In my example, I try to create an ASCII table from a sequence of characters. I managed to do it with a List
of strings but failed with an array of chars.
I get an error that Character::hashCode
cannot be resolved in Collectors.toMap()
.
Error:(26, 17) java: method collect in interface java.util.stream.IntStream cannot be applied to given types;
required: java.util.function.Supplier<R>,java.util.function.ObjIntConsumer<R>,java.util.function.BiConsumer<R,R>
found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.Map<java.lang.Object,java.lang.Object>>
reason: cannot infer type-variable(s) R
(actual and formal argument lists differ in length)
Error:(26, 42) java: incompatible types: cannot infer type-variable(s) T,K,U,T
(argument mismatch; invalid method reference
incompatible types: java.lang.Object cannot be converted to char)
Is there a way to do it?
public class JavaCollectToMapEx2 {
public static void main(String[] args) {
// list of ASCII characters
var chars = List.of("a", "b", "c", "d", "e", "f",
"g", "h", "i", "j", "k", "l", "m", "n",
"o", "p", "q", "r", "s", "t", "u", "v",
"w", "x", "y", "z");
// CharSequence chars2 = "abcdefghijklmnopqrstuvwxyz";
char[] letters = "abcdefghijklmnopqrstuvwxyz".toCharArray();
// Map to represent ASCII character table
Map<Integer, String> asciiMap = chars.stream()
.collect(Collectors.toMap(String::hashCode, Function.identity()));
Map<Integer, Character> asciiMap2 = CharBuffer.wrap(letters).chars()
.collect(Collectors.toMap(Character::hashCode, Function.identity()));
System.out.println(asciiMap);
System.out.println(asciiMap2);
}
}
.chars()
is giving you an IntStream
, which is a stream of primitive int
, and not a stream of characters (more info). This is why no method references on Character
will work.
To achieve what you're looking for, you'll need a Stream<Character>
first:
Map<Integer, Character> asciiMap2 = CharBuffer.wrap(letters)
.chars()
.mapToObj(e -> (char) e)
.collect(Collectors.toMap(e -> e.hashCode(), Function.identity()));
Now, you still have the issue of using a method reference for getting the hash code. You can't use Character::hashCode
because it's ambiguous as to which method you want, as there are two that are possible:
- The override of Object#hashCode,
- The static method
int hashCode(char value)
You can see from this code, that both satisfy the first argument to toMap()
:
Function<Character, Integer> f1 = e -> Character.hashCode(e);
Function<Character, Integer> f2 = e -> e.hashCode();
To resolve this, you can use Object::hashCode
for the non-static method call.