compiler giving generic error in Collectors toMap

Bharat Bhagat :

I was writing one simple expression wherein I have to collect Map of String vs the list of indexes from an array. To do that I am trying to use

 Collectors.toMap(keyMapper, valueMapper, mergeFunction).

Gist of it is as follow.

    Map<String, List<Integer>> sortedStringToIndex = IntStream.range(0, strs.length)
 .mapToObj(i -> new AbstractMap.SimpleEntry<String,Integer>(sortString(strs[i]),i))
 .collect(Collectors.toMap((Map.Entry<String,Integer> pair) -> pair.getKey(),
            (Map.Entry<String,Integer> pair) -> {
        List<Integer> val = new ArrayList<>(){{add(pair.getValue());}};
        return val;
        }, (List<Integer> index1, List<Integer> index2) ->  index1.addAll(index2)));

But It gives me following error.

method java.util.stream.Collectors.toMap(java.util.function.Function,java.util.function.Function,java.util.function.BinaryOperator) is not applicable (inference variable U has incompatible bounds equality constraints: java.util.List lower bounds: java.lang.Boolean,java.util.List)

Can someone please explain the compiler error and how to fix this. Thanks in advance

Andronicus :

Take a look at javadoc. It's because List#addAll produces boolean, it cannot be used as downstream function. You can use streams:

Stream.concat(index1.stream(), index2.stream())
                         .collect(Collectors.toList())

Or using apache commons collections:

ListUtils.union(index1, index2)

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