Java sort hashmap by double value not working

Question

I am trying to sort a hashmap containing an object and a double value but it's not working as expected. Here is my code.

HashMap<Route, Double> unsortedRoutes = new HashMap<>();

unsortedRoutes.add(new Route("Route Z"),9.49307305463114167);
unsortedRoutes.add(new Route("Route A"),9.546311415463114167);
unsortedRoutes.add(new Route("Route K"),9.37307305463114167);
unsortedRoutes.add(new Route("Route Y"),9.89730305463114167);
unsortedRoutes.add(new Route("Route Z"),3.49307305463114167);
unsortedRoutes.add(new Route("Route A"),3.41870305463114167);
unsortedRoutes.add(new Route("Route K"),4.49307305463114167);
unsortedRoutes.add(new Route("Route Y"),5.34307305463114167);

HashMap<Route, Double> sortedRoutes = new HashMap<>();
        unsortedRoutes.entrySet()
                .stream()
                .sorted(comparingByValue())
                .forEachOrdered(x -> sortedRoutes.put(x.getKey(), x.getValue()));

When I ran the code these are the results I get

Name: Route D Distance = 3.4187030546311417
Name: Route J Distance = 5.343073054631142
Name: Route A Distance = 9.546311415463114
Name: Route Y Distance = 9.897303054631141
Name: Route U Distance = 4.493073054631141
Name: Route K Distance = 9.373073054631142
Name: Route Z Distance = 9.493073054631141
Name: Route Q Distance = 3.4930730546311417

I expected the results to be listed in ascending order.

You don't sort a `HashMap`, because insertion order isn't guaranteed to be preserved, that's why it doesn't work. If you really need a map, you have to use a `TreeMap` with a custom `Comparator` instead. — BackSlash, Sep 19 '18 at 12:22
HashMaps are by nature unsorted, and it makes no difference if you put in items in any particular order. If you need an ordered map then use an ordered map — Hovercraft Full Of Eels, Sep 19 '18 at 12:24

score 3 · Accepted Answer · answered Sep 19 '18 at 12:23

3

HashMaps make no guarantee on the order of elements stored in them, so even if you call put in a sorted fashion, the resulting HashMap will have no intrinsic order. If you want to preserve the order that put was called in, you should use a LinkedHashMap:

Map<Route, Double> sortedRoutes = new LinkedHashMap<>();

answered Sep 19 '18 at 12:23

Mureinik

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@Nikolas you experienced a downvote of your upvote? IMHO this answer is fine, just altering the implementing map class. – Joop Eggen Sep 19 '18 at 12:29

Java sort hashmap by double value not working

1 Answers1