ERROR: Type mismatch: cannot convert from Map<Object,List<Object>> to Map<String,List<>>

I have a list to group by using an attribute. I'm not sure why I'm getting an error

Type mismatch: cannot convert from Map<Object,List> to Map<String,List<MyClass.Leader>>

when calling the Leader class. I'm using Java 8.

Map<String, List<Leader>> reports = vals.stream()
    .collect(
        Collectors.mapping(val -> val.split("\\|"),
            Collectors.mapping(leaderArgs -> new Reports(leaderArgs[0], leaderArgs[1], leaderArgs[2], leaderArgs[3]),
                Collectors.groupingBy(leader -> leader.country))));

```java
public class Leader {
    String name;
    String country;
    String age;
    String sex;

    Leader(String name, String country, String age, String sex) {
        this.name = name;
        this.country = country;
        this.age = age;
        this.sex = sex;
    }
}

The following code does the same thing as your code but splits up the collect in to multiple map steps, this may work better for you:

Pattern splitPattern = Pattern.compile("\\|");

Map<String, List<Leader>> result =
      vals.stream()
          .map(splitPattern::split)
          .map(leaderArgs -> new Leader(leaderArgs[0], leaderArgs[1], leaderArgs[2], leaderArgs[3]))
          .collect(Collectors.groupingBy(leader -> leader.country));

I have also used Pattern which will be a bit faster than repeated calls to String.split.

Simple XML output for this might be:

result.entrySet()
    .forEach(entry -> {
        System.out.println("<LeaderList country=\"" + entry.getKey() + "\">");

        entry.getValue()
             .forEach(leader -> System.out.println("<Leader name=\"" + leader.name + "\" age=\"" + leader.age + "\" sex=\"" + leader.sex + "\">"));

        System.out.println("</LeaderList>");
    });

The collect code is assuming the input vals is a stream of String with one entry per line. For example to read a file:

Path path = ... file path

try (Stream<String> vals = Files.lines(path))
  {
    ... the collect code
  }
catch (final IOException ex)
 {
   // TODO error handling
 }

Note: It is important to use a try-with-resources block with Files.lines so that the stream is closed properly.