Create arraylist from list of maps for certain key value with java 8

I have an object with a list of maps. Each map looks like this:

{
  id: "something",
  link: "someLink"
}

I am trying to create an array list of all the id’s in the maps. I can do this with a simple loop

 List<String> Ids = new ArrayList<>();
        
 List<Map<String, String>> maps = pojo.getMaps();
        
 for(Map<String, String> map: maps) {
       Ids.add(map.get("id"));
    }

But how is this done in one line using java 8 streams? I have never used it with maps so I am lost. I assume it would be along the lines of something like this but i honestly dont know

List<String> ids = pojo.getMaps().stream().map(Map.Entry::  ???? ).collect(Collectors.toList())

Answer

pojo.getMaps().stream()

So far, so good. You now have a stream of Map<String, String> objects. We need to just get the keys from this. So, given one of your weird map things, how do we turn that into the key value?

Looks like a trivial map.get("id") does that job, no?

So let’s put that in the lambda:

pojo.getMaps().stream().map(theMap -> theMap.get("id"))

and now we have a Stream<String> with ids.

HOWEVER, big note!

The fact that you start out with a map object that looks like an object is a giant code smell. Most likely you should go back a few steps in your project and fix that instead. You really ought to have a class that represents this link concept:

@Value
public class Link {
    String id, link;
}

and go from there:

listOfLinks.stream().map(Link::getId).distinct().collect(...);

NB: The above uses Lombok’s @Value.

functional is just a tool

Note that there is no need to rewrite your code to ‘use java 8 features’. Compare:

    var ids = pojo.getMaps().stream()
      .map(m -> m.get("id"))
      .collect(Collectors.toList());

with:

    var ids = new ArrayList<String>();
    for (var m : pojo.getMaps()) ids.add(m.get("id"));

it’s about the same amount of code.