Java custom comparator with different sort options

Question

I have a an Item class with two properties - id and timestamp. There is a custom comparator class to sort an itemList according to the timestamp.

Is there a way to use the comparator class such that I can specify sort by timestamp or sort by id?

Item class:

   public class  Item {

      private Integer id;
      private Date timestamp;

}

Comparator :

public class ItemComparator implements Comparator<Item>{
  @Override
  public int compare(Item mdi1, Item mdi2) {

    return mdi1.getTimestamp().compareTo(mdi2.getTimestamp());

   }

}

Sort code:

 Collections.sort(itemList, new ItemComparator());

Can I use the same comparator to sort the list by Id too?

rgettman · Accepted Answer · 2013-10-30 16:22:47Z

3

With some modifications, yes.

Add a constructor with an enum argument to define which field to use:

public class ItemComparator implements Comparator<Item>{
    public enum Field {
        ID, TIMESTAMP;
    }

    private Field field;

    public ItemComparator(Field field) {
        this.field = field;
    }

Then in the compare method, switch on the field chosen:

@Override
public int compare(Item mdi1, Item mdi2) {
    int comparison = 0;
    switch(field) {
    case TIMESTAMP:
        comparison = mdi1.getTimestamp().compareTo(mdi2.getTimestamp());
    case ID:
        comparison = mdi1.getID() - mdi2.getID();
    }
    return comparison;
}

answered Oct 30, 2013 at 16:22

rgettman

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1 Comment

Louis Wasserman Over a year ago

This works, but it's probably preferable to have your enum extend Comparator<Item> directly and use a different Comparator for different cases, rather than hacking one Comparator to work differently in different cases.

Jon Skeet · Accepted Answer · 2013-10-30 16:18:46Z

2

Can I use the same comparator to sort the list by Id too?

Well you could have a constructor parameter to say which property you want to sort by - but personally I'd create a different class for that instead. You can have as many classes implementing Comparator<Item> as you like...

answered Oct 30, 2013 at 16:18

Jon Skeet

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Comments

camickr · Accepted Answer · 2013-10-30 16:37:05Z

1

You can use the Bean Comparator. Then you can sort on any property in your class. You just specify the method name you want to use to get the property to sort. For example:

BeanComparator byId = new BeanComparator(Item.class, "getId");
Collections.sort(items, byId);

This comparator can be used on any class.

answered Oct 30, 2013 at 16:37

camickr

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Comments

Jonathan Rosenne · Accepted Answer · 2013-10-30 16:19:42Z

0

Use two separate comparators. Select which one you want to use in the sort call.

answered Oct 30, 2013 at 16:19

Jonathan Rosenne

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