Today in the NixMashup Links App I’m building we have a cached list of NixMashup Links from which we want to display only those links that have a certain tag. We’re not going to go to the database, but rather will use Java 8 Streams.
Pictures first. Here’s a sample list of links displayed for the tag “Streams.”
Collection Selection by List<Integer>
In essence what we’re doing is filtering our NixMashupLinks Collection by a List of Link IDs, or all of the link IDs for the selected tag. We’re going to retrieve those IDs from our NixMashupTagLinks Collection in the form of a List<Integer>.
Before we do that, here’s a representation of the NixMashupTagLink object from PostgreSQL. We’re going to make a List of the link_id field for the selected tag.
Onto the code.
public static List<Integer> getNixMashupTagLinkIds(String tag) {
return getNixMashupTagLinks()
.stream()
.filter(l -> l.tag.equals(tag))
.map(l -> l.linkId)
.collect(Collectors.toList());
}
Extracting the NixMashupLink Objects from the Link_ID List
Now for the interesting task of retrieving a subset of NixMashupLink objects. Below is the source on that. As you see we’re passing the List<Integer> of Link IDs we just created. Each NixMashupLink has a unique LinkId key which we’ll compare to the LinkId list, using a linkIds.contains(nixMashupLink.linkId) test to set a NixMashupLink.hasTag boolean property while iterating through the NixMashupLinks Collection.
With NixMashupLink.hasTag set we can use Java Streams to filter out only those items set to TRUE. As a bonus we’ll use a reverseSort Java Comparator “byPostDate” to display the most recent links for the tag first.
public static List<NixMashupLink> getNixMashupTagLinks(List<Integer> linkIds) {
List<NixMashupLink> allNixMashupLinks = getNixMashupLinks();
for (NixMashupLink nixMashupLink : allNixMashupLinks) {
nixMashupLink.setHasTag(linkIds.contains(nixMashupLink.linkId));
}
return allNixMashupLinks
.stream()
.filter(l -> l.getHasTag() == true)
.sorted(byPostDate)
.collect(Collectors.toList());
}
Damn fine looking Java Code, and no trip to the database.