String newAuthor, double newPrice )
 {
  title = newTitle;
  author = newAuthor;
  price  = newPrice;
 }

 /** getTitle method
 *   @return the title
 */
 public String getTitle( )
 {
  return title;
 }

 /** getAuthor method
 *   @return the author
 */
 public String getAuthor( )
 {
  return author;
 }

 /** getPrice method
 *   @return the price
 */
 public double getPrice( )
 {
  return price;
 }

 /** toString
 * @return title, author, and price
 */
 public String toString( )
 {
  return ( "title: " + title + "\t"
           + "author: " + author + "\t"
           + "price: " + price );
 }
}

Overall, I’m wondering how I get rid of the

double cannot be dereferenced error

since I need to search for a double rather than a String.

Sorry if this was long.

Answer 1

Don’t use .indexOf() to find if the price is less than a certain value. .indexOf(s) finds the starting index of the first instance of a string in another string.

You are looking for the less-than comparison operator: <.

Change your logic to:

public ArrayList<JAmos_Chapter09_exercise_94> searchForPrice( String searchString ) {  
  ArrayList<JAmos_Chapter09_exercise_94> searchResult = new ArrayList<JAmos_Chapter09_exercise_94>( ); 
  //Converts the search string price into a double price.
  double maxPrice = Double.parseDouble(searchString);
  for ( JAmos_Chapter09_exercise_94 currentJAmos_Chapter09_exercise_94 : library ) {
    //If itemPrice < maxPrice, add it to the list.
    if ( currentJAmos_Chapter09_exercise_94.getPrice( ) < maxPrice)
      searchResult.add( currentJAmos_Chapter09_exercise_94 );
  } 
  searchResult.trimToSize( ); 
  return searchResult; 
}

If your searchString is not a nicely formatted double, I suggest writing a routine to convert searchString into a double.

EDIT: If this wasn’t clear, please ask me about it in the comments…