HashMap底层

public class HashSet<E>{
    //重要属性
    private transient HashMap<E,Object> map;
    private static final Object PRESENT = new Object();
    //构造器
    public HashSet() {
            map = new HashMap<>();//HashSet底层就是利用HashMap来完成的
        }
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }    
}