gistfile1.txt

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.temporal.Temporal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class HeightIn8 {
  static class Person {
    private final String name;

    public Person(String name) {
      this.name = name;
    }
    
    public String getName() {
      return name;
    }
    
    @Override
    public String toString() {
      return name + ' ' + Integer.toHexString(super.hashCode());
    }
  }
  
  public static void main(String[] args) throws IOException {
    // 1
    List<String> list = Arrays.asList(args);
    list.sort((s1, s2) -> s1.compareToIgnoreCase(s2));
    list.sort(String::compareToIgnoreCase);
    
    // 2
    /*Iterator<String> it = list.iterator();
    while(it.hasNext()) {
      String s = it.next();
      if (s.length() %2 == 0) {
        it.remove();
      }
    }*/
    list.removeIf(s -> s.length() %2 == 0);
    
    // 3
    Map<String, Long> map = new HashMap<>();
    for(String s: args) {
      map.put(s, 1 + map.getOrDefault(s, 0L));
    }
    
    // 4
    List<Person> people = Arrays.asList(new Person("Paul"), new Person("John"), new Person("Paul"));
    Map<String, List<Person>> byNameMap = new HashMap<>();
    for(Person person: people) {
      byNameMap.computeIfAbsent(person.getName(), name -> new ArrayList<>()).add(person);
    }
    
    // 5
    //Map<String, List<Person>> byNameMap = ...
    byNameMap.forEach((name, persons) -> {
      System.out.println(name + ' ' + persons);
    });
    
    // 6
    Map<String, List<Person>> byNameMap2 =
        people.stream().collect(Collectors.groupingBy(Person::getName));
    System.out.println(byNameMap2);
    
    // 7
    Path dictionnary = Paths.get("dict.txt");
    Map<String, Long> histoMap =
        Files.lines(dictionnary)
             .flatMap(line -> Arrays.stream(line.split(" ")))
             .collect(Collectors.groupingBy(Function.identity(),
                 Collectors.counting()));
    System.out.println(histoMap);
    
    // 8
    List<Temporal> temporals = Arrays.asList(LocalDate.now(), LocalDateTime.now());
    temporals.stream().map(temporal -> temporal.query(DayOfWeek::from)).forEach(System.out::println);
  }
}

Great post. I noticed two bugs in your snippets:

for(Person person: people) {
    byNameMap.getOrDefault(person.getName(), new ArrayList<>()).add(person);
}

should be

for(Person person: people) {
    List<Person> persons = byNameMap.getOrDefault(person.getName(), new ArrayList<>());
    persons.add(person);
    byNameMap.put(persons);
}

Otherwise you would end up with an empty map, if I'm not mistaken, since getOrDefault() doesn't put back the default value in the map automatically like computeIfAbsent() does.

And

for(Map.Entry<String, List<Person>> entry: byNameMap.entrySet()) {
    System.out.println(name + ' ' + persons);
}

should of course be

for(Map.Entry<String, List<Person>> entry: byNameMap.entrySet()) {
    System.out.println(entry.getKey() + ' ' + entry.getValue());
}

which, BTW, makes it much less readable than the Java 8 version.