java21学习笔记-序列集合

引入新的接口来表示具有定义的相遇顺序的集合。每个这样的集合都有一个明确定义的第一个元素、第二个元素，依此类推，直到最后一个元素。它还提供统一的 API 来访问其第一个和最后一个元素，并以相反的顺序处理其元素。

Sequenced collections

interface SequencedCollection<E> extends Collection<E> {
    // new method
    SequencedCollection<E> reversed();
    // methods promoted from Deque
    void addFirst(E);
    void addLast(E);
    E getFirst();
    E getLast();
    E removeFirst();
    E removeLast();
}

        List<String> list = new ArrayList<>();
        list.add("1");        list.add("4");
        list.add("6");        list.add("2");
        list.add("9");        list.add("3");
        System.out.println(list.getFirst());    //1
        System.out.println(list.getLast());     //3
        System.out.println(list.removeFirst()); //1
        System.out.println(list);               //[4, 6, 2, 9, 3]
        System.out.println(list.removeLast());  //3
        System.out.println(list);               //[4, 6, 2, 9]     
        list.addFirst("6");
        System.out.println(list);               //[6, 4, 6, 2, 9]
        System.out.println(list.reversed());    //[9, 2, 6, 4, 6]

Sequenced sets

interface SequencedSet<E> extends Set<E>, SequencedCollection<E> {
    SequencedSet<E> reversed();    // covariant override
}

        LinkedHashSet<String> set = new LinkedHashSet<>();
        set.add("1");        set.add("5");
        set.add("9");        set.add("2");
        set.add("4");        set.add("7");
        Set<String> collect = new HashSet<>(set);
        System.out.println(collect);        //[1, 2, 4, 5, 7, 9]
        LinkedHashSet<String> set1 = new LinkedHashSet<>(collect);
        System.out.println(set1.reversed());//[9, 7, 5, 4, 2, 1]

Sequenced maps

interface SequencedMap<K,V> extends Map<K,V> {
    // new methods
    SequencedMap<K,V> reversed();
    SequencedSet<K> sequencedKeySet();
    SequencedCollection<V> sequencedValues();
    SequencedSet<Entry<K,V>> sequencedEntrySet();
    V putFirst(K, V);
    V putLast(K, V);
    // methods promoted from NavigableMap
    Entry<K, V> firstEntry();
    Entry<K, V> lastEntry();
    Entry<K, V> pollFirstEntry();
    Entry<K, V> pollLastEntry();
}

        SequencedMap<String, String> map = new LinkedHashMap<>();
        map.put("name", "张三");              map.put("age", "18");
        map.put("nickname", "法外狂徒");       map.put("sex", "男");
        System.out.println(map);                //{name=张三, age=18, nickname=法外狂徒, sex=男}
        System.out.println(map.reversed());     //{sex=男, nickname=法外狂徒, age=18, name=张三}
        SequencedSet<String> sequencedKeySet = map.sequencedKeySet();
        System.out.println(sequencedKeySet);    //[name, age, nickname, sex]
        SequencedCollection<String> sequencedValues = map.sequencedValues();
        System.out.println(sequencedValues);    //[张三, 18, 法外狂徒, 男]
        SequencedSet<Map.Entry<String,String>> sequencedEntrySet = map.sequencedEntrySet();
        System.out.println(sequencedEntrySet);  //[name=张三, age=18, nickname=法外狂徒, sex=男]
        map.putFirst("first","first");
        System.out.println(map);                //{first=first, name=张三, age=18, nickname=法外狂徒, sex=男}
        map.putLast("last","last");
        System.out.println(map);                //{first=first, name=张三, age=18, nickname=法外狂徒, sex=男, last=last}
        System.out.println(map.firstEntry());   //first=first
        System.out.println(map.lastEntry());    //last=last
        System.out.println(map.pollFirstEntry());//first=first
        System.out.println(map);                //{name=张三, age=18, nickname=法外狂徒, sex=男, last=last}
        System.out.println(map.pollLastEntry());//last=last
        System.out.println(map);                //{name=张三, age=18, nickname=法外狂徒, sex=男}

改造

上面定义的三个新接口完全适合现有的集合类型层次结构

List 现在将 SequencedCollection 作为其直接超接口

Deque 现在将 SequencedCollection 作为其直接的超级接口

LinkedHashSet 还实现了 SequencedSet

SortedSet 现在将 SequencedSet 作为其直接超接口

LinkedHashMap 还实现了 SequencedMap

SortedMap 现在将 SequencedMap 作为其直接的超级接口

 Collections 实用程序类添加了新方法，为三种新类型创建不可修改的包装器

Collections.unmodifiableSequencedCollection(sequencedCollection)

Collections.unmodifiableSequencedSet(sequencedSet)

Collections.unmodifiableSequencedMap(sequencedMap)

       var unmodifiableSequencedCollection = Collections.unmodifiableSequencedCollection(sequencedValues);
        System.out.println(unmodifiableSequencedCollection);    //[张三, 18, 法外狂徒, 男]

        var unmodifiableSequencedSet = Collections.unmodifiableSequencedSet(sequencedEntrySet);
        System.out.println(unmodifiableSequencedSet);           //[name=张三, age=18, nickname=法外狂徒, sex=男]

        var unmodifiableSequencedMap =  Collections.unmodifiableSequencedMap(map.reversed());
        System.out.println(unmodifiableSequencedMap);           //{sex=男, nickname=法外狂徒, age=18, name=张三}