JDK 21新特性---有序集合

概述

• 在JDK 21中，正式加入了序列集合的功能，新增了SequencedCollection，SequencedSet，SequencedMap接口，使得Java中的有序集合类可以按照统一的方法来进行集合的操作。
• 架构图：
  

三大接口

• SequencedCollection

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

• SequencedSet

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

• SequencedMap

       interface SequencedMap<K,V> extends Map<K,V> {
           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();
       }
  

示例

	List<Integer> originalList = Lists.newArrayList(1, 2, 3, 4, 5);
	List<Integer> reversedList=originalList.reversed();
	System.out.println("original list : " + originalList);
	System.out.println("reversed list : " + reversedList);
	originalList.addLast(6);
	System.out.println("modified original list : " + originalList);
	System.out.println("modified reversed list : " + reversedList);
	SequencedMap<String,Integer> sequencedMap=Maps.newLinkedHashMap();
	sequencedMap.put("a",1);
	sequencedMap.put("b",2);
	Map.Entry<String,Integer> firstEntry=sequencedMap.entrySet().iterator().next();
	System.out.println("map first value : "+firstEntry.getValue());
	firstEntry.setValue(100);
	System.out.println("modified map first value : "+firstEntry.getValue());
	Map.Entry<String,Integer> firstEntry2=sequencedMap.firstEntry();
	System.out.println("map first value : "+firstEntry2.getValue());
	//抛出UnsupportedOperationException
	firstEntry2.setValue(200);