Tool class for manipulating Collection and Map-Collections

Collections: Tool classes for manipulating Collection and Map

The difference between Collections and Collection

Collections is a tool class used to manipulate Collection and Map.

• Common methods in the Collections class.
  reverse(List): Reverse the order of the elements in
  List shuffle(List): Randomly arrange the order in the set (every time is random)
  sort(list): Sort according to natural order, if you want to change it The method needs to use sort (list, Comparator)
  swap (i, j); exchange the position of element i and element j in the List collection.
  frequency(i); Returns the number of occurrences of the specified element in the collection.
  copy(list1,list2); copy list2 to list1

    public static void main(String[] args) {
    
    
        List list = new ArrayList();
        list.add("1");
        list.add("1");
        list.add("1");
        list.add("1");
        list.add("1");
        list.add("1");

//        ArrayList<Object> objects = new ArrayList<Object>();
//直接复制会导致出错，是因为objects集合的大小小于list集合。
        List dest = Arrays.asList(new Object[list.size()]);
        System.out.println(dest);
        Collections.copy(dest,list);
        System.out.println(dest);	
        //返回线程安全的list
   		List list1 = Collections.synchronizedList(list);
    }