java - 如何使Set中的元素始终是第一个元素

Question

我有一个树集，里面有很多元素。我把它们按字母顺序排列。我真的可以让它的元素始终是第一个元素而不干扰排序顺序吗？

例如:我希望“partnumber”元素始终是集合中的第一个元素。能做到吗？预先感谢!

Answer 1

这应该允许您将头元素添加到任何Set。

public class HeadSet<T> extends AbstractSet<T> implements Set<T> {
    // The extra item to add at the head.
    final T head;
    // The rest of the items.
    final Set<T> body;

    public HeadSet(T head, Set<T> body) {
        this.head = head;
        this.body = body;
    }

    @Override
    public Iterator<T> iterator() {
        // Iterator starts with the head element and then delegates to the body iterator.
        return new Iterator<T>() {
            private T next = head;
            private Iterator<T> i = body.iterator();

            @Override
            public boolean hasNext() {
                if (next == null && i.hasNext()) {
                    next = i.next();
                }
                return next != null;
            }

            @Override
            public T next() {
                if (hasNext()) {
                    T n = next;
                    next = null;
                    return n;
                } else {
                    throw new NoSuchElementException();
                }
            }

        };
    }

    @Override
    public int size() {
        // Size is one more than the size of the body.
        return 1 + body.size();
    }
}

public void test() {
    Set<String> bodySet = new TreeSet<>(Arrays.asList("One", "Two"));
    Set<String> mySet = new HeadSet("Choose part number ...", bodySet);
    System.out.println(mySet);
}