Here's a solution I came up with:
Stream<Document> stream = Stream.of(
new Document("A", "v1"),
new Document("A", "v2"),
new Document("B", "v1"),
new Document("C", "v1"),
new Document("C", "v2")
);
Iterator<Document> iterator = stream.iterator();
Stream<GroupedDocument> result = Stream.generate(new Supplier<GroupedDocument>() {
Document lastDoc = null;
@Override
public GroupedDocument get() {
try {
Document doc = Optional.ofNullable(lastDoc).orElseGet(iterator::next);
String id = doc.getId();
GroupedDocument gd = new GroupedDocument(doc.getId());
gd.getVersions().add(doc.getVersion());
if (!iterator.hasNext()) {
return null;
}
while (iterator.hasNext() && (doc = iterator.next()).getId().equals(id)) {
gd.getVersions().add(doc.getVersion());
}
lastDoc = doc;
return gd;
} catch (NoSuchElementException ex) {
return null;
}
}
});
Here are the Document
and GroupedDocument
classes:
class Document {
private String id;
private String version;
public Document(String id, String version) {
this.id = id;
this.version = version;
}
public String getId() {
return id;
}
public String getVersion() {
return version;
}
}
class GroupedDocument {
private String id;
private List<String> versions;
public GroupedDocument(String id) {
this.id = id;
versions = new ArrayList<>();
}
public String getId() {
return id;
}
public List<String> getVersions() {
return versions;
}
@Override
public String toString() {
return "GroupedDocument{" +
"id='" + id + '\'' +
", versions=" + versions +
'}';
}
}
Note that the resulting stream is an infinite stream. After all the groups there will be an infinite number of null
s. You can take all the elements that are not null by using takeWhile
in Java 9, or see this post.