HashSet

类关系

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{}

主要参数

	// 底层通过HashMap来实现
	private transient HashMap<E,Object> map;
	
    // Dummy value to associate with an Object in the backing Map
    // 底层借助HashMap来实现去重,那么value存储这里的空对象
	private static final Object PRESENT = new Object();

构造方法

构造方法在底层会创建不同的Map，前几个是HashMap，最后一个是LinkedHashMap，这个主要是给LinkedHashSet提供的

	public HashSet() {
        map = new HashMap<>();
    }
	
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
	
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        // 创建一个LinkedHashMap,这样LinkedHashSet底层借助其实现排序去重
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

基础方法

add()

	public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

remove()

	public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

LinkedHashSet

LinkedHashSet类的内容很简单，就这么几行，LinkedHash和HashSet的相比，不但元素去重，而且还能保持元素顺序，那么底层就是借助LinkedHashMap

那么LinkedHashMap如何来的，就在构造方法中，LinkedHashSet调用的都是父类HashSet3个参数的构造方法，这样通过上面就可以看到，直接创建了LinkedHashMap

public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = -2851667679971038690L;

    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, true);
    }

    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }

    public LinkedHashSet() {
        super(16, .75f, true);
    }

    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }

    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}