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

Answer 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.