Convert List<List<Object>> to Map<String,String> Java 8

Question

I have a List where OcrImageLocation contain List and String s3_id;

I just wanted to convert list to map which contain s3_id as key and image_location as value using java 8

public class PageLocationInfo {
   @JsonProperty("page_index")
   private String page_index;
   @JsonProperty("location")
   private String location;
   @JsonProperty("image_location")
   private String image_location;
}
public class OcrImageLocation {
   private List<PageLocationInfo> page_info;
   @JsonProperty("s3_id")
   private String s3_id;
}

What are you going to do with duplicates? You can't have multiple values for the same s3_id key in your output Map. — Eran
– Eran, Commented Apr 30, 2020 at 4:52
Maybe you want a map of s3_id => list of image_locations, in other words Map<String, List<String>>? — xdhmoore
– xdhmoore, Commented Apr 30, 2020 at 4:57
In your OcrImageLocation s3_id maps to a list of PageLocationInfo. that means one s3_id map with multiple image_locations. Since Map doesn't allow duplicate keys what you are asking is confusing. — Ravinda Lakshan
– Ravinda Lakshan, Commented Apr 30, 2020 at 5:05
Actually i am thinking to make s3_id + page_index as key as it would be unique always — Rahul Anand
– Rahul Anand, Commented Apr 30, 2020 at 5:11
Map<String,String> as per this, May I know one s3_id fill contain only one image location. if is true the it has sense otherwise data will be overridden. It is is not then Map<String, List<String>> — Sachin Kumar
– Sachin Kumar, Commented Apr 30, 2020 at 6:24

Eran · Accepted Answer · 2020-04-30 06:48:40Z

2

You can map each pair of OcrImageLocation and PageLocationInfo to a Map.Entry<String,String> of the corresponding s3_id + page_index and image_location:

Map<String, String> map =
    input.stream()
         .flatMap(oil -> oil.getPageInfo()
                            .stream()
                            .map(pli -> new SimpleEntry<>(oil.getS3Id() + pli.getPageIndex(),
                                                          pli.getImageLocation())))
         .collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));

answered Apr 30, 2020 at 6:48

Eran

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Vikas · Accepted Answer · 2020-04-30 06:32:00Z

0

You can do with Stream as below,

Map<String, String> result = locationList.stream().map(e -> e.getPage_info().stream()
          .collect(Collectors.toMap(e1 -> e.getS3_id() + e1.getPage_index(),
                  PageLocationInfo::getImage_location)))
          .flatMap(e -> e.entrySet().stream())
          .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

answered Apr 30, 2020 at 6:32

Vikas

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3 Comments

ETO Over a year ago

Collecting twice is not a good idea. Actually the first collect is redundant. You can flatmap the uncollected stream.

ETO Over a year ago

And if you do that, you'll end up with Eran's asnwer

Vikas Over a year ago

I came to this solution after spending almost 15 mins. Yes, I agree that collecting twice is not a good idea. Erans answer is perfect.

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