Custom Sort Maps in Java 8

Question

I have a list of maps like below. I want to sort the maps inside the list in custom order in Java 8.

For Example, Below is the list of maps .

[{model=Ferrari},
{model=Tesla},
{model=Benz},
{model=Honda}]

If I sort the above list of maps using the below code, Its sorting in Alphabetical order.

l.sort(Comparator.comparing((Map<String,String> mp) -> mp.get("model")));

gives the output below.

[{model=Benz}, {model=Ferrari}, {model=Honda}, {model=Tesla}]

But I need to sort this map based on the order(Not Alphabetical Order) : Honda,Tesla,Benz, Ferrari

Is there a better approach we can achieve the custom sort in Java 8 ?

Answer 1

You should define your preferred order somewhere outside of this code. Put it into some kind of list and then transform it to Map of modelName -> index.

Map<String,Integer> modelOrder = .....

then, you can add one more lookup for getting ordinal for comparison.

l.sort(Comparator.comparing((Map<String,String> mp) ->
    modelOrder.get(mp.get("model"))));

This will explode in your face if unknown model is ever encountered. You can do quick fix by something like

l.sort(Comparator.comparing((Map<String,String> mp) -> 
    modelOrder.getOrDefault(mp.get("model"),Integer.MAX_VALUE)));

to put unknown models at the very end, but they won't be sorted between themselves (so even same unknown model might be interlaced by other unknown ones).

Edit:

As people suggested in comments, it can be solved nicely with .thenComparing

l.sort(Comparator
    .comparing((Map<String,String> mp) -> modelOrder.getOrDefault(mp.get("model"),Integer.MAX_VALUE))
    .thenComparing((Map<String,String> mp) -> mp.get("model")));