Using CompareTo to compare both a string and a double value within an array?

Question

I am given an assignment to sort an array of inputted values using both insertion sort and selection sort. I am having some trouble with overwriting the compareTo method with in the Item class. I would like it to sort so that if two items are priced the same it then compares based on Category. So Child (C) items come first and then M and then Women. This is my code and every time that I attempt to compile it I get an error saying that double cannot be referenced.

public int compareTo (Object other) {
      int result;

      double otherPrice = ((Item)other).getClothPrice();
      String otherCategory = ((Item)other).getClothCategory();

      if (clothPrice == otherPrice)
         result = clothCategory.compareTo(otherCategory);
      else
         result = clothPrice.compareTo(otherPrice);

      return result;
}

castletheperson · Accepted Answer · 2017-10-21 05:22:50Z

7

First, your compareTo method should take an Item as a parameter. If this results in a compiler error, make sure Item is implementing Comparable<Item>.

To compare primitive double values, use Double.compare:

public int compareTo(Item other) {
    int result = Double.compare(clothPrice, other.clothPrice);
    if (result == 0) {
        result = clothCategory.compareTo(other.clothCategory);
    }
    return result;
}

If you're using Java 8, you may prefer to use some of the more advanced features of Comparator which make extending this code much simpler:

private static final Comparator<Item> naturalOrdering = Comparator
        .comparingDouble(Item::getClothPrice)
        .thenComparing(Item::getClothCategory);

public int compareTo(Item other) {
    return naturalOrdering.compare(this, other);
}

edited Oct 21, 2017 at 5:22

answered Oct 21, 2017 at 1:25

castletheperson

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5 Comments

Tommy Harvey Over a year ago

I was forgetting to add the Comparable, thank you for that I ended up getting the code to work by just changing the else to result = Double.compare(clothPrice, otherPrice); Would that accomplish the same thing?

castletheperson Over a year ago

@TommyHarvey Yes, that would accomplish the same thing. I've just rewritten your code in the way I've seen most people write it.

Tommy Harvey Over a year ago

Ok thank you I was just going off the example we were given in class. Thank you very much for your help

Basil Bourque Over a year ago

Wow, I didn't know about Comparator. Thanks! Shouldn't that Comparator object be cached for the sake of performance rather than re-instantiated on each call to compareTo?

castletheperson Over a year ago

@BasilBourque Yes, it would be much more efficient to cache the Comparator object using a static field.

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