集合视图与快照：改一处，另一处会变吗

import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
public class Main {
    record Item(String id, int rank) {
        Item { if (id == null || id.isBlank()) throw new IllegalArgumentException("id"); }
    }
    static final Comparator<Item> ORDER = Comparator.comparingInt(Item::rank).thenComparing(Item::id);
    static final class Snapshot {
        private final List<Item> items;
        Snapshot(List<Item> source) {
            if (source == null) throw new IllegalArgumentException("source");
            HashSet<String> ids = new HashSet<>();
            List<Item> copy = new ArrayList<>();
            for (Item item : source) {
                if (item == null || !ids.add(item.id())) throw new IllegalArgumentException("duplicate/null");
                copy.add(item);
            }
            copy.sort(ORDER);
            items = List.copyOf(copy); // 容器快照与外部列表脱离。
        }
        List<Item> page(Item after, int limit) {
            if (limit < 1 || limit > 100) throw new IllegalArgumentException("limit");
            int lo = 0, hi = items.size();
            if (after != null) {
                while (lo < hi) {
                    int mid = lo + (hi-lo)/2;
                    if (ORDER.compare(items.get(mid),after) <= 0) lo = mid+1;
                    else hi = mid;
                }
            }
            int end = lo + Math.min(limit,items.size()-lo);
            return List.copyOf(items.subList(lo,end)); // 不暴露内部范围视图。
        }
    }
    static void require(boolean condition) {
        if (!condition) throw new AssertionError();
    }
    public static void main(String[] args) {
        Item a = new Item("a",1), b = new Item("b",1), c = new Item("c",2);
        List<Item> source = new ArrayList<>(List.of(b,a,c));
        Snapshot snapshot = new Snapshot(source);
        // 同分平价、后续页、尾部、空快照与来源修改。
        require(snapshot.page(null,2).equals(List.of(a,b)));
        require(snapshot.page(b,2).equals(List.of(c)));
        require(snapshot.page(c,2).isEmpty());
        require(new Snapshot(List.of()).page(null,2).isEmpty());
        source.clear();
        require(snapshot.page(null,3).equals(List.of(a,b,c)));
        boolean rejected = false;
        try { new Snapshot(List.of(a,a)); } catch (IllegalArgumentException e) { rejected = true; }
        require(rejected);
        System.out.println("PASS 6");
    }
}