Java list comparison resulting in error I can't pin down

Question

I'm working on this script to compare elements sequentially in a list/array in Java. Essentially the code takes in a CSV, converts it to a list and iterates over the list's size. When I attempt a comparison between the values I run into an error I can't figure out in this case:

Exception in thread "main" java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Float (java.lang.String and java.lang.Float are in module java.base of loader 'bootstrap')
at list.main(list.java:22)

The code is:

public class list{

    public static void main(String[] args){
        System.out.println("Enter your filename");
        Scanner inputValue = new Scanner(System.in);
        String fileLocation = inputValue.nextLine();
        try {
            String checkvalue = new String(Files.readAllBytes(Paths.get(fileLocation)));
            ArrayList<Float> listValues = new ArrayList(Arrays.asList(checkvalue.split("[\\r\\n]+")));
            System.out.println(listValues);
            for (int i = 0; i < listValues.size(); i++){
                System.out.println(listValues.get(i));
                float valueA = listValues.get(i);
                float valueB = listValues.get(i+1);
                if (valueA <= valueB){
                    System.out.println("True");
                }
            }
        }
        catch (IOException e){
            e.printStackTrace();
        }
    }
}

String split returns a string. so `listValues` is actually an ArrayList but you've declared it an `ArrayList`. — Roddy of the Frozen Peas, Nov 06 '18 at 18:48
Ah, ok, that makes sense. I'm coming from Python if that clarifies the mistake, type conversions are very straightforward compares to here. In this case, I guess I generate the list, then convert the list elements into floats afterward? Or is there a doc where I can read up on how to convert directly to a float by this method? — , Nov 06 '18 at 18:50
`new ArrayList` instantiates a raw type. You should have a warning on that line. You want `new ArrayList` or just `new ArrayList<>`. — Daniel Pryden, Nov 06 '18 at 18:52
@DanielPryden: Technically true but there's an issue with conversion between the type of lists which would be better explained in an answer. — Makoto, Nov 06 '18 at 18:52

Sweeper · Accepted Answer · 2018-11-06T19:41:44.533

4

Replace this line

ArrayList<Float> listValues = new ArrayList(Arrays.asList(checkvalue.split("[\\r\\n]+")));

with this:

ArrayList<Float> listValues = new ArrayList<>(
    Arrays.stream(
        checkvalue.split("[\\r\\n]+"))
            .map(Float::parseFloat).collect(Collectors.toList()
    )
);

Your have two main mistakes

new ArrayList creates a raw type. And you should not use raw types. This is why I have added <>.
Arrays.asList(checkvalue.split("[\\r\\n]+")) creates a List<String>, which can't be converted to a ArrayList<Float>. This is why I mapped everything in the String[] with Float::parseFloat.

Or more simply, as suggested by littleLouito,

ArrayList<Float> listValues = Arrays.stream(
        checkvalue.split("[\\r\\n]+"))
            .map(Float::parseFloat).collect(Collectors.toCollection(ArrayList::new));

edited Nov 06 '18 at 19:41

answered Nov 06 '18 at 19:01

Sweeper

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The creation of the `ArrayList` is pointless when you create a list with `toList`, since the list is never modified.' – Lii Nov 06 '18 at 19:11
@Lii Without creating a `new ArrayList` it won't compile. – Roshana Pitigala Nov 06 '18 at 19:19
2

you can use Collectors.toCollection(ArrayList::new) to collect directly into an ArrayList – EduSanCon Nov 06 '18 at 19:38
1

@RoshanaPitigala: When you are saying that something doesn't compile it is helpful to say *what* is and what error message it gives. It this case the problem probably is that the type of the `listValues` variable must be changed to `List`. But that's okay in the posters code, because they doesn't need the list to be an `ArrayList`. – Lii Nov 06 '18 at 19:39

Roddy of the Frozen Peas · Answer 2 · 2018-11-06T19:41:37.853

0

 ArrayList<Float> listValues = new ArrayList(Arrays.asList(checkvalue.split("[\\r\\n]+")));

checkvalue.split(...) returns a string array. So the right hand side of this expression builds up an ArrayList<String[]>. You've declared listValues as ArrayList<Float>, though, so it's trying to tell you that you have a type mismatch.

To solve, you'll need to parse the Strings into floats. Java's Float class has a nice parseFloat method you can use.

edited Nov 06 '18 at 19:41

answered Nov 06 '18 at 18:50

Roddy of the Frozen Peas

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You'll need to stream it and convert using `Float.parseFloat`. – Roddy of the Frozen Peas Nov 06 '18 at 19:41

Java list comparison resulting in error I can't pin down

2 Answers2