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Sort a HashMap based on a Value object's attribute

Arnav Velimala
Ranch Hand

Joined: Jun 04, 2007
Posts: 37

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?

Rob Spoor

Joined: Oct 27, 2005
Posts: 19958

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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Sebastian Janisch
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Joined: Feb 23, 2009
Posts: 1183
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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