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Sort a HashMap based on a Value object's attribute  RSS feed

 
Ranch Hand
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Hi,

Am trying to sort a Map based on a value object's attribute.
The Map has supplierId(String) as Keys and Supplier object as value. The Map has to be sorted on the Supplier object's name.

But the following code doesn't work!!






Can anyone tell me what's wrong in my code?

Thanks,
Arnav
 
Sheriff
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The list is sorted, but then you put it in a "sorted" map. However, HashMap is not sorted, and it will never be sorted. There is currently only one map that is sorted, and that is TreeMap.

The problem is that any comparator for a TreeMap needs a reference to the TreeMap to be able to sort it. However, it needs to provide the comparator to the TreeMap constructor, so you can't put a reference to the TreeMap right away.

The following may work (I haven't tested it):
The TreeMapComparator should be a static nested class in your current class; its definition is not needed outside of it, only the fact that it is a Comparator.


Edit: of course you can use a LinkedHashMap; that keeps the insertion order. It won't stay ordered if you put new items though, but I'm not quite sure my above solution will handle that well.
 
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I would rather use this form of implementation:




and thats it.

You can also take away the Comparable from the Supplier class and use a Comparator when sorting.
 
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OUTPUT=

 
Sheriff
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You will have seen from the earlier posts that it is impossible to sort a hash map. What you are doing is sort a List and then finding the Map.Entry in the Map with the corresponding “V”. What you are actually doing is sorting a List<String>, which is a completely different task.

By the way: Why are you using a Comparator to sort something which already implements Comparable?
 
Arun Singh Raaj
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Ya, it is true that Map cannot be sorted by values straight.
And you are right, it could have been done by implementing Comparable interface to the Employee, But  i actually had a case in my mind that if the 'value' of the map object is user-defined type then you could have had Comparator implemented.

By the way, if i had the requirement to sort Map by value then does my way of converting Map values into List then finding it through the EntrySet, slow down the performance?

 
Campbell Ritchie
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Arun Singh Raaj wrote:. . . does my way of converting Map values into List then finding it through the EntrySet, slow down the performance?

What do you think? Look at how many loops you have written.
 
Master Rancher
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Why not have a List<Map.Entrry> with a suitable Comparator?
 
Saloon Keeper
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Such a comparator can easily be created like this:

Or even simpler, when you want to compare the values themselves and not a property of the values:
 
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