无法弄清楚如何以正确的顺序打印列表

Question

Basically I have an arraylist of objects which have a name and startDate and they are in a similar initial order to :

Frank 10/20/1990

Bill 1/2/1990

Frank 2/2/1990

John 9/8/1990

Bill 4/4/1990

Frank 5/1/1990

I'm trying to print them in order of the first names declared and then by date, eg, the final printing should print :

Frank 1/1/1990

Frank 5/1/1990

Frank 10/20/1990

Bill 1/2/1990

Bill 4/4/1990

John 9/8/1990

Answer 1

Assuming you object class looks like this:

private static class MyObject {

  private final String name;
  private final Date date;

  private MyObject(String name, Date date) {
    this.name = name;
    this.date = date;
  }

  public String getName() {
    return name;
  }

  public Date getDate() {
    return date;
  }
}

You could print your objects using the following method:

@Test
public void printInOrder() {
  List<MyObject> list = new ArrayList<>(); // List containing your objects
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 10, 20).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Bill", Date.from(LocalDate.of(1990, 1, 2).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 2, 2).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("John", Date.from(LocalDate.of(1990, 9, 8).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Bill", Date.from(LocalDate.of(1990, 4, 4).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 5, 1).atStartOfDay(ZoneId.systemDefault()).toInstant())));

  list.stream()
      .sorted(Comparator.comparing(MyObject::getName)
                        .thenComparing(Comparator.comparing(MyObject::getDate)))
      .map(obj -> obj.getName() + " " + obj.getDate())
      .forEach(System.out::println);
}

Output:

Bill Tue Jan 02 00:00:00 CET 1990
Bill Wed Apr 04 00:00:00 CEST 1990
Frank Fri Feb 02 00:00:00 CET 1990
Frank Tue May 01 00:00:00 CEST 1990
Frank Sat Oct 20 00:00:00 CET 1990
John Sat Sep 08 00:00:00 CEST 1990

---

This uses Java 8 streams and a comparator chain to sort your objects. The comparators compare by a given attribute, the thenComparing is used to chain them.

---

UPDATE:

As I am not sure if I possibly misunderstood you, here is a second version for the following case:

You want to sort by name, but in the order the names are appearing in the original list , so names first appearing will be first.

In this case you will have to remember the order of the names in a separate, temporary list (only containing the unique names in the original order) that can be used inside the comparator afterwards.

This list is produced by transforming the original list into a set (so have only the unique names) and then transforming this set back into a list (to be able to use indexOf inside the comparator).

@Test
public void printInOrder() {
  List<MyObject> list = new ArrayList<>();
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 10, 20).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Bill", Date.from(LocalDate.of(1990, 1, 2).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 2, 2).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("John", Date.from(LocalDate.of(1990, 9, 8).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Bill", Date.from(LocalDate.of(1990, 4, 4).atStartOfDay(ZoneId.systemDefault()).toInstant())));
  list.add(new MyObject("Frank", Date.from(LocalDate.of(1990, 5, 1).atStartOfDay(ZoneId.systemDefault()).toInstant())));

  List<String> declaredNameOrder = new ArrayList<>(list.stream()
                                      .map(MyObject::getName)
                                      .collect(Collectors.toCollection(LinkedHashSet::new)));

  list.stream()
      .sorted(Comparator.<MyObject>comparingInt(o -> declaredNameOrder.indexOf(o.getName()))
                  .thenComparing(Comparator.comparing(MyObject::getDate)))
      .map(obj -> obj.getName() + " " + obj.getDate())
      .forEach(System.out::println);
}

Output:

Frank Fri Feb 02 00:00:00 CET 1990
Frank Tue May 01 00:00:00 CEST 1990
Frank Sat Oct 20 00:00:00 CET 1990
Bill Tue Jan 02 00:00:00 CET 1990
Bill Wed Apr 04 00:00:00 CEST 1990
John Sat Sep 08 00:00:00 CEST 1990

Answer 2

You should do it in two steps:

First order by name using custom comparator like:

    Collections.sort(myList, new Comparator<MyObject>() {
       public int compare(MyObject o1, MyObject o2) {
          return o1.getName().compareTo(o2.getName());
       }
    });

And store the output in a temp list

Second use temp list to sort by data:

    Collections.sort(myList, new Comparator<MyObject>() {
       public int compare(MyObject o1, MyObject o2) {
           return o1.getDate().compareTo(o2.getDate());
       }
    });

then your list will be sorted as you want.