I have code like this, which is supposed to create a Map from an array of integers. The key represents the number of digits.

public static Map<Integer, List<String>> groupByDigitNumbersArray(int[] x) {
    return Arrays.stream(x)  // array to stream
        .filter(n -> n >= 0) // filter negative numbers
        .collect(Collectors.groupingBy(n -> Integer.toString((Integer) n).length(), // group by number of digits
            Collectors.mapping(d -> (d % 2 == 0 ? "e" : "o")   d,
                Collectors.toList()))); // if even e odd o add to list
}

The problem is in the line with mapping(). I'm getting an error:

Operator '%' cannot be applied to 'java.lang.Object', 'int'

Does someone know how to solve this?

CodePudding user response：

The flavor of collect() that expects a Collector as an argument isn't available with primitive streams. Even without a modulus operator %, your code will not compile - comment out the downstream collector of groupingBy() to see what I'm talking about.

You need to apply boxed() operation in order to convert an IntStream into a stream of objects Stream<Integer>.

Your method might look like this:

public static Map<Integer, List<String>> groupByDigitNumbersArray(int[] x) {
    
    return Arrays.stream(x)  // creates a stream over the given array
        .filter(n -> n >= 0) // retain positive numbers and zero
        .boxed()             // <- converting IntStream into a Stream<Integer>
        .collect(Collectors.groupingBy(
            n -> String.valueOf(n).length(), // group by number of digits
            Collectors.mapping(d -> (d % 2 == 0 ? "e" : "o")   d, // if even concatinate 'e', if odd 'o'
                Collectors.toList()))); // collect to list
} 

I've changed the classifier function of groupingBy() to be more readable.

CodePudding user response：

You have to secure that the elements of the array you are working on is a Reference type and not a primitive type. So you have two options:

1. Convert your argument int[ ] x to an Integer[ ] or

    public static Map<Integer, List<String>> groupByDigitNumbersArray(int[] x){
   
    return Arrays.stream(y) //array to stream
            .filter(n -> n >= 0) //filter negative numbers
             .boxed() //int stream to wrapper Integer stream
             .collect(Collectors.groupingBy(n -> n.toString().length(),
                    //group by number of digits
                    mapping(d -> (d % 2 == 0 ? "e" : "o")   d,
                            Collectors.toList()))); //if even e odd o add to list
   }
   

2. Change the signatur of your methode to Integer[ ]

    public static Map<Integer, List<String>> groupByDigitNumbersArray(Integer[] x){
    return Arrays.stream(x) //array to stream
            .filter(n -> n >= 0) //filter negative numbers
            .collect(Collectors.groupingBy(n -> n.toString().length(),
                    //group by number of digits
                    mapping(d -> (d % 2 == 0 ? "e" : "o")   d,
                            Collectors.toList()))); //if even e odd o add to list
   }