How to group all the Objects in a List with the same Object property? Without mentioning the Object property value.
Model Class:
public class Item {
private String id;
private String name;
private String team
}
List<item> items = new ArrayList();
I have tried this:
items.stream().filter(item -> "Elites".equals(item.team)).collect(Collectors.toList());
But this requires passing the team name as a parameter.
How can we group the items without specifying a team value?
And Making a HashMap with Key as the item. team and value as a list of key-value pairs with that team.name & item.id
Like this:
"item.team":{
"item.id":"item.name",
"item.id":"item.name",
"item.id":"item.name",
.....
}
If we can return a Map<String, List<Item>>
, where the key is the team
and the value is a List<Item>
belonging to that team, we can use
final Map<String, List<Item>> itemsByTeam =
items.stream().collect(Collectors.groupingBy(item -> item.team));
Remark: This solution was first posted in a comment by another user and they deleted the comment shortly after. I do not remember the user's name. If they post an answer, I will delete mine. If they do not want to post an answer, but contact me, I will credit them by name.
A comment on the code: I would recommend to introduce getters for the attributes since the stream
-operation is most likely to be called outside of class Item
itself, hence attribute team
will not be visible. Also, this would lead to an implementation like
final Map<String, List<Item>> itemsByTeam =
items.stream().collect(Collectors.groupingBy(Item::getTeam));
which may or may not be regarded as "more pleasing" to the reader.
From the accepted answer by Turing85 .
I have created a complete solution for the Question I asked.
To create an output with the following structure:
"item.team":{
"item.id":"item.name",
"item.id":"item.name",
"item.id":"item.name",
.....
}
Source data:
List<Item> itemsListData = //Get the data
Function to group the Items:
public static Map<String, List<Item>> groupItemsByTeam(Collection<Item> itemsList) {
return itemsList.stream().collect(Collectors.groupingBy(Item::team));
}
Structure the items list returned by groupItemsByTeam :
//GET THE GROUPED DATA
Map<String, List<Item>> result = groupItemsByTeam(itemsListData);
//STRUCTURE THE GROUPED DATA
for (Entry<String, List<Item>> parentItem : result .entrySet()) {
System.out.println(parentItem .getKey() + " : "); // item.team value
for (Item childItem : parentItem.getValue()) {
System.out.println(childItem.getKEY() + " = " + childItem.getVALUE());
}
System.out.println("-------------------------------------------");
}
OUTPUT:
Team A :
Item 1= Item 1 name
Item 2= Item 2 name
-------------------------------------------
Team G :
Item 456= Item 456 name
Item 254= Item 254 name
-------------------------------------------
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