I have a list of objects like this:

MyObject {

   Date date;
   int value;
}

I want to iterate the list and only keeps the objects where the dates differ. If there are two objects with the same date then I want to keep the object with the bigger value. Can this be achieved by using streams and predicates somehow?

CodePudding user response：

Yes, you can use stream to filter distinct elements:

public class MyObject {
    
    private final LocalDate date;
    
    private final int value;
    
    public MyObject(LocalDate date, int value) {
        this.date = date;
        this.value = value;
    }
    
    public final LocalDate getDate() {
        return date;
    }
    
    public final int getValue() {
        return value;
    }
    
    @Override
    public String toString() {
        return "[date="   date   ", value="   value  "]";
    }
}

To get distinct elements you can do:

public static Collection<MyObject> getDistinct(List<MyObject> list) {
    return list.stream().collect(
            Collectors.groupingBy(MyObject::getDate, 
                    Collectors.collectingAndThen(
                            Collectors.maxBy(Comparator.comparingInt(MyObject::getValue)), 
                            Optional::get)))
            .values();
}

Then:

List<MyObject> list = List.of(new MyObject(LocalDate.of(2021, 1, 1), 1),
            new MyObject(LocalDate.of(2021, 1, 1), 2),
            new MyObject(LocalDate.of(2022, 2, 2), 2));
    
Collection<MyObject> distinct = getDistinct(list);
    
distinct.forEach(System.out::println);

Output:

[date=2022-02-02, value=2]
[date=2021-01-01, value=2]

Note: I would use LocalDate instead of Date

CodePudding user response：

You should use Map and Map.merge() instread of Stream.

record MyObject(Date date, int value) {}

public static void main(String[] args) {
    List<MyObject> list = List.of(
        new MyObject(new Date(0), 0),
        new MyObject(new Date(0), 1),
        new MyObject(new Date(1000), 2));

    Map<Date, MyObject> map = new LinkedashMap<>();
    for (MyObject e : list)
        map.merge(e.date(), e, (a, b) -> a.value() > b.value() ? a : b);
    list = new ArrayList<>(map.values());

    for (MyObject e : list)
        System.out.println(e);
}

output:

MyObject[date=Thu Jan 01 09:00:00 JST 1970, value=1]
MyObject[date=Thu Jan 01 09:00:01 JST 1970, value=2]