ClassCastException when using custom map supplier in grouping-by

7
votes

While I was doing a little programming exercise i stumbled upon a ClassCastException. As background i'm giving a simplified version of the exercise to demonstrate the problem:

Given a string which contains only the characters A or B compute a map with the characters as keys and the number of occurrences as values. Additionally the map should always contain both characters as key (with value zero if a character is missing in the input string).

Examples:

  • "A" => {A=1, B=0}
  • "AAB" => {A=2, B=1}

My first solution was the following:

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;

public Map<Character, Long> createResult(String input) {
    Map<Character, Long> map = input.chars()
        .mapToObj(c -> (char) c)
        .collect(groupingBy(c -> c, counting()));

    map.putIfAbsent('A', 0L);
    map.putIfAbsent('B', 0L);
    return map;
}

This solution worked but i wanted to try if it was possible to supply a map with default values to the groupingBy function:

public HashMap<Character, Long> createResult2(String input) {
    return input.chars()
        .mapToObj(c -> (char) c)
        .collect(groupingBy(c -> c, this::mapFactory, counting()));
}

private HashMap<Character, Long> mapFactory() {
    HashMap<Character, Long> map = new HashMap<>();
    map.put('A', 0L);
    map.put('B', 0L);
    return map;
}

When calling createResult2 with input A a ClassCastException is thrown at runtime:

java.lang.ClassCastException: java.lang.Long cannot be cast to [Ljava.lang.Object;
    at java.util.stream.Collectors.lambda$groupingBy$45(Collectors.java:909)
    at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.util.stream.IntPipeline$4$1.accept(IntPipeline.java:250)
    at java.lang.CharSequence$1CharIterator.forEachRemaining(CharSequence.java:149)
    at java.util.Spliterators$IntIteratorSpliterator.forEachRemaining(Spliterators.java:1908)
    at java.util.Spliterator$OfInt.forEachRemaining(Spliterator.java:693)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)

Can anyone explain why this is happening?

2
javajava-8java-stream
Streams look like overkill for this simple problem. Even worse, the stacktrace contains nothing about where your code caused an error. That would be reason enough for me to use a simple loop. - user6999902
The last line in the stacktrace corresponds to the collect call in my code snippet. - eee
@LutzHorn This question is more about why this is happening and less about how to solve the exercise. - eee
The map supplier should never provide a non-empty map. Besides, this answer may shed some light on the issue. - Holger
Note when using input.chars().partitioningBy(c -> c=='A', counting()), you’ll get a Map in which both keys are always present, mapping to zero if no matching characters were found, but the keys will be Boolean.TRUE for 'a' and Boolean.FALSE for 'b' (actually “not 'a'”)… - Holger

2 Answers

2
votes

There is type inference magic involved, but if you want solution here it is:

Replace 

map.put('A', 0L);
map.put('B', 0L);

by

map.put('A', new Object[]{0L});
map.put('B', new Object[]{0L});

But I strongly discourage you of using this solution in practice. Implementation details can be changed in any update and this hack stop works.

Here more explanation about "why" from groupingBy javadoc

mapFactory - a function which, when called, produces a new empty Map of the desired type

mapFactory is second parameter. Word "empty" is very important. Implementation use created map to store arrays of long while iterating and then convert them to long. It works because of massive amount of casting inside.

0
votes

Why not a simple loop?

private static Map<Character, Integer> count(String input) {
    Map<Character, Integer> result = new HashMap<>();
    result.put('A', 0);
    result.put('B', 0);
    for (Character c : input.toCharArray()) {
        result.put(c, result.get(c) + 1);
    }
    return result;
}