Java's three major collections (two): Set

Set: Represents an unordered (stored in and taken out) non-repeatable collection, which can save null elements.
Under the interface: HashSet , TreeSet , LinkedHashSet

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HashSet : Based on HashMap, the bottom layer is key-value pair + linked list + red-black tree (after jdk1.8)
initial length: 16, load factor: 0.75

Mechanism for judging duplication: hashcode()—>equals()

1. First determine the hashCode value of the element
2. In the case of equal hashCode, continue to judge equals comparison, if they are equal, it is considered the same element

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LinkedHashSet : is an ordered HashSet, a linked list is maintained on the basis of HashSet

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TreeSet : Based on TreeMap, the bottom layer of TreeMap is based on red-black tree (it is a special binary tree with small left fork value and large right fork value, with self-balancing mechanism)

1. Natural sorting: The element type must implement the Comparable interface, and the compareTo() method must be rewritten
   . Positive return value: ascending order
   Negative return value: descending order
   Return value 0: the same element will not be added

2. Custom sorting: when creating a TreeSet object, pass in an implementation class of the Comparator interface, which must override the compare method
   . Positive return value: ascending order,
   negative return value: descending order,
   return value 0: the same element

Note:
1. When natural sorting and custom sorting are used at the same time, custom sorting has a higher priority.
2. Normally, natural sorting is used for default sorting , and custom sorting is used for special sorting.

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Because Set is unordered and no subscript, there are only two traversal methods: foreach and one-way iterator

		//1、foreach
		for (Object object : hashSet) {
    
    
			System.out.println(object);
		}
		
		//单向迭代器
		Iterator iterator = hashSet.iterator();
		while (iterator.hasNext()) {
    
    
			Object object = (Object) iterator.next();
			System.out.println(object);
		}