Using streams to collect to Map

Question

I have the following TreeMap:

TreeMap<Long,String> gasType = new TreeMap<>(); // Long, "Integer-Double"
gasType.put(1L, "7-1.50");
gasType.put(2L, "7-1.50");
gasType.put(3L, "7-3.00");
gasType.put(4L, "8-5.00");
gasType.put(5L, "8-7.00");
Map<Integer,TreeSet<Long>> capacities = new TreeMap<>);

The key is of the form 1L (a Long), and value of the form "7-1.50" (a String concatenation of an int and a double separated by a -).

I need to create a new TreeMap where the keys are obtained by taking the int part of the values of the original Map (for example, for the value "7-1.50", the new key will be 7). The value of the new Map would be a TreeSet containing all the keys of the original Map matching the new key.

So, for the input above, the value for the 7 key will be the Set {1L,2L,3L}.

I can do this without Streams, but I would like to do it with Streams. Any help is appreciated. Thank you.

Eran · Accepted Answer · 2018-05-30 07:34:09Z

5

Here's one way to do it:

Map<Integer,TreeSet<Long>> capacities = 
  gasType.entrySet()
         .stream ()
         .collect(Collectors.groupingBy (e -> Integer.parseInt(e.getValue().substring(0,e.getValue ().indexOf("-"))),
                                         TreeMap::new,
                                         Collectors.mapping (Map.Entry::getKey,
                                                             Collectors.toCollection(TreeSet::new))));

I modified the original code to support integers of multiple digits, since it appears you want that.

This produces the Map:

{7=[1, 2, 3], 8=[4, 5]}

If you don't care about the ordering of the resulting Map and Sets, you can let the JDK decide on the implementations, which would somewhat simplify the code:

Map<Integer,Set<Long>> capacities = 
  gasType.entrySet()
         .stream ()
         .collect(Collectors.groupingBy (e -> Integer.parseInt(e.getValue().substring(0,e.getValue ().indexOf("-"))),
                                         Collectors.mapping (Map.Entry::getKey,
                                                             Collectors.toSet())));

edited May 30, 2018 at 7:34

answered May 30, 2018 at 7:08

Eran

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3 Comments

Eran Over a year ago

@Aominè see comment near the end that eliminates that assumption.

Ousmane D. Over a year ago

yup just about got to it :)

Nizamuddin Shaikh Over a year ago

Yeah, Integers aren't always single digit and you have provided both possibilities.This is an elegant solution! Grateful to you! Thanks.

Ravindra Ranwala · Accepted Answer · 2018-05-30 08:23:35Z

4

You may try this out,

final Map<Integer, Set<Long>> map = gasType.entrySet().stream()
        .collect(Collectors.groupingBy(entry -> Integer.parseInt(entry.getValue().substring(0, 1)),
                Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));

UPDATE

If you want to split the value based on "-" since there may be more that one digit, you can change it like so:

final Map<Integer, Set<Long>> map = gasType.entrySet().stream()
        .collect(Collectors.groupingBy(entry -> Integer.parseInt(entry.getValue().split("-")[0]),
                Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));

edited May 30, 2018 at 8:23

answered May 30, 2018 at 7:14

Ravindra Ranwala

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3 Comments

Nizamuddin Shaikh Over a year ago

Is there any way to split the value based on "-" so that other than single digit also can be utilized?

Ravindra Ranwala Over a year ago

I have changed the answer according to your new requirement.

Nizamuddin Shaikh Over a year ago

Thanks @Ravindra Sir.

Hadi · Accepted Answer · 2018-05-30 07:48:04Z

2

3 Comments

Nizamuddin Shaikh Over a year ago

I will try this also, Thanks! much appreciated Sir.

Ousmane D. Over a year ago

Why do you need to map to SimpleEntry? That just adds further overhead which can be avoided and also collecting to a list only to create a stream from it in the middle of a stream pipeline is suboptimal. I’d do all the key extraction and value extraction in the groupingBy.

Hadi Over a year ago

You are right, but i thought it shouldn't be same with other answer.

gasType.entrySet()                 .stream()             .collect(Collectors.groupingBy(m->Integer.valueOf(m.getValue().split("-")[0]),                         Collectors.mapping(Map.Entry::getKey,Collectors.toCollection(TreeSet::new))));

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