how to reorder a list of objects?

Question

Firstly, this is going to sound like homework, but it ain't. Just a problem I'm trying to solve at work.

I have a list of objects, the objects have a sequence number which indicates their order in the list in the UI. Example:

public class Task {
  Long id;
  String name;
  Long seq;
}

The table in my UI has "up" and "down" links on each row of the table for moving the tasks up and down in the list.

I'm implementing two methods to handle the reordering. These methods are call by ajax in the web UI.

public void incTaskSeq(List<Task> allTasks, Task taskToMove)

For example; if I have t1.seq=1, t2.seq=2, t3.seq=3, t4.seq=4, t5.seq=5 and I want to increment the place of t3, then t3.seq becomes 4, and t4.seq must become 3.

public void decTaskSeq(List<Task> allTasks, Task taskToMove)

Similarly; if I have t1.seq=1, t2.seq=2, t4.seq=3, t3.seq=4, t5.seq=5 and I want to decrement the place of t4, then t4.seq becomes 2, and t2.seq must become 3. Resulting in:

t1.seq=1, t4.seq=2, t2.seq=3, t3.seq=4, t5.seq=5

I'm a little stuck on the best way to do this.

I was thinking of putting all the tasks in a HashMap and then sort the map by the sequence number. Then locate the taskToMove in the map, change the sequence number, and then change all the affected tasks sequences.

But this approach seems inelegant. Does anyone have any ideas how I should do this?

Thanks, Rob

Of course it is inelegant, you are putting inside the class an attribute (sequence number) that really depends of the container. It is like building a dictionary and saying that "alphabet" is the word 17. It does not depend of the word itself, but of the collection to which it belongs (which ordering has been chosen, how may words are before). — SJuan76, Jun 18 '12 at 16:33

score 1 · Accepted Answer · answered Jun 18 '12 at 16:21

1

Use a Comparator, which is the Java interface for sorting non-naturally.

public TaskSequenceComparator implements Comparator<Task> {

  public int compare(Task one, Task two) {
    return one.getSequence() - two.getSequence();
  }

}

...

List<Task> tasks = ...;
Collections.sort(tasks, new TaskSquenceComaprator());
// tasks is now sorted by sequence.

You can create multiple Comparator classes to implement each kind of supported sort. Then you can select from them when you need the list sorted in a particular manner.

answered Jun 18 '12 at 16:21

Edwin Buck

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But I don't just need to sort the items, I need to renumber the sequences of the items. I'll add more details. – Jun 18 '12 at 16:25
Reordering the list is sorting it, merely pulling out some items in a non-standard order is not reordering the list. – Edwin Buck Jun 18 '12 at 16:26
@RobertHume if the `seq` field is not related to the `Task` but external to it, it should not be in the class. – SJuan76 Jun 18 '12 at 16:27
@SJuan76 The sequence field is in the task. – Jun 18 '12 at 16:29
@EdwinBuck Maybe I'm not being clear. If I move and item in the list by inc or dec it's seq value, it can have the same value of another task. So I need to change the values of the other task accordingly. I don't think a simple comparator is what I need, unless I'm missing something. – Jun 18 '12 at 16:31
1

@RobertHume What SJuan76 is saying is that if the Task can conceptually survive without a sequence number, then the maintenance and handling of sequence numbers probably should be in a different class, like a TaskPriority class that refers to lots of Task classes. – Edwin Buck Jun 18 '12 at 16:31
1

@RobertHume If you are chaninging sequence numbers, and expecting them to be reordered, you shouldn't be using a plain List. Extend the List to reorder based on listening to Tasks, or write a Schedule data structure which knows the intimate details of the schedule tasks. Anything else will be putting the nearby List maintenance code external to the data structure, the opposite of encapsulation. – Edwin Buck Jun 18 '12 at 16:33
@EdwinBuck Thx. Is there a List implementation that has "moveItemUp" and "moveItDown" methods? Something like listOfTasks.moveUp(myTask) would be great, then I could interate through the list and number the sequences correctly, then persist. – Jun 18 '12 at 16:37
Hmm, similar question here: http://stackoverflow.com/questions/4938626/moving-items-around-in-an-arraylist – Jun 18 '12 at 16:39
Looks like you can do this: `Collections.swap(arrayList, i, i-1);` – Jun 18 '12 at 16:40

score 1 · Answer 2 · answered Jun 18 '12 at 16:21

1

Implement a Comparator for each criteria you want to sort for.

Create a new ordered Collection object (maybe a TreeSet) passing it the Comparator needed. Do .addAll of the objects.

answered Jun 18 '12 at 16:21

SJuan76

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+1 using self-balancing data structure like TreeMap or TreeSet with custom Comparator is the best way to do it. – sperumal Jun 18 '12 at 16:52

score 1 · Answer 3 · answered Jun 18 '12 at 16:34

1

Just let the List keep tarck of ordering. (Except You always need to update the seqence number).

public void incTaskSeq(List<Task> allTasks, Task taskToMove){
     int movTaskNum = 0;
     for(int i=0;i<allTasks.size();i++){
          if(allTasks.get(i).equals(taskToMove)) 
               movTaskNum = i;
     }
     allTasks.remove(taskToMove);
     allTasks.add(i-1, taskToMove);
}

sorry for typos

answered Jun 18 '12 at 16:34

jvataman

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1

Thanks for the help, found this which is similar: `Collections.swap(arrayList, i, i-1);` – Jun 18 '12 at 16:48

score 0 · Answer 4 · answered Jun 18 '12 at 16:31

0

Use a doubly linked list and swap the sequence numbers when you move an item.

answered Jun 18 '12 at 16:31

smp7d

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how to reorder a list of objects?

4 Answers4