Reduce a set of objects by a common field in Java-8

Question

I have a list of objects Obdefined as

class Ob {
   private String type;
   private List<String> attire;
   // standard getter and setters

   public Ob (String type){
        this.type=type;
    }

    public Ob addAttrire(String att){
        if(attire == null){
            attire = new ArrayList<>();
        }
        attire.add(att);
        return this;
    }
}

I get objects like

[{
"type" : "upper"
attires : [{"t1","t2"}]
},
{
"type" : "upper"
attires : ["t3","t4"]
},
{
"type" : "lower"
attires : ["l1","l2"]
}]

which I have to combine as

[{
"type" : "upper"
attires : ["t1","t2","t3","t4"]
},{
"type" : "lower"
attires : ["l1","l2"]
}]

How can I use a thread for this. Reduces help? The stream that can be used is

List<Ob> coll = new ArrayList<>();


 coll.add(new Ob("a").addAttrire("1").addAttrire("2").addAttrire("3"));
        coll.add(new Ob("a").addAttrire("1").addAttrire("2").addAttrire("3"));
        coll.add(new Ob("a").addAttrire("1").addAttrire("2").addAttrire("3"));
        coll.add(new Ob("b").addAttrire("1").addAttrire("2").addAttrire("3"));
        coll.add(new Ob("b").addAttrire("1").addAttrire("2").addAttrire("3"));
        coll.add(new Ob("b").addAttrire("1").addAttrire("2").addAttrire("3"));



Collection<Ob> values = coll.stream()
                .collect(toMap(Ob::getType, Function.identity(), (o1, o2) -> {
                    o1.getAttire().addAll(o2.getAttire());
                    return o1;
                })).values();

Updated question with Ruben solution. There is no need to remove duplicates, but this can be done using the set in Ob for the outfit. The current solution works flawlessly.

Answer 1

You can compile toMap with a merge function that merges lists

    Collection<Ob> values = coll.stream()
     .collect(toMap(Ob::getType, Function.identity(), (o1, o2) -> {
        o1.getAttire().addAll(o2.getAttire());
        return o1;
    })).values();

Answer 2

groupingBy, , Ob, Ob, type.

, Rubens , . , , Ob , , .

public static void testTrickyStreamSet() {
    Stream<Ob> s = Stream.of(
        new Ob("a", "1", "2"),
        new Ob("b", "1", "4"),
        new Ob("a", "1", "3"),
        new Ob("b", "1", "5"));

     List<Ob> os = s.collect(groupingBy(o -> o.type))
         .entrySet().stream()
         .map(e -> new Ob(e.getKey(), 
             e.getValue().stream().flatMap(o -> o.attire.stream()).collect(toList())))
         .collect(toList());

     // Prints [<Ob type=a, attire=[1, 2, 3]>, <Ob type=b, attire=[1, 4, 5]>]
     System.out.println(os);
}

public static class Ob {
    public String type;
    public List<String> attire;

    public Ob(String type, String... attire) {
        this.type = type;
        this.attire = Arrays.asList(attire);
    }

    public Ob(String type, List<String> attire) {
        this.type = type;
        this.attire = new ArrayList<>(attire);
    }

    @Override
    public String toString() {
        return "<Ob type=" + type + ", attire=" + attire + ">";
    }
}